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磁共振(磁谐振耦合)无线充电技术鼻祖级文章-英文原文

Wireless Power Transfer via Strongly Coupled Magnetic ResonancesAndré Kurs,1* Aristeidis Karalis,2 Robert Moffatt,1 J. D. Joannopoulos,1 Peter Fisher,3Marin Soljačić11Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 2Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 3Department of Physics and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.*To whom correspondence should be addressed. E-mail: akurs@Using self-resonant coils in a strongly coupled regime, we experimentally demonstrate efficient non-radiative power transfer over distances of up to eight times the radius of the coils. We demonstrate the ability to transfer 60W with approximately 40% efficiency over distances in excess of two meters. We present a quantitative model describing the power transfer which matches the experimental results to within 5%. We discuss practical applicability and suggest directions for further studies. At first glance, such power transfer is reminiscent of the usual magnetic induction (10); however, note that the usual non- resonant induction is very inefficient for mid-range applications.Overview of the formalism. Efficient mid-range power transfer occurs in particular regions of the parameter space describing resonant objects strongly coupled to one another. Using coupled-mode theory to describe this physical system (11), we obtain the following set of linear equationsIn the early 20th century, before the electrical-wire grid, Nikola Tesla (1) devoted much effort towards schemes to a&m(t)=(iωm-Γm)a m(t)+∑iκmn a n(t)+F m(t)n≠m(1)transport power wirelessly. However, typical embodiments (e.g. Tesla coils) involved undesirably large electric fields. During the past decade, society has witnessed a dramatic surge of use of autonomous electronic devices (laptops, cell- phones, robots, PDAs, etc.) As a consequence, interest in wireless power has re-emerged (2–4). Radiative transfer (5), while perfectly suitable for transferring information, poses a number of difficulties for power transfer applications: the efficiency of power transfer is very low if the radiation is omnidirectional, and requires an uninterrupted line of sight and sophisticated tracking mechanisms if radiation is unidirectional. A recent theoretical paper (6) presented a detailed analysis of the feasibility of using resonant objects coupled through the tails of their non-radiative fields for mid- range energy transfer (7). Intuitively, two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off- resonant objects. In systems of coupled resonances (e.g. acoustic, electro-magnetic, magnetic, nuclear, etc.), there is often a general “strongly coupled” regime of operation (8). If one can operate in that regime in a given system, the energy transfer is expected to be very efficient. Mid-range power transfer implemented this way can be nearly omnidirectional and efficient, irrespective of the geometry of the surrounding space, and with low interference and losses into environmental objects (6).Considerations above apply irrespective of the physical nature of the resonances. In the current work, we focus on one particular physical embodiment: magnetic resonances (9). Magnetic resonances are particularly suitable for everyday applications because most of the common materials do not interact with magnetic fields, so interactions with environmental objects are suppressed even further. We were able to identify the strongly coupled regime in the system of two coupled magnetic resonances, by exploring non-radiative (near-field) magnetic resonant induction at MHzfrequencies. where the indices denote the different resonant objects. The variables a m(t) are defined so that the energy contained in object m is |a m(t)|2, ωm is the resonant frequency of thatisolated object, and Γm is its intrinsic decay rate (e.g. due to absorption and radiated losses), so that in this framework anuncoupled and undriven oscillator with parameters ω0 and Γ0 would evolve in time as exp(iω0t –Γ0t). The κmn= κnm are coupling coefficients between the resonant objects indicated by the subscripts, and F m(t) are driving terms.We limit the treatment to the case of two objects, denoted by source and device, such that the source (identified by the subscript S) is driven externally at a constant frequency, and the two objects have a coupling coefficient κ. Work is extracted from the device (subscript D) by means of a load (subscript W) which acts as a circuit resistance connected to the device, and has the effect of contributing an additional term ΓW to the unloaded device object's decay rate ΓD. The overall decay rate at the device is therefore Γ'D= ΓD+ ΓW. The work extracted is determined by the power dissipated in the load, i.e. 2ΓW|a D(t)|2. Maximizing the efficiency η of the transfer with respect to the loading ΓW, given Eq. 1, is equivalent to solving an impedance matching problem. One finds that the scheme works best when the source and the device are resonant, in which case the efficiency isThe efficiency is maximized when ΓW/ΓD= (1 + κ2/ΓSΓD)1/2. It is easy to show that the key to efficient energy transfer is to have κ2/ΓSΓD> 1. This is commonly referred to as the strongcoupling regime. Resonance plays an essential role in thisDS S D'' power transfer mechanism, as the efficiency is improved by approximately ω2/ΓD 2 (~106 for typical parameters) compared to the case of inductively coupled non-resonant objects. Theoretical model for self-resonant coils. Ourexperimental realization of the scheme consists of two self- resonant coils, one of which (the source coil) is coupled inductively to an oscillating circuit, while the other (the device coil) is coupled inductively to a resistive load (12) (Fig. 1). Self-resonant coils rely on the interplay between distributed inductance and distributed capacitance to achieve resonance. The coils are made of an electrically conducting wire of total length l and cross-sectional radius a wound into Given this relation and the equation of continuity, one finds that the resonant frequency is f 0 = 1/2π[(LC )1/2]. We can now treat this coil as a standard oscillator in coupled-mode theory by defining a (t ) = [(L /2)1/2]I 0(t ).We can estimate the power dissipated by noting that the sinusoidal profile of the current distribution implies that the spatial average of the peak current-squared is |I 0|2/2. For a coil with n turns and made of a material with conductivity σ, we modify the standard formulas for ohmic (R o ) and radiation (R r ) µ0ω l a helix of n turns, radius r , and height h . To the best of our knowledge, there is no exact solution for a finite helix in the literature, and even in the case of infinitely long coils, the solutions rely on assumptions that are inadequate for our R o = 2σ 4πa µ πωr 42 ωh 2 (6)system (13). We have found, however, that the simple quasi- R =0 n 2 + (7)static model described below is in good agreementr ε 12 c3π3 c(approximately 5%) with experiment.We start by observing that the current has to be zero at the ends of the coil, and make the educated guess that the resonant modes of the coil are well approximated bysinusoidal current profiles along the length of the conducting wire. We are interested in the lowest mode, so if we denote by s the parameterization coordinate along the length of the conductor, such that it runs from -l /2 to +l /2, then the time- dependent current profile has the form I 0 cos(πs /l ) exp(i ωt ). It follows from the continuity equation for charge that the linear charge density profile is of the form λ0 sin(πs /l ) exp(i ωt ), so the two halves of the coil (when sliced perpendicularly to its axis) contain charges equal in magnitude q 0 = λ0l /π but opposite in sign.As the coil is resonant, the current and charge density profiles are π/2 out of phase from each other, meaning that the real part of one is maximum when the real part of the other is zero. Equivalently, the energy contained in the coil is 0The first term in Eq. 7 is a magnetic dipole radiation term(assuming r << 2πc /ω); the second term is due to the electric dipole of the coil, and is smaller than the first term for our experimental parameters. The coupled-mode theory decay constant for the coil is therefore Γ = (R o + R r )/2L , and its quality factor is Q = ω/2Γ.We find the coupling coefficient κDS by looking at the power transferred from the source to the device coil,assuming a steady-state solution in which currents and charge densities vary in time as exp(i ωt ).P =⎰d rE (r )⋅J (r ) =-⎰d r (A&S (r )+∇φS (r ))⋅J D (r ) at certain points in time completely due to the current, and at other points, completely due to the charge. Usingelectromagnetic theory, we can define an effective inductance L and an effective capacitance C for each coil as follows:=-1⎰⎰d r d r ' µJ &S(r ')+ρS(r ') 4π |r -r |ε0≡-i ωMI S I Dr '-r|r '-r |3⋅J D (r )(8)L =µ04π |I 0 |⎰⎰d r d r 'J (r )⋅J (r ')|r -r '|where the subscript S indicates that the electric field is due to the source. We then conclude from standard coupled-mode theory arguments that κDS = κSD = κ = ωM /2[(L S L D )1/2]. When 1 1 ρ(r )ρ(r ') the distance D between the centers of the coils is much larger= C 4πε 0 |q 0 | ⎰⎰d r d r ' |r -r '|(4)than their characteristic size, κ scales with the D -3dependence characteristic of dipole-dipole coupling. Both κ and Γ are functions of the frequency, and κ/Γ and the where the spatial current J (r ) and charge density ρ(r ) are obtained respectively from the current and charge densities along the isolated coil, in conjunction with the geometry of the object. As defined, L and C have the property that the efficiency are maximized for a particular value of f , which is in the range 1-50MHz for typical parameters of interest. Thus, picking an appropriate frequency for a given coil size, as we do in this experimental demonstration, plays a major role in optimizing the power transfer.1 2Comparison with experimentallydeterminedU =2 L |I 0 |parameters. The parameters for the two identical helical coils built for the experimental validation of the power 1 2 transfer scheme are h = 20cm, a = 3mm, r = 30 cm, and n = =2C|q 0 | (5)5.25. Both coils are made of copper. The spacing between loops of the helix is not uniform, and we encapsulate theuncertainty about their uniformity by attributing a 10% (2cm) uncertainty to h . The expected resonant frequency given these22dimensions is f0 = 10.56 ± 0.3MHz, which is about 5% off from the measured resonance at 9.90MHz.The theoretical Q for the loops is estimated to be approximately 2500 (assuming σ = 5.9 × 107 m/Ω) but the measured value is Q = 950±50. We believe the discrepancy is mostly due to the effect of the layer of poorly conductingcopper oxide on the surface of the copper wire, to which the current is confined by the short skin depth (~20μm) at this frequency. We therefore use the experimentally observed Q and ΓS= ΓD= Γ = ω/2Q derived from it in all subsequent computations.We find the coupling coefficient κ experimentally by placing the two self-resonant coils (fine-tuned, by slightly adjusting h, to the same resonant frequency when isolated) a distance D apart and measuring the splitting in the frequencies of the two resonant modes. According to coupled-mode theory, this splitting should be ∆ω = 2[(κ2-Γ2)1/2]. In the present work, we focus on the case where the two coils are aligned coaxially (Fig. 2), although similar results are obtained for other orientations (figs. S1 and S2).Measurement of the efficiency. The maximum theoretical efficiency depends only on the parameter κ/[(L S L D)1/2] = κ/Γ, which is greater than 1 even for D = 2.4m (eight times the radius of the coils) (Fig. 3), thus we operate in the strongly- coupled regime throughout the entire range of distances probed.As our driving circuit, we use a standard Colpitts oscillator whose inductive element consists of a single loop of copper wire 25cm in radius(Fig. 1); this loop of wire couples inductively to the source coil and drives the entire wireless power transfer apparatus. The load consists of a calibrated light-bulb (14), and is attached to its own loop of insulated wire, which is placed in proximity of the device coil and inductively coupled to it. By varying the distance between the light-bulb and the device coil, we are able to adjust the parameter ΓW/Γ so that it matches its optimal value, given theoretically by (1 + κ2/Γ2)1/2. (The loop connected to the light-bulb adds a small reactive component to ΓW which is compensated for by slightly retuning the coil.) We measure the work extracted by adjusting the power going into the Colpitts oscillator until the light-bulb at the load glows at its full nominal brightness.We determine the efficiency of the transfer taking place between the source coil and the load by measuring the current at the mid-point of each of the self-resonant coils with a current-probe (which does not lower the Q of the coils noticeably.) This gives a measurement of the current parameters I S and I D used in our theoretical model. We then compute the power dissipated in each coil from P S,D=ΓL|I S,D|2, and obtain the efficiency from η = P W/(P S+ P D+P W). To ensure that the experimental setup is well described by a two-object coupled-mode theory model, we position the device coil such that its direct coupling to the copper loop attached to the Colpitts oscillator is zero. The experimental results are shown in Fig. 4, along with the theoretical prediction for maximum efficiency, given by Eq. 2. We are able to transfer significant amounts of power using this setup, fully lighting up a 60W light-bulb from distances more than 2m away (figs. S3 and S4).As a cross-check, we also measure the total power going from the wall power outlet into the driving circuit. The efficiency of the wireless transfer itself is hard to estimate in this way, however, as the efficiency of the Colpitts oscillator itself is not precisely known, although it is expected to be far from 100% (15). Still, the ratio of power extracted to power entering the driving circuit gives a lower bound on the efficiency. When transferring 60W to the load over a distance of 2m, for example, the power flowing into the driving circuit is 400W. This yields an overall wall-to-load efficiency of 15%, which is reasonable given the expected efficiency of roughly 40% for the wireless power transfer at that distance and the low efficiency of the Colpitts oscillator.Concluding remarks. It is essential that the coils be on resonance for the power transfer to be practical (6). We find experimentally that the power transmitted to the load drops sharply as either one of the coils is detuned from resonance. For a fractional detuning ∆f/f0 of a few times the inverse loaded Q, the induced current in the device coil is indistinguishable from noise.A detailed and quantitative analysis of the effect of external objects on our scheme is beyond the scope of the current work, but we would like to note here that the power transfer is not visibly affected as humans and various everyday objects, such as metals, wood, and electronic devices large and small, are placed between the two coils, even in cases where they completely obstruct the line of sight between source and device (figs. S3 to S5). External objects have a noticeable effect only when they are within a few centimeters from either one of the coils. While some materials (such as aluminum foil, styrofoam and humans) mostly just shift the resonant frequency, which can in principle be easily corrected with a feedback circuit, others (cardboard, wood, and PVC) lower Q when placed closer than a few centimeters from the coil, thereby lowering the efficiency of the transfer.When transferring 60W across 2m, we calculate that at the point halfway between the coils the RMS magnitude of the electric field is E rms= 210V/m, that of the magnetic field isH rms= 1A/m, and that of the Poynting vector is S rms=3.2mW/cm2 (16). These values increase closer to the coils, where the fields at source and device are comparable. For example, at distances 20cm away from the surface of the device coil, we calculate the maximum values for the fields to be E rms= 1.4kV/m, H rms= 8A/m, and S rms= 0.2W/cm2. The power radiated for these parameters is approximately 5W, which is roughly an order of magnitude higher than cell phones. In the particular geometry studied in this article, the overwhelming contribution (by one to two orders of magnitude) to the electric near-field, and hence to the near- field Poynting vector, comes from the electric dipole moment of the coils. If instead one uses capacitively-loaded single- turn loop design (6) - which has the advantage of confining nearly all of the electric field inside the capacitor - and tailors the system to operate at lower frequencies, our calculations show (17) that it should be possible to reduce the values cited above for the electric field, the Poynting vector, and the power radiated to below general safety regulations (e.g. the IEEE safety standards for general public exposure(18).) Although the two coils are currently of identical dimensions, it is possible to make the device coil small enough to fit into portable devices without decreasing the efficiency. One could, for instance, maintain the product of the characteristic sizes of the source and device coils constant, as argued in (6).We believe that the efficiency of the scheme and the power transfer distances could be appreciably improved by silver-plating the coils, which should increase their Q, or by working with more elaborate geometries for the resonant objects (19). Nevertheless, the performance characteristics of the system presented here are already at levels where they could be useful in practical applications.References and Notes1. N. Tesla, U.S. patent 1,119,732 (1914).2.J. M. Fernandez, J. A. Borras, U.S. patent 6,184,651(2001).3.A. Esser, H.-C. Skudelny, IEEE Trans. Indust. Appl. 27,872(1991).4.J. Hirai, T.-W. Kim, A. Kawamura, IEEE Trans. PowerElectron. 15, 21(2000).5.T. A. Vanderelli, J. G. Shearer, J. R. Shearer, U.S. patent7,027,311(2006).6.A. Karalis, J. D. Joannopoul os, M. Soljačić, Ann. Phys.,10.1016/j.aop.2007.04.017(2007).7.Here, by mid-range, we mean that the sizes of the deviceswhich participate in the power transfer are at least a few times smaller than the distance between the devices. For example, if the device being powered is a laptop (size ~ 50cm), while the power source (size ~ 50cm) is in thesame room as the laptop, the distance of power transfer could be within a room or a factory pavilion (size of the order of a fewmeters).8. T. Aoki, et al., Nature 443, 671 (2006).9.K. O’Brien, G. Scheible, H. Gueldner, 29th AnnualConference of the IEEE 1, 367(2003).10.L. Ka-Lai, J. W. Hay, P. G. W., U.S. patent7,042,196(2006).11.H. Haus, Waves and Fields in Optoelectronics(Prentice- Supporting Online Material/cgi/content/full/1143254/DC1SOM TextFigs. S1 to S530 March 2007; accepted 21 May 2007Published online 7 June 2007; 10.1126/science.1143254 Include this information when citing this paper.Fig. 1. Schematic of the experimental setup. A is a single copper loop of radius 25cm that is part of the driving circuit, which outputs a sine wave with frequency 9.9MHz. S and D are respectively the source and device coils referred to in the text. B is a loop of wire attached to the load (“light-bulb”). The various κ’s represent direct couplings between the objects indicated by the arrows. The angle between coil D and the loop A is adjusted to ensure that their direct coupling is zero, while coils S and D are aligned coaxially. The direct couplings between B and A and between B and S are negligible.Fig. 2. Comparison of experimental and theoretical values for κ as a function of the separation between coaxially aligned source and device coils (the wireless power transfer distance.) Fig. 3. Comparison of experimental and theoretical values for the parameter κ/Γ as a function of the wireless power transfer distance. The theory values are obtained by using the theoretical κ and the experimentally measured Γ. The shaded area represents the spread in the theoretical κ/Γ due to the 5% uncertainty in Q.Fig. 4. Comparison of experimental and theoretical efficiencies as functions of the wireless power transfer distance. The shaded area represents the theoretical prediction for maximum efficiency, and is obtained by inserting theHall, Englewood Cliffs, NJ, 1984).12.The couplings to the driving circuit and the load donot theoretical values from Fig. 3 into Eq. 2 [with Γκ2/Γ2 1/2 W /ΓD= (1 +have to be inductive. They may also be connected by awire, for example. We have chosen inductive coupling in the present work because of its easier implementation. 13.S. Sensiper, thesis, Massachusetts Institute of Technology(1951).14.We experimented with various power ratings from 5W to75W.15.W. A. Edson, Vacuum-Tube Oscillators (Wiley, NewYork,1953).16.Note that E ≠cμ0H, and that the fields are out of phaseand not necessarily perpendicular because we are not in a radiativeregime.17.See supporting material on Science Online.18.IEEE Std C95.1—2005 IEEE Standard for Safety Levelswith Respect to Human Exposure to Radio FrequencyElectromagnetic Fields, 3 kHz to 300 GHz (IEEE,Piscataway, NJ,2006).19. J. B. Pendry, Science 306, 1353 (2004).20. The authors would like to thank John Pendry forsuggesting the use of magnetic resonances, and Michael Grossman and Ivan Čelanović for technical assistance.This work was supported in part by the Materials Research Science and Engineering Center program of the National Science Foundation under Grant No. DMR 02-13282, by the U.S. Department of Energy under Grant No. DE-FG02-99ER45778, and by the Army Research Officethrough the Institute for Soldier Nanotechnologies under Contract No. DAAD-19-02-D0002.) ]. The black dots are the maximum efficiency obtained from Eq. 2 and the experimental values of κ/Γ from Fig. 3. The red dots present the directly measured efficiency,as described in thetext.。

2001年考研英语真题及解析（黄皮书）2

2001年考研英语真题及解析（黄皮书）2第一部分英语知识应运试题解析一、文章总体分析本文是一篇报道性的文章，介绍了自露丝玛莉·韦斯特案件发生后，政府、法院、媒体各方面对于付款给证人的反应。

文章第一段介绍了政府的反应：要禁止报界买断证人新闻的举动。

第二至六段介绍了以大法官埃尔温勋爵为代表的法院在这个问题上的态度。

最后一段介绍了露丝玛莉·韦斯特案件的始末。

在该案件中由于很多证人通过讲述他们的经历而从媒体获得报酬，结果导致被告数罪并罚，被判十项无期徒刑。

结论为付款给证人的做法成为一个颇有争议的问题。

二、试题具体解析1. [A] as to关于，至于[B] for instance举例[C] in particular特别地[D] such as例如[答案] D[解析]本题考核的知识点是：逻辑关系。

解答该题时，考生需要判断空格前后部分prominent cases和The trial of Rosemary West 之间的关系，前者泛指“一些著名的案件”，后者是一个具体的案件，即“对露丝玛莉·韦斯特案件的审判”，可见两者是例证关系。

因此，所填入的选项应是一个表示“例如”或“像……一样”的连接词。

首先排除as to和in particular。

for instance（或for example）可表示“举例”，但放在句中多为插入语，且后面不可直接加宾语。

如：Here in Chicago, for instance, the movement was growing by leaps and bounds.（比如在芝加哥，运动正在迅猛发展）。

选项中只有介词短语such as可以接名词做宾语，表达“例如…，象这种的”的含义。

首段第一句话的结构比较复杂，中心句为The government is to ban payments to witnesses by newspapers（政府要禁止报界付钱给证人），现在分词结构seeking to buy up... Rosemary West 做后置定语，用来修饰newspapers，意为“试图收买涉及一些要案证人的报纸”。

Singularity of the density of states in the two-dimensional Hubbard model from finite size

a r X i v :c o n d -m a t /9503139v 1 27 M a r 1995Singularity of the density of states in the two-dimensional Hubbard model from ﬁnitesize scaling of Yang-Lee zerosE.Abraham 1,I.M.Barbour 2,P.H.Cullen 1,E.G.Klepﬁsh 3,E.R.Pike 3and Sarben Sarkar 31Department of Physics,Heriot-Watt University,Edinburgh EH144AS,UK 2Department of Physics,University of Glasgow,Glasgow G128QQ,UK 3Department of Physics,King’s College London,London WC2R 2LS,UK(February 6,2008)A ﬁnite size scaling is applied to the Yang-Lee zeros of the grand canonical partition function for the 2-D Hubbard model in the complex chemical potential plane.The logarithmic scaling of the imaginary part of the zeros with the system size indicates a singular dependence of the carrier density on the chemical potential.Our analysis points to a second-order phase transition with critical exponent 12±1transition controlled by the chemical potential.As in order-disorder transitions,one would expect a symmetry breaking signalled by an order parameter.In this model,the particle-hole symmetry is broken by introducing an “external ﬁeld”which causes the particle density to be-come non-zero.Furthermore,the possibility of the free energy having a singularity at some ﬁnite value of the chemical potential is not excluded:in fact it can be a transition indicated by a divergence of the correlation length.A singularity of the free energy at ﬁnite “exter-nal ﬁeld”was found in ﬁnite-temperature lattice QCD by using theYang-Leeanalysisforthechiral phase tran-sition [14].A possible scenario for such a transition at ﬁnite chemical potential,is one in which the particle den-sity consists of two components derived from the regular and singular parts of the free energy.Since we are dealing with a grand canonical ensemble,the particle number can be calculated for a given chem-ical potential as opposed to constraining the chemical potential by a ﬁxed particle number.Hence the chem-ical potential can be thought of as an external ﬁeld for exploring the behaviour of the free energy.From the mi-croscopic point of view,the critical values of the chemical potential are associated with singularities of the density of states.Transitions related to the singularity of the density of states are known as Lifshitz transitions [15].In metals these transitions only take place at zero tem-perature,while at ﬁnite temperatures the singularities are rounded.However,for a small ratio of temperature to the deviation from the critical values of the chemical potential,the singularity can be traced even at ﬁnite tem-perature.Lifshitz transitions may result from topological changes of the Fermi surface,and may occur inside the Brillouin zone as well as on its boundaries [16].In the case of strongly correlated electron systems the shape of the Fermi surface is indeed aﬀected,which in turn may lead to an extension of the Lifshitz-type singularities into the ﬁnite-temperature regime.In relating the macroscopic quantity of the carrier den-sity to the density of quasiparticle states,we assumed the validity of a single particle excitation picture.Whether strong correlations completely distort this description is beyond the scope of the current study.However,the iden-tiﬁcation of the criticality using the Yang-Lee analysis,remains valid even if collective excitations prevail.The paper is organised as follows.In Section 2we out-line the essentials of the computational technique used to simulate the grand canonical partition function and present its expansion as a polynomial in the fugacity vari-able.In Section 3we present the Yang-Lee zeros of the partition function calculated on 62–102lattices and high-light their qualitative diﬀerences from the 42lattice.In Section 4we analyse the ﬁnite size scaling of the Yang-Lee zeros and compare it to the real-space renormaliza-tion group prediction for a second-order phase transition.Finally,in Section 5we present a summary of our resultsand an outlook for future work.II.SIMULATION ALGORITHM AND FUGACITY EXPANSION OF THE GRAND CANONICALPARTITION FUNCTIONThe model we are studying in this work is a two-dimensional single-band Hubbard HamiltonianˆH=−t <i,j>,σc †i,σc j,σ+U i n i +−12 −µi(n i ++n i −)(1)where the i,j denote the nearest neighbour spatial lat-tice sites,σis the spin degree of freedom and n iσis theelectron number operator c †iσc iσ.The constants t and U correspond to the hopping parameter and the on-site Coulomb repulsion respectively.The chemical potential µis introduced such that µ=0corresponds to half-ﬁlling,i.e.the actual chemical potential is shifted from µto µ−U412.(5)This transformation enables one to integrate out the fermionic degrees of freedom and the resulting partition function is written as an ensemble average of a product of two determinantsZ ={s i,l =±1}˜z = {s i,l =±1}det(M +)det(M −)(6)such thatM ±=I +P ± =I +n τ l =1B ±l(7)where the matrices B ±l are deﬁned asB ±l =e −(±dtV )e −dtK e dtµ(8)with V ij =δij s i,l and K ij =1if i,j are nearestneigh-boursand Kij=0otherwise.The matrices in (7)and (8)are of size (n x n y )×(n x n y ),corresponding to the spatial size of the lattice.The expectation value of a physical observable at chemical potential µ,<O >µ,is given by<O >µ=O ˜z (µ){s i,l =±1}˜z (µ,{s i,l })(9)where the sum over the conﬁgurations of Ising ﬁelds isdenoted by an integral.Since ˜z (µ)is not positive deﬁnite for Re(µ)=0we weight the ensemble of conﬁgurations by the absolute value of ˜z (µ)at some µ=µ0.Thus<O >µ= O ˜z (µ)˜z (µ)|˜z (µ0)|µ0|˜z (µ0)|µ0(10)The partition function Z (µ)is given byZ (µ)∝˜z (µ)N c˜z (µ0)|˜z (µ0)|×e µβ+e −µβ−e µ0β−e −µ0βn (16)When the average sign is near unity,it is safe to as-sume that the lattice conﬁgurations reﬂect accurately thequantum degrees of freedom.Following Blankenbecler et al.[1]the diagonal matrix elements of the equal-time Green’s operator G ±=(I +P ±)−1accurately describe the fermion density on a given conﬁguration.In this regime the adiabatic approximation,which is the basis of the ﬁnite-temperature algorithm,is valid.The situa-tion diﬀers strongly when the average sign becomes small.We are in this case sampling positive and negative ˜z (µ0)conﬁgurations with almost equal probability since the ac-ceptance criterion depends only on the absolute value of ˜z (µ0).In the simulations of the HSﬁelds the situation is dif-ferent from the case of fermions interacting with dynam-ical bosonﬁelds presented in Ref.[1].The auxilary HS ﬁelds do not have a kinetic energy term in the bosonic action which would suppress their rapidﬂuctuations and hence recover the adiabaticity.From the previous sim-ulations on a42lattice[3]we know that avoiding the sign problem,by updating at half-ﬁlling,results in high uncontrolledﬂuctuations of the expansion coeﬃcients for the statistical weight,thus severely limiting the range of validity of the expansion.It is therefore important to obtain the partition function for the widest range ofµ0 and observe the persistence of the hierarchy of the ex-pansion coeﬃcients of Z.An error analysis is required to establish the Gaussian distribution of the simulated observables.We present in the following section results of the bootstrap analysis[17]performed on our data for several values ofµ0.III.TEMPERATURE AND LATTICE-SIZEDEPENDENCE OF THE YANG-LEE ZEROS The simulations were performed in the intermediate on-site repulsion regime U=4t forβ=5,6,7.5on lat-tices42,62,82and forβ=5,6on a102lattice.The ex-pansion coeﬃcients given by eqn.(14)are obtained with relatively small errors and exhibit clear Gaussian distri-bution over the ensemble.This behaviour was recorded for a wide range ofµ0which makes our simulations reli-able in spite of the sign problem.In Fig.1(a-c)we present typical distributions of theﬁrst coeﬃcients correspond-ing to n=1−7in eqn.(14)(normalized with respect to the zeroth power coeﬃcient)forβ=5−7.5for diﬀer-entµ0.The coeﬃcients are obtained using the bootstrap method on over10000conﬁgurations forβ=5increasing to over30000forβ=7.5.In spite of diﬀerent values of the average sign in these simulations,the coeﬃcients of the expansion(16)indicate good correspondence between coeﬃcients obtained with diﬀerent values of the update chemical potentialµ0:the normalized coeﬃcients taken from diﬀerentµ0values and equal power of the expansion variable correspond within the statistical error estimated using the bootstrap analysis.(To compare these coeﬃ-cients we had to shift the expansion by2coshµ0β.)We also performed a bootstrap analysis of the zeros in theµplane which shows clear Gaussian distribution of their real and imaginary parts(see Fig.2).In addition, we observe overlapping results(i.e.same zeros)obtained with diﬀerent values ofµ0.The distribution of Yang-Lee zeros in the complexµ-plane is presented in Fig.3(a-c)for the zeros nearest to the real axis.We observe a gradual decrease of the imaginary part as the lattice size increases.The quantitative analysis of this behaviour is discussed in the next section.The critical domain can be identiﬁed by the behaviour of the density of Yang-Lee zeros’in the positive half-plane of the fugacity.We expect toﬁnd that this density is tem-perature and volume dependent as the system approaches the phase transition.If the temperature is much higher than the critical temperature,the zeros stay far from the positive real axis as it happens in the high-temperature limit of the one-dimensional Ising model(T c=0)in which,forβ=0,the points of singularity of the free energy lie at fugacity value−1.As the temperature de-creases we expect the zeros to migrate to the positive half-plane with their density,in this region,increasing with the system’s volume.Figures4(a-c)show the number N(θ)of zeros in the sector(0,θ)as a function of the angleθ.The zeros shown in theseﬁgures are those presented in Fig.3(a-c)in the chemical potential plane with other zeros lying further from the positive real half-axis added in.We included only the zeros having absolute value less than one which we are able to do because if y i is a zero in the fugacity plane,so is1/y i.The errors are shown where they were estimated using the bootstrap analysis(see Fig.2).Forβ=5,even for the largest simulated lattice102, all the zeros are in the negative half-plane.We notice a gradual movement of the pattern of the zeros towards the smallerθvalues with an increasing density of the zeros nearθ=πIV.FINITE SIZE SCALING AND THESINGULARITY OF THE DENSITY OF STATESAs a starting point for theﬁnite size analysis of theYang-Lee singularities we recall the scaling hypothesis forthe partition function singularities in the critical domain[11].Following this hypothesis,for a change of scale ofthe linear dimension LLL→−1),˜µ=(1−µT cδ(23)Following the real-space renormalization group treatmentof Ref.[11]and assuming that the change of scaleλisa continuous parameter,the exponentαθis related tothe critical exponentνof the correlation length asαθ=1ξ(θλ)=ξ(θ)αθwe obtain ξ∼|θ|−1|θ|ναµ)(26)where θλhas been scaled to ±1and ˜µλexpressed in terms of ˜µand θ.Diﬀerentiating this equation with respect to ˜µyields:<n >sing =(−θ)ν(d −αµ)∂F sing (X,Y )ν(d −αµ)singinto the ar-gument Y =˜µαµ(28)which deﬁnes the critical exponent 1αµin terms of the scaling exponent αµof the Yang-Lee zeros.Fig.5presents the scaling of the imaginary part of the µzeros for diﬀerent values of the temperature.The linear regression slope of the logarithm of the imaginary part of the zeros plotted against the logarithm of the inverse lin-ear dimension of the simulation volume,increases when the temperature decreases from β=5to β=6.The re-sults of β=7.5correspond to αµ=1.3within the errors of the zeros as the simulation volume increases from 62to 82.As it is seen from Fig.3,we can trace zeros with similar real part (Re (µ1)≈0.7which is also consistentwith the critical value of the chemical potential given in Ref.[22])as the lattice size increases,which allows us to examine only the scaling of the imaginary part.Table 1presents the values of αµand 1αµδ0.5±0.0560.5±0.21.3±0.3∂µ,as a function ofthe chemical potential on an 82lattice.The location of the peaks of the susceptibility,rounded by the ﬁnite size eﬀects,is in good agreement with the distribution of the real part of the Yang-Lee zeros in the complex µ-plane (see Fig.3)which is particularly evident in the β=7.5simulations (Fig.4(c)).The contribution of each zero to the susceptibility can be singled out by expressing the free energy as:F =2n x n yi =1(y −y i )(29)where y is the fugacity variable and y i is the correspond-ing zero of the partition function.The dotted lines on these plots correspond to the contribution of the nearby zeros while the full polynomial contribution is given by the solid lines.We see that the developing singularities are indeed governed by the zeros closest to the real axis.The sharpening of the singularity as the temperature de-creases is also in accordance with the dependence of the distribution of the zeros on the temperature.The singularities of the free energy and its derivative with respect to the chemical potential,can be related to the quasiparticle density of states.To do this we assume that single particle excitations accurately represent the spectrum of the system.The relationship between the average particle density and the density of states ρ(ω)is given by<n >=∞dω1dµ=ρsing (µ)∝1δ−1(32)and hence the rate of divergence of the density of states.As in the case of Lifshitz transitions the singularity of the particle number is rounded at ﬁnite temperature.However,for suﬃciently low temperatures,the singular-ity of the density of states remains manifest in the free energy,the average particle density,and particle suscep-tibility [15].The regular part of the density of states does not contribute to the criticality,so we can concentrate on the singular part only.Consider a behaviour of the typedensity of states diverging as the−1ρsing(ω)∝(ω−µc)1δ.(33)with the valueδfor the particle number governed by thedivergence of the density of states(at low temperatures)in spite of theﬁnite-temperature rounding of the singu-larity itself.This rounding of the singularity is indeedreﬂected in the diﬀerence between the values ofαµatβ=5andβ=6.V.DISCUSSION AND OUTLOOKWe note that in ourﬁnite size scaling analysis we donot include logarithmic corrections.In particular,thesecorrections may prove signiﬁcant when taking into ac-count the fact that we are dealing with a two-dimensionalsystem in which the pattern of the phase transition islikely to be of Kosterlitz-Thouless type[23].The loga-rithmic corrections to the scaling laws have been provenessential in a recent work of Kenna and Irving[24].In-clusion of these corrections would allow us to obtain thecritical exponents with higher accuracy.However,suchanalysis would require simulations on even larger lattices.The linearﬁts for the logarithmic scaling and the criti-cal exponents obtained,are to be viewed as approximatevalues reﬂecting the general behaviour of the Yang-Leezeros as the temperature and lattice size are varied.Al-though the bootstrap analysis provided us with accurateestimates of the statistical error on the values of the ex-pansion coeﬃcients and the Yang-Lee zeros,the smallnumber of zeros obtained with suﬃcient accuracy doesnot allow us to claim higher precision for the critical ex-ponents on the basis of more elaborateﬁttings of the scal-ing behaviour.Theﬁnite-size eﬀects may still be signiﬁ-cant,especially as the simulation temperature decreases,thus aﬀecting the scaling of the Yang-Lee zeros with thesystem rger lattice simulations will therefore berequired for an accurate evaluation of the critical expo-nent for the particle density and the density of states.Nevertheless,the onset of a singularity atﬁnite temper-ature,and its persistence as the lattice size increases,areevident.The estimate of the critical exponent for the diver-gence rate of the density of states of the quasiparticleexcitation spectrum is particularly relevant to the highT c superconductivity scenario based on the van Hove sin-gularities[25],[26],[27].It is emphasized in Ref.[25]thatthe logarithmic singularity of a two-dimensional electrongas can,due to electronic correlations,turn into a power-law divergence resulting in an extended saddle point atthe lattice momenta(π,0)and(0,π).In the case of the14.I.M.Barbour,A.J.Bell and E.G.Klepﬁsh,Nucl.Phys.B389,285(1993).15.I.M.Lifshitz,JETP38,1569(1960).16.A.A.Abrikosov,Fundamentals of the Theory ofMetals North-Holland(1988).17.P.Hall,The Bootstrap and Edgeworth expansion,Springer(1992).18.S.R.White et al.,Phys.Rev.B40,506(1989).19.J.E.Hirsch,Phys.Rev.B28,4059(1983).20.M.Suzuki,Prog.Theor.Phys.56,1454(1976).21.A.Moreo, D.Scalapino and E.Dagotto,Phys.Rev.B43,11442(1991).22.N.Furukawa and M.Imada,J.Phys.Soc.Japan61,3331(1992).23.J.Kosterlitz and D.Thouless,J.Phys.C6,1181(1973);J.Kosterlitz,J.Phys.C7,1046(1974).24.R.Kenna and A.C.Irving,unpublished.25.K.Gofron et al.,Phys.Rev.Lett.73,3302(1994).26.D.M.Newns,P.C.Pattnaik and C.C.Tsuei,Phys.Rev.B43,3075(1991);D.M.Newns et al.,Phys.Rev.Lett.24,1264(1992);D.M.Newns et al.,Phys.Rev.Lett.73,1264(1994).27.E.Dagotto,A.Nazarenko and A.Moreo,Phys.Rev.Lett.74,310(1995).28.A.A.Abrikosov,J.C.Campuzano and K.Gofron,Physica(Amsterdam)214C,73(1993).29.D.S.Dessau et al.,Phys.Rev.Lett.71,2781(1993);D.M.King et al.,Phys.Rev.Lett.73,3298(1994);P.Aebi et al.,Phys.Rev.Lett.72,2757(1994).30.E.Dagotto, A.Nazarenko and M.Boninsegni,Phys.Rev.Lett.73,728(1994).31.N.Bulut,D.J.Scalapino and S.R.White,Phys.Rev.Lett.73,748(1994).32.S.R.White,Phys.Rev.B44,4670(1991);M.Veki´c and S.R.White,Phys.Rev.B47,1160 (1993).33.C.E.Creﬃeld,E.G.Klepﬁsh,E.R.Pike and SarbenSarkar,unpublished.Figure CaptionsFigure1Bootstrap distribution of normalized coeﬃcients for ex-pansion(14)at diﬀerent update chemical potentialµ0for an82lattice.The corresponding power of expansion is indicated in the topﬁgure.(a)β=5,(b)β=6,(c)β=7.5.Figure2Bootstrap distributions for the Yang-Lee zeros in the complexµplane closest to the real axis.(a)102lat-tice atβ=5,(b)102lattice atβ=6,(c)82lattice at β=7.5.Figure3Yang-Lee zeros in the complexµplane closest to the real axis.(a)β=5,(b)β=6,(c)β=7.5.The correspond-ing lattice size is shown in the top right-hand corner. Figure4Angular distribution of the Yang-Lee zeros in the com-plex fugacity plane Error bars are drawn where esti-mated.(a)β=5,(b)β=6,(c)β=7.5.Figure5Scaling of the imaginary part ofµ1(Re(µ1)≈=0.7)as a function of lattice size.αm u indicates the theﬁt of the logarithmic scaling.Figure6Electronic susceptibility as a function of chemical poten-tial for an82lattice.The solid line represents the con-tribution of all the2n x n y zeros and the dotted line the contribution of the six zeros nearest to the real-µaxis.(a)β=5,(b)β=6,(c)β=7.5.。

article

Journal of the Korean Physical Society,Vol.35,No.1,July1999,pp.16∼20CsI(Tl)Scintillator Telescope Measurement of Charged Particles Extracted from the KCCH MC-50CyclotronS.H.Park∗,C.Lee,H.Park,J.H.Kim,J.H.Ha,E.Seo,J.S.Kim,H.Bhang and J.C.KimDepartment of Physics,Seoul National University,Seoul151-742Y.D.KimDepartment of Physics,SeJong University,Seoul143-747J.H.Lee,Y.K.Kwon and C.S.LeeDepartment of Physics,Chung-Ang University,Seoul156-756J.H.ChangKorea Atomic Energy Research Institute,Taejon305-600H.Y.Lee and S.A.ShinDepartment of Physics,Ewha Womans University,Seoul120-750J.S.Chai and Y.S.KimCyclotron Application Laboratory,Korea Cancer Center Hospital,Korea Atomic Energy Research Institute,Seoul139-240K.H.YooDaebul University,Young-Am,JeonNam526-890(Received7September1998)A telescope was made with a150-µm-thick silicon surface barrier detector(∆E)and a cylindricalCsI(Tl)scintillator(E).A50-MeV proton beam from the MC-50cyclotron was incident on a nat Agfoil,and outgoing particles were measured with the telescope.Particle identiﬁcation was performedwith the telescope,and particles were identiﬁed to be protons,deuterons,and tritons with the helpof the range-energy relation.The diﬀerential cross sections for the nat Ag(p,p)reaction leading to theground state of nat Ag were measured and compared with the result of an optical model calculation.This result showed some possibility for charged particle experiments using the cyclotron at theKorea Cancer Center Hospital.I.INTRODUCTIONMany eﬀorts have been attempted to do nuclear exper-iments using the azimuthally varyingﬁeld-type(AVF) MC-50cyclotron at the KCCH(Korea Cancer Center Hospital),including measurements of the characteristics of the beam[1],in-beamγ-ray spectroscopy[2],the de-tection of neutrons[3].Charged-particle experiments us-ing this facility,however,have been limited because of ∗E-mail:psh@phya.snu.ac.kr,Telefax:02-871-1085the sensitivity of the energy measurement to the beam-energy resolution.Since the cyclotron was designed for medical treatment instead of science experiment,the beam optics and the emittance are not high enough to carry out physics experiment.This results in certain limits on the experimental energy and the position reso-lution.Detecting charged particles in air leads to addi-tional uncertainties generated by the energy losses and the stragglings.Given the limited conditions,systematic studies of the optical parameters[4]and studies of the spectra of sec-ondary particles[5]can be carried out at the the MC-50 -16-CsI(Tl)Scintillator Telescope Measurement of ···–S.H.PARK et al.-17-Fig.1.Sytematic diagram of the detector setup.cyclotron in KCCH.We attempted to measure the an-gular distributions of the scatterings of the beam oﬀa heavy target for the ﬁrst time in Korea by using the KCCH MC-50cyclotron,so as to enhance the feasibility of the facility for nuclear physics experiments.In charged-particle spectroscopy,the telescope method has usually been applied to identify the detected par-ticle.The energy loss per unit distance traveled by a non-relativistic particle of charge z and mass m can be expressed by the Bethe formula [6]asdE =C 1mz 2ln C 2Ewhere C 1and C 2are constants,and E is the particle energy.From the relation of the signals from the ∆E and the E detectors,it is possible to determine the mass and the charge of the scattered particle.A thin silicon detector can be used as a possible ∆E detector because of its small stopping power and because of its resolution high enough to resolve the dE diﬀerence for the given particle energy.An inorganic scintillator be used as a possible E de-tector.NaI(Tl)coupled to a photomultiplier tube has wide application among inorganic scintillators in nuclear physics,especially in γ-ray detection.NaI(Tl),though,has some drawbacks as a particle detector.It usually tends to be bulky and is sensitive to a magneticﬁeld.Fig.2.Drawing of the MC-50cyclotron facility at the Ko-rea Cancer Center Hospital.The experiment was performed on the zero-degreeline.Fig.3.Block diagram of the electronics used in this exper-iment.(TFA:Timing Fast Ampliﬁer,CFD:Constant Fraction Discriminator,GDG:Gate Delay Generator,ADC:Analog-to-Digital Converter,DAQ Pro:Data Acquisition Program)Moreover,the crystal is hygroscopic and should be en-capsulated.This lead to a loss of energy resolution and to a higher energy threshold.CsI(Tl)has some superior properties compared to NaI(Tl).The crystal has good mechanical and thermal stability so that it is easy to fab-ricate the crystal into various shapes.The crystal is less hygroscopic so that sealing it like NaI(Tl)is not necces-sary.The scintillator can be used without high-quality polishing.Moreover,the longer wavelength of the scin-tillation emission (570nm)is outside the peak response of most photomultipliers,and that can be overcome by using a photodiode readout.A photodiode is verycom-Fig.4.Two-dimensional plot of ∆E versus E of scattered particles.-18-Journal of the Korean Physical Society,Vol.35,No.1,July1999Fig. 5.Gaussianﬁttings for the energy spectrum of the protons.pact and is insensitive to a magneticﬁeld without using a higher voltage.The energy resolution of a CsI(Tl)scin-tillator with a PIN diode readout has been reported to be1%for50-MeV protons[7].In this work,the diﬀerential cross-sections of the pro-tons scattering oﬀa natural silver target(nat Ag(p,p)) were measured using a telescope made of a silicon sur-face barrier(SSB)detector and a CsI(Tl)scintillator. The results were compared with optical-model calcula-tions.The performance of the CsI(Tl)scintillator was investigated experimentally.Also,the extended usage of the KCCH MC-50cyclotron was carefully examined in the current study.II.EXPERIMENTWe fabricated a CsI(Tl)scintillator with a diameter of25mm,and a length of30mm,which was compa-rable to the range of about100-MeV protons.The pin silicon photodiode,Hamamatsu S2744,was coupled to the rearﬂat surface of the scintillator because the spec-tral response of the photodiode was better suited to the use of a CsI(Tl)scintillator.The sensitive area of the PIN diode had a rectangular shape of20.0mm by10.0Fig. 6.Deuteron energy spectra at diﬀerent angles:(a) 30◦,(b)40◦,(c)50◦,(d)60◦,(e)70◦,and(f)80◦.mm.The PIN diode was directly coupled to the scintil-lator.All surfaces of the scintillator were sanded,andboth the front and the rear surfaces of the scintillatorwere polished.The front was wrapped with1-µm-thickaluminized mylar,and the side of the scintillator wascovered with teﬂon tape.The scintillator and the PINdiode were coupled in the back and wrapped by usingblack tape for light tightness.In order to check the re-sponse of the detector toγ-rays,the energy spectra of 60Co and132Csγ-sources were taken and compared with the NaI(Tl)scintillator spectra.The FWHM at the1.33-MeV peak from60Co was85keV using NaI(Tl)scintil-lator,and it was93keV with the CsI(Tl)scintillator.A particle detector telescope was made by using a thinsilicon surface barrier detector(∆E)and a CsI(Tl)scin-tillator(E).The∆E detector used was an ORTEC TB-015-050-150SSB detector with full depletion length of150µm.High voltages of+30V and+150V were ap-plied to the PIN diode and the∆E detector,respectively.The silicon detector and the CsI(Tl)scintillator were in-serted into a cylindrically shaped aluminum mount.Acopper collimator with a diameter of5mm was put inthe front of the silicon detector.A systematic diagramof the telescope is shown in Fig.1.Table.1.The optical potential parameters used for the optical-model-calculation.V r o a4W D r o a V s r s a s (MeV)(fm)(fm)(MeV)(fm)(fm)(MeV)(fm)(fm) Previous[10]54.3 1.210.6837.2 1.240.71 6.0 1.250.68CsI(Tl)Scintillator Telescope Measurement of···–S.H.PARK et al.-19-Fig.7.Angular distribution of nat Ag(p,p)leading to the ground state of nat Ag.The curve is the result of an optical-model calculation.The experiment was carried out using the AVF MC-50 cyclotron,and its schematic beam transport features are shown in Fig.2.The target chamber was placed at the zero-degree beam line in the gantry room.The cham-ber was made of stainless steel.It had a window made of0.0762-mm-thick mylar on the left side of the cham-ber.The particles penetrated the mylar foil and were detected outside the chamber.A0.013-mm-thick natu-ral Ag target was placed in the center of the chamber. The electronics diagram for∆E−E coincidence is displayed in Fig.3.The pre-ampliﬁer,which was con-nected to the pin diode of the CsI(Tl)scintillator,had been specially designed for this purpose.The main gate for the ADC was made by the coincidence of the timing signals of the PIN diode for the CsI(Tl)scintillator and the pre-ampliﬁer for the∆E detector.The signals from the CsI(Tl)scintillator induced byγ-rays were rejected by the coincidence.The data from the analog-to-digital converter(ADC)were processed through the computer automated measurement and control(CAMAC)system, and the data acquisition was done using the KODAQ data sorting code[8].The count rate in our data acquisition was maintained at a rate of500counts per second to avoid energy spread-out in the CsI(Tl)scintillator caused by the highγ-ray background activity.Data were taken at angles from35◦to80◦in increments of5◦.A plastic scintillator was set at45◦with respect to the beam direction to monitor the condition of the incident beam at each detection angle. The anode signal from the plastic scintillator was atten-uated to one tenth and changed to a logic signal through the octal discriminator.The discrimination levels were set to be200mV and transferred to the TTL signal by passing through the gate delay generator(GDL),and the number of signal counts was measured by the counter.III.EXPERIMENTAL RESULTS ANDOPTICAL MODEL CALCULATIONProton,deuteron,and triton events were well resolved in a2-dimensional plot of∆E and E,as shown in Fig.4.The proton and the deuteron gates are drawn in this ﬁgure.The nat Ag foil was reported to consist of51.8%107Ag, and48.2%109Ag[9].The ground states and low-lying excited states(0.311MeV and0.415MeV states in109Ag [10];0.423MeV,and0.324MeV states in107Ag[11]) appeared to be too close to be resolved with the CsI(Tl) scintillator and photodiode,for which the FWHM of the detected signal for35-MeV proton was assumed to be 350keV[7].Hence,an additional Gaussian relating the mentioned excited states and to the main ground-state structure was used inﬁtting the ground-state peak of the current data for each angle,and the elastic-scattering cross section of the protons oﬀthe natural Ag target was extracted based on theﬁt.Theﬁts are shown in Fig.5. The FWHM from the elastic peak of the nat Ag(p,p) reaction was observed to be620keV.Considering the beam energy resolution,which was estimated to be500 keV for the35-MeV proton beam[3],we think that the detector resolution itself(Γ=370keV)appears to be acceptable for charged-particle experiments.Anderson et al.[12]investigated the level structure of 106Ag through the107Ag(p,d)reaction.They suggested low-lying excited106Ag states were populated so closely that they could hardly be resolved in the deuteron en-ergy spectrum in the current work,as is shown in Fig.6. We also found that the ratio of continuum events to the elastic scatterings increased as the detection angle was moved backward.Theoretical calculations were done for the diﬀerential cross-section for the elastic scatterings.Optical-model calculations were carried out using the DWUCK4code [13].The optical-model potential used in the calculations wasU(r)=−Ve+1+4iW Dddx1e+1+ ¯h mπc2V s1rddr1e s+1(L·σ)+V c,where x x=r−r xa x,and V c is the Coulomb potential due to a uniformly charged sphere of radius1.25A13fm.The optical model parameters extracted by Ford et al.[10,11] were used as references.Allowing the parameters V and W D to vary in order to search for the appropriate param-eters giving the bestﬁt,we could use an optical-model calculation to regenerate angular distribution matching-20-Journal of the Korean Physical Society,Vol.35,No.1,July1999in optimized way to the experimental results.The elastic scattering oﬀthe107Ag and the109Ag nuclei were calcu-lated independently.The sum of the elastic cross sections oﬀthe Ag isotopes for each angle was then extracted by weighting with their respective natural abundunce frac-tion.The optical potential parameters extracted from the current work are listed in Table1.We normalized the measured diﬀerential cross-section by using the one calculated with the optical model.Fig.7displays the agreement of the experimental angular distribution with the theoretical calculation in elastic scattering.IV.SUMMARYWe studied the performance of a∆E−E Si-CsI(Tl) telescope for charged-particle detection by using the nat Ag(p,p)reaction.A CsI(Tl)scintillator with a PINdiode connection was used for the E detector.The CsI(Tl)scintillator was easily machined,and it was less hygroscopic.Since the ranges of charged particles through CsI(Tl)crystal are shorter than with other scin-tillators,it was possible to make a compact telescope. Protons,deuterons,and tritons were clearly identiﬁed by using this telescope.The detected energy resolution of CsI(Tl)was revealed to be good enough to be used for charged particle experiments.The angular distribution of the proton elastic-scattering by the nat Ag(p,p)reaction was measured by using this telescope,and optical-model calculations suc-cessfully regenerated the distribution with optical pa-rameters which had been newly searched for in the cur-rent experiment.The results also provide encouragement to keep on performing extensive physics reaction research using the KCCH AVF MC-50accelerator facility.ACKNOWLEDGMENTSThis work was supported by the Basic Science Re-search Institute Program,Ministry of Education,Korea 1997(Project No.BSRI-97-2417),and by the Nuclear R &D Program,Ministry of Science and Technology.REFERENCES[1]C.S.Lee,Y.S.Kim,J.H.Lee,J.C.Kim,J.Ha,J.H.Park,I.C.Kim,S.H.Park,Z.Jang,Y.B.Lee,Y.K.Kim,J.S.Chai and Y.S.Kim,J.Korean Phys.Soc.32, 20(1998).[2]J.H.Ha,Ph.D.Dissertation,Seoul National University,1998,unpublished.[3]J.H.Kim,H.Bhang,J.H.Ha,J.C.Kim,M.J.Kim,Y.D.Kim,H.Park,J.S.Chai,Y.S.Kim,H.Y.Lee,S.A.Shin,J.Y.Huh,C.S.Lee and J.H.Lee,J.KoreanPhys.Soc.32,462(1998).[4]E.Fabrici,S.Micheletti,M.Pignanelli and F.G.Resmini,Phys.Rev.C21,844(1980).[5]F.E.Bertrand and R.W.Peelle,Phys.Rev.C8,1045(1973).[6]Glenn F.Knoll,Radiation Detection and Measurement(Wiley,New York,1989),Chap.11.[7]W.G.Gong,Y. D.Kim,G.Poggi,Z.Chen, C.K.Gelbke,W.G.Lynch,M.R.Maier,T.Murakami,M.B.Tsang and H.M.Xu,Nucl.Inst.Meth.A268,190(1988).[8]Y.D.Kim,H.Bhang,O.Hashimoto,K.Maeda,K.Omata,H.Outa,H.Park and M.Youn,Nucl.Inst.Meth.A372,431(1996),and references therein.[9]K.S.Krane,Introductory Nuclear Physics(Wiley,NewYork,1988).[10]J.L.C.Ford,Jr.,Cheuk-Yin Wong,Taro Tamura,R.L.Robinson and P.H.Stelson,Phys.Rev.158,1194 (1967).[11]J.L.C.Ford,Jr.,R.L.Robinson,P.H.Stelson,TaroTamura and Cheuk-Yin Wong,Nucl.Phys.A142,525 (1970).[12]R.E.Anderson,R.L.Bunting,J.D.Burch,S.R.Chinn,J.J.Kraushaar,R.J.Peterson,D.E.Prull,B.W.Ridley and R.A.Ristinen,Nucl.Phys.A242,93(1975). [13]P.D.Kunz and E.Rost,unpublished.。

简谐振动运动方程的推导

v2+
1 2
kx2 =
E( v
表示物体的振
动速度, x 表示物体离开平衡位置的位移) 变形
图
( 2) 式两边平方有: 2kEx2= Cos2U, ,( 3) ( 2) 式微分 dx= 2kESinUdU,,( 4) ( 3) ( 4) 两式代入( 1) 式后化简:
X 收稿日期: 2000- 11- 05 作者简介: 蔡群( 1963- ) , 女, 浙江温州人, 副教授, 主要从事物理教学及研究
k m
,
可得二阶常系数线性齐次微分方程
dd2tx2 + X2x= 0 , ,( 1)
方程( 1) 的特征方程为: r2 + X2 = 0 它有两
个复根 r= ? iX
方程( 1) 式的两个特解为
x1= e+ iXt , x2= = e- iXt
其通解为: x = C1e+ iXt + C2e- iXt , ,( 2)
第 3卷
=
A21+ A22(
A1 A 21+
A22 CosXt
+ A2 SinXt) , ,( 3) A21+ A22
( A1= C1+ C2, A2= C1- C2 为常数)
令 A=
A 21+ A22 CosU0=
A1 A21+ A22
SinU0
=
A2 A21+ A22
代入( 3) 式得
x= A( CosU0CosXt+ SinU0SinXt ) 根据三角函数和角公式, 同样得到弹簧振
( C1, C2 为常数)
根据欧拉公式
e(A? iB) x= ( CosBx ? iSinBx) eAx

英语申请信函写作

考研英语申请信写作介绍及经典范文当一个人想得到某种机会或某些东西，如申请得到奖学金、求学、申请出国签证等，往往通过申请信来请求。

申请信如何分段，没有严格的规定，其内容一般应包括以下三个部分：申请的具体内容和缘由；自己的情况和条件；提出要求，如回信、面试等等。

对于不同的申请内容，在介绍自己情况的时候也要有不同的侧重。

例如在求学信中，应比较侧重介绍自己已有的学位及专业情况；在申请留学经济资助的信中，应着重介绍自己的学业及学术水平，因为国外很多学校颁发奖学金是以此为标准的。

写申请信应注意：第一，语气要诚挚友好千万不要表现出强求的意思；第二，对于申请的内容和原因一定要写得非常清楚；第三，尽管不能表现出强求的意思，但是期望得到的心情一定要表达出来。

以下是两封申请信，第一封是求学信，另一封是申请留学资助的，第一封信用的是齐头式，第二封信用的是缩进式，信头和信内地址都已省略。

［Directions］: You are a student of Huabei University,. Write an application to get a scholarship for your graduate study in another university in America. You should write about 100 words. Do not sign your own name at the end of your letter,using “Li Ming” instead. You do not need to write the address.［参考范文之一］Dear Sir or Madam:I am writing to apply for admission to your university to pursue my M.S. degree. I have read the annual prospectus issued by your university and found that it has the best graduate program of chemistry. I am greatly interested in the program.I graduated in 2004 from Huabei University, majoring in Chemistry and holding a B. S. degree. At university, I took many fundamental courses in Chemistry and my English is excellent as I had served as the head of English Association for two years. Since then I have been teaching Chemistry in Beijing Normal University. Through my teaching experience, I have not only deepened my understanding in this field, but mastered many complicated research skills as well.Two of my former professors and the present dean of our department have kindly written letters of recommendation for me, as enclosed with this letter.Thank you very much. I look forward to hearing from you soon. Sincerely yours,Li Ming［参考范文之二］Dear Sir or Madam:I wish to apply for admission to your department as a graduate student.I am writing to ask whether it will be possible for you to grant me a fullscholarship, considering my academic record and the fact that I have no relatives or friends in America who can act as my sponsor.I completed a four-year course in chemistry at Beijing University last June. During my four years in the university, I have passed all the required courses of study with satisfactory marks. With Chemistry as my major, I minored in Physics and Mathematics. Enclosed herewith is my transcript from the department concerned.My English is very good. I have been learning English since early childhood with the help of my father who is a professor of English in Fudan University, Shanghai. I therefore believe that I will not have language difficulties while studying in the United States.I should be most grateful if you would give my request favorable consideration. Thank you very much and look forward to your reply.Li LI英语留学申请信五例【例一】Post Office Box 2418Branch 28-5Post Code 100081Beijing, P.R.C.November 12, 1997The Registrar of AdmissionThe Graduate School ofthe Pennsylvania State UniversityU.S.A. 16802Dear Sir:I am deeply interested in your graduate school in the Department of MechanicalEngineering, and plan to apply for admission for the fall of 1998. My GPA in the university was 3.5, and I plan to take TOEFL and GRE in October and December, 1997, respectively.Please send catalog and application forms to me. I shall be greatly appreciated.Very truly yours,Wang Yang【例二】Department of Applied PhysicsTsinghua University, 100084Beijing, ChinaSept. 11th, 1999RE: Applying for AdmittanceOffice of Graduate AdmissionsBoston UniversityMassachusetts, U.S.A.Dear Sir,I am writing in the hope that I may obtain an opportunity to further my study in Applied Physics toward Master degree in your university.My name is Li Jin, an undergraduate student of the Department of Applied Physics, Tsinghua University(China). Next year in the summer, I will graduate and get my BS degree. I plan to continue my study and researchin this field under the instructions of first class professors and ina dynamic academic atmosphere. I chose Boston University because there are a congenial team of researchers, an array of databases and research projects in your school of Physics. I believe my interests are extremely congruent with the strengths of the school. And my solid academic background will meet your general entrance requirements for graduate study.I will appreciate it very much if you could send me the Graduate Application Forms, the Application Form for Scholarships/Assistantships, a detailed introduction to the School of Physics, and other relevant information. My mailing address is shown on the top of this letter.I am looking forward to hearing from you soon.Sincerely yours,Li Jin【例三】English CollegeBeijing Foreign Language UniversityBeijing 100083P.R. ChinaJune 16, 1999Office of AdmissionHuntington UniversityDear Sir,Thank you for your letter and the application forms. Enclosed you will find the completed forms and three checks (80 dollars) for the applicationfee and the first month s rent for dormitory. The additional 5 dollarswill be deposited at your college. I shall appreciate any favorable action you might take in admitting me to your college for the spring semester of 1998. I am most anxious to hear from you soon.Sincerely yours，Zhao Pengfei【例四】Department of Finance and EconmicsShanghai Jiaotong UniversityShanghai 200043May 1, 1999Graduate SchoolOxford UniversityLondonDear Sir:Thank you for your email and kind offer! Definitely, I would like to try to apply for a visa for the winter quarter. I understand that both you and Professor Black have tried very hard to help me with my situation and I appreciate your efforts.As it is true that nobody really knows why the visa officers refuse certain applicants and allow others, I have been told by several successful visareapplicants that reapplying with a new I 20 with a slightly differentoffer from the US schools would help. For example, a slight increase in financial aid and so on. I don't know whether this is something possible for you to consider for my situation. But I would really appreciate if you could consider some possibilities. I know that I have given you enough trouble and please ignore my request if it is too difficult for you to do. I will simply try my best to get my visa and come to your school. 英文留学申请信四例【例一】Communication DepartmentFudan UniversityShanghai 200056P.R.ChinaMarch 23, 2001Graduate SchoolUniversity of TorontoToronto, OntarioDear Sir,I have just received from your office an application form and information for international students. However, I have completed and submitted the same form and paid the admission/evaluation fee before.Would you please tell me if my application and fee have reached you or if the recent materials were sent to me by mistake?Thanks for your assistance.Yours sincerely,Luan Jinfu【例二】Department of ComputerBeijing UniversityBeijing 100087P.R. China1st May, 2000Office of AdmissionUniversity of AucklandPrivate Bag AucklandNew ZealandDear Sir,I am going to graduate from Department of Computer in Beijing University in June next year. I am very eager to enter the Graduate School of your university next fall to study applied computer for Ph.D. degree.I would appreciate it very much if you would send me a graduate catalog of your university and any other necessary information, and also a set of application forms for admission. Thank you for your kind assistance. Sincerely yours，Wang Feng【例三】Department of Applied MathematicsWuhan UniversityWuhan 430000P.R. ChinaMarch 16, 1999Office of Foreign AdmissionUniversity of WaterlooWaterloo, OntarioCanadaDear Admission Officer:I am ×××, the Chinese applicant who desires to pursue my Ph.D. study in your honored program.I am very sorry to inform you that I presently cannot afford the application fee you demand because of the following reasons:1 The high foreign currency exchange rate, US = 8.9 RMB;2.My relative low monthly income, about 200 RMB (equal to only );3.The extremely strict limitation in China to obtain foreign currency, esp. U.S. Dollar in near future. According to current foreign currency regulation, only citizens who get visa can have at most ,000 exchanged.I got excellent scores on TOEFL (63/62/67, total: 640, May, 1998) and GRE (V: 700, Q: 800, A: 800, total: 2,300, Nov. 5th, 1998). My graduate GPA is 3.7. I sincerely hope you can consider my situation and review my application first.Thanks a lot. Your kindness is greatly appreciated.Yours truly，Fang Lin【例四】Department of Chemical EngineeringTsinghua UniversityBeijing 100084ChinaJan. 6, 1994Office of Foreign AdmissionUniversity of BirminghamP.O. Box 363Birmingham B152TTU.K.Dear Sir:I wish to pursue a doctoral degree in Chemical Engineering at your institution. My desired date of entrance is fall, 1994. Please send me necessary application forms at your early convenience.If possible, I also wish to obtain a graduate assistantship so that I may support myself and obtain more practical experience while pursuing graduate study.I obtained my B. E. (Chemical Engineering) in 1989 and M. S. (Chemical Engineering) in 1992 from Tsinghua University. At present, I work as a teacher at the same university.考研英语作文留学和奖学金申请信范文留学申请信和奖学金申请信中须写明下列几点：（1）写明申请学校和所学专业。

植物中活性氧的产生及清除机制

17卷2期2001年3月生　物　工　程　学　报Chinese Journal of Biotechnology Vol.17No.2March 2001收稿日期:2000207226,修回日期:2000212218。

基金项目:国家海洋863资助项目(819208203)。

3联系作者。

济南军区总医院检验科(250031),Tel :86253122187681转66314。

33北京协和医科大学基础医学研究院博士生。

植物中活性氧的产生及清除机制杜秀敏3　殷文璇33　赵彦修　张　慧(山东师范大学逆境植物实验室,济南250014)摘　要　环境胁迫使植物细胞中积累大量的活性氧,从而导致蛋白质、膜脂、DNA 及其它细胞组分的严重损伤。

植物体内有效清除活性氧的保护机制分为酶促和非酶促两类。

酶促脱毒系统包括超氧化物歧化酶(SOD )、抗坏血酸过氧化物酶(APX )、过氧化氢酶(CA T )和谷胱甘肽过氧化物酶(GPX )等。

非酶类抗氧化剂包括抗坏血酸、谷胱甘肽、甘露醇和类黄酮。

利用基因工程策略增加这些物质在植物体内的含量,从而获得耐逆转基因植物已取得一定的进展。

关键词　活性氧,氧化损伤,酶促脱毒系统中图分类号　Q943 文献标识码　C 文章编号100023061(2001)022******* 全球由于环境胁迫给作物造成的品质下降,产量降低的损失是惊人的。

当作物生长的外在条件如温度、湿度、土壤中的水分、盐浓度等发生急剧变化或当大气污染(如SO 2、臭氧)、紫外线辐射、某些农药如Paraquat (一种光动除草剂)及病原体等作用于植物时,都会使植物体内产生大量的活性氧(Reactive Oxygen Species ,ROS ),形成氧化损伤。

这些比氧活泼的含氧化合物包括:超氧根阴离子(O 2・-)、氢氧根离(OH -)、羟自由基(・OH )、过氧化氢(H 2O 2)等。

产生的活性氧可导致蛋白质、膜脂和其它细胞组分的损伤[2]。

sample letters

Sample letters:Invitation Letter for the Conference on MathematicsSchool of Mathematics andSystem ScienceShandong University27 Shanda NanluJinan, Shandong 250100P. R. ChinaMarch 20, 2007Professor George SmithSchool of Mathematical SciencesThe University of NottinghamUniversity ParkNottinghamNG7 2RD UKDear Professor George Smith,I am pleased to invite you to attend the Ninth International Conference on Finite or Infinite Dimensional Complex Analysis to be held from July 24 to 28, 2007 in Jinan and Tai’an city, China. The conference is jointly organized by Shandong University and Shandong University of Science and Technology.The conference will first be opened in Jinan, and then move to Tai’an. And you are cordially invited to attend the Jinan part of the conference, July 24-26, 2007, at the Academic Exchange Center of Shandong University. You will be provided with local expenses, including hotel accommodation and meals for the duration of the conference.If you have any enquires, please contact our Conference Convener Ms Song Mei at (86)531 8836 **** or visit .I look forward to seeing you in Jinan.Yours sincerely,(Signature)Liu DonglinOrganizing CommitteeLetter of AcceptanceJuly 15, 2005Dear Dr. Pan Feng,I have received your letter dated June 22, inviting me to speak at the Opening Ceremony of the Fifth National Council Meeting of Translators Association of China to be held on November 4-6, 2005, in Beijing. I will speak on “Globalization and Diversity: What Do They Mean for Translators?”Thank you for your kind invitation.I wish the meeting a very fruitful and successful one.I look forward to seeing you at the Opening Ceremony in Beijing.Yours sincerely,(signature)Betty CohenLetter of RefusalDear Mr. Snow,Thank you very much for your kind invitation to speak before the Western Cartographers Association (WCA) Next month.Unfortunately, prior commitments make it impossible for me to accept your flattering offer. My work takes me abroad quite frequently, and I will be in Upper Volta the week your association meets.Please accept my sincere apologies, and I hope you will think of me again if the WCA needs a guest speaker at some future date.Yours sincerely,(signature)Jack VanceApplication Letter for a Working Group(Your address)January 22, 2012Dr. Melba RipleyThe School of ArchitectureUniversity of Louisiana100 Webber HallBaton Rouge, LA 70814Dear Dr. Ripley,I am a researcher at school of Materials Engineering, Purdue University. I have devoted nearly five years to the study of how inexpensive housing can be made of native materials in Third World countries; I was intrigued by the recent accomplishments of you and your students in designing and providing “instant” housing to victims of the recent hurricane in Central America. Therefore, I wish to learn about construction techniques of developing new structures in your working group for six months, preferably during April through September.During the past three years, I have been an assistant to Dr. Graham Benson who has shared the rostrum with you at many academic and global conferences on housing. Together we, along with a group of students, have spent the past two summers showing people in South Central Africa how to mix a specially prepared binder developed by our research lab with local materials to make durable homes in a matter of days. I would truly like to have the opportunity to do research work in your group so as to learn state-of-the-art technology in housing-building materials.I would greatly appreciate it if you could send me more information about application at your earliest convenience.Sincerely yours,Thomas Lamb Application Letter for a Graduate ProgramDepartment of PhysicsNanjing University22 Hankou Roan, Nanjing 21009P. R. ChinaNovember 3, 2009Mrs. Elizabeth WilliamsGraduate Adamissions OfficeUniversity OfficesWellington SquareOxford, OX1 2JDUKDear Mrs. Elizabeth Williams,I am a student in the Department of Physics, expecting to graduate with a bachelor’s degree in July next year. I am very much interested in pursuing a master’s degree in Particle Physics Department of Oxford University. From my review of graduate programs and discussion with my professors, I find that Oxford University has the largest particle physics group in the UK, with a large academic and support staff. I intend to enter in the autumn of 2010.In my undergraduate years, I have worked hard. As you can see from my curriculum vitae, my GPA in major courses is 3.8/4.0 and I have remained top 5% of about 100 students. I have also worked with Dr. Liu Wei and Professor Luo Lei on research topics like the applied physics at the enterprise level. Because of my excellence in study and research, I have been awarded scholarships three times. In addition, I am well-prepared linguistically to further my studies in the UK. My TELTS is 7.5 and my GRE is 130.I would be grateful if you would send me the application forms for admission and financial support at your earliest convenience. Thank you for your consideration.I look forward to hearing from you soon.Sincerely yoursLin Pengpeng Application Letter for Attending a Conference(Your address)March 15, 2007Dear Dr. Nelson,I am a Ph. D. candidate in School of Environmental Science and Engineering of Shandong University, P. R. China. I have heard from Prof. Wang Dongliang that the International Conference on Cities and Conservation will be held in the University of California, September 1-5, 2007. I am writing to you about the possibility of attending the forthcoming conference.I am currently doing research in Prof. Wang’s working group on water pollution prevention technology and water reuse, the area that I have always been interested in. I believe that attending such a conference will provide me with a very good opportunity to exchange ideas and expertise with attendees from different parts of the world and, more importantly, to learn from them.I would be grateful if you could send me more information about application form and financial support.I look forward to your early reply.Sincerely yours,Wei LinExercise:1.Fill in the blanks with suitable words:You are cordially (1) to participate in the Second International Conference (2) the Telemedical Information Society (TIS) 2001 which has (3) with the Second International Conference on Information Technology Applications in Biomedicine (ITAB) 2001 (4) be held in Amsterdam, the Netherlands, from April 12th to 14th, 2001.TIS-ITAB 2001 is the first combined International conference. It (5) be an opportunity for the international community to (6) ideas and develop a common vision for the future of the world healthcare. Contributions (7) the progress of developing a global telemetrically information society will (8) scientific papers, demonstrations, forums and future vision papers. The (9) committee has been working hard to make TIS-ITAB 2001 a truly (10) experience for all participants.2.The following is the body of a Letter of Acceptance, please translate it into English.我已收到您2002年3月9日邀请我参加于2002年11月5-8日在中国地质大学(武汉)学术交流中心举行的地球科学与可持续发展策略会议的来信。

Observation of long-lived interlayer excitons in monolayer MoSe2–WSe2 heterostructures

ARTICLEReceived1Apr2014|Accepted9Jan2015|Published24Feb2015Observation of long-lived interlayer excitonsin monolayer MoSe2–WSe2heterostructuresPasqual Rivera1,John R.Schaibley1,Aaron M.Jones1,Jason S.Ross2,Sanfeng Wu1,Grant Aivazian1,Philip Klement1,Kyle Seyler1,Genevieve Clark2,Nirmal J.Ghimire3,4,Jiaqiang Yan4,5,D.G.Mandrus3,4,5, Wang Yao6&Xiaodong Xu1,2Van der Waals bound heterostructures constructed with two-dimensional materials,such asgraphene,boron nitride and transition metal dichalcogenides,have sparked wide interest indevice physics and technologies at the two-dimensional limit.One highly coveted hetero-structure is that of differing monolayer transition metal dichalcogenides with type-II bandalignment,with bound electrons and holes localized in individual monolayers,that is,interlayer excitons.Here,we report the observation of interlayer excitons in monolayerMoSe2–WSe2heterostructures by photoluminescence and photoluminescence excitationspectroscopy.Weﬁnd that their energy and luminescence intensity are highly tunable by anapplied vertical gate voltage.Moreover,we measure an interlayer exciton lifetime of B1.8ns,an order of magnitude longer than intralayer excitons in monolayers.Our work demonstratesoptical pumping of interlayer electric polarization,which may provoke further explorationof interlayer exciton condensation,as well as new applications in two-dimensional lasers,light-emitting diodes and photovoltaic devices.1Department of Physics,University of Washington,Seattle,Washington98195,USA.2Department of Materials Science and Engineering,University of Washington,Seattle,Washington98195,USA.3Department of Physics and Astronomy,University of T ennessee,Knoxville,T ennessee37996,USA.4Materials Science and T echnology Division,Oak Ridge National Laboratory,Oak Ridge,T ennessee37831,USA.5Department of Materials Science and Engineering,University of T ennessee,Knoxville,T ennessee37996,USA.6Department of Physics and Center of Theoretical and Computational Physics, University of Hong Kong,Hong Kong,China.Correspondence and requests for materials should be addressed to P.R.(email:pasqual@)or to X.X. (email:xuxd@).T he recently developed ability to vertically assemble different two-dimensional(2D)materials heralds a newrealm of device physics based on van der Waals heterostructures(HSs)1.The most successful example to date is the vertical integration of graphene on boron nitride.Such novel HSs not only markedly enhance graphene’s electronic properties2, but also give rise to superlattice structures demonstrating exotic physical phenomena3–5.A fascinating counterpart to gapless graphene is a class of monolayer direct bandgap semiconductors, namely transition metal dichalcogenides(TMDs)6–8.Due to the large binding energy in these2D semiconductors,excitons dominate the optical response,exhibiting strong light–matter interactions that are electrically tunable9,10.The discovery of excitonic valley physics11–15and strongly coupled spin and pseudospin physics16,17in2D TMDs opens up new possibilities for device concepts not possible in other material systems. Monolayer TMDs have the chemical formula MX2where the M is tungsten(W)or molybdenum(Mo),and the X is sulfur(S) or selenium(Se).Although these TMDs share the same crystalline structure,their physical properties,such as bandgap,exciton resonance and spin–orbit coupling strength,can vary signiﬁ-cantly.Therefore,an intriguing possibility is to stack different TMD monolayers on top of one another to form2D HSs.First-principle calculations show that heterojunctions formed between monolayer tungsten and molybdenum dichalcogenides have type-II band alignment18–20.Recently,this has been conﬁrmed by X-ray photoelectron spectroscopy and scanning tunnelling spectroscopy21.Since the Coulomb binding energy in2D TMDs is much stronger than in conventional semiconductors, it is possible to realize interlayer excitonic states in van der Waals bound heterobilayers,that is,bound electrons and holes that are localized in different layers.Such interlayer excitons have been intensely pursued in bilayer graphene for possible exciton condensation22,but direct optical observation demonstrating the existence of such excitons is challenging owing to the lack of a sizable bandgap in graphene.Monolayer TMDs with bandgaps in the visible range provide the opportunity to optically pump interlayer excitons,which can be directly observed through photoluminescence(PL)measurements.In this report,we present direct observation of interlayer excitons in vertically stacked monolayer MoSe2–WSe2HSs.We show that interlayer exciton PL is enhanced under optical excitation resonant with the intralayer excitons in isolated monolayers,consistent with the interlayer charge transfer resulting from the underlying type-II band structure.We demonstrate the tuning of the interlayer exciton energy by applying a vertical gate voltage,which is consistent with the permanent out-of-plane electric dipole nature of interlayer excitons.Moreover,weﬁnd a blue shift in PL energy at increasing excitation power,a hallmark of repulsive dipole–dipole interac-tions between spatially indirect excitons.Finally,time-resolved PL measurements yield a lifetime of1.8ns,which is at least an order of magnitude longer than that of intralayer excitons.Our work shows that monolayer semiconducting HSs are a promising platform for exploring new optoelectronic phenomena.ResultsMoSe2–WSe2HS photoluminescence.HSs are prepared by standard polymethyl methacrylate(PMMA)transfer techniques using mechanically exfoliated monolayers of WSe2and MoSe2(see Methods).Since there is no effort made to match the crystal lattices of the two monolayers,the obtained HSs are considered incom-mensurate.An idealized depiction of the vertical MoSe2–WSe2HS is shown in Fig.1a.We have fabricated six devices that all show similar results as those reported below.The data presented here are from two independent MoSe2–WSe2HSs,labelled device1and device2.Figure1b shows an optical micrograph of device1,which has individual monolayers,as well as a large area of vertically stacked HS.This device architecture allows for the comparison of the excitonic spectrum of individual monolayers with that of the HS region,allowing for a controlled identiﬁcation of spectral changes resulting from interlayer coupling.We characterize the MoSe2–WSe2monolayers and HS using PL measurements.Inspection of the PL from the HS at room temperature reveals three dominant spectral features(Fig.1c). The emission at1.65and1.57eV corresponds to the excitonic states from monolayer WSe2and MoSe2(refs10,15),respectively. PL from the HS region,outlined by the dashed white line in Fig.1a,reveals a distinct spectral feature at1.35eV(X I).Two-dimensional mapping of the spectrally integrated PL from X I shows that it is isolated entirely to the HS region(inset,Fig.1c), with highly uniform peak intensity and spectral position (Supplementary Materials1).Low-temperature characterization of the HS is performed with 1.88eV laser excitation at20K.PL from individual monolayer WSe2(top),MoSe2(bottom)and the HS area(middle)are shown with the same scale in Fig.1d.At low temperature,the intralayer neutral(X M o)and charged(X MÀ)excitons are resolved10,15,where M labels either W or parison of the three spectra shows that both intralayer X M o and X MÀexist in the HS with emission at the same energy as from isolated monolayers,demonstrating the preservation of intralayer excitons in the HS region.PL from X I becomes more pronounced and is comparable to the intralayer excitons at low temperature.We note that the X I energy position has variation across the pool of HS samples we have studied (Supplementary Fig.1),which we attribute to differences in the interlayer separation,possibly due to imperfect transfer and a different twisting angle between monolayers.We further perform PL excitation(PLE)spectroscopy to investigate the correlation between X I and intralayer excitons.A narrow bandwidth(o50kHz)frequency tunable laser is swept across the energy resonances of intralayer excitons(from1.6to 1.75eV)while monitoring X I PL response.Figure2a shows an intensity plot of X I emission as a function of photoexcitation energy from device2.We clearly observe the enhancement of X I emission when the excitation energy is resonant with intralayer exciton states(Fig.2b).Now we discuss the origin of X I.Since X I has never been observed in our exfoliated monolayer and bilayer samples,if its origin were related to defects,they must be introduced by the fabrication process.This would result in sample-dependent X I properties with non-uniform spatial dependence.However,our data show that key physical properties of X I,such as the resonance energy and intensity,are spatially uniform and isolated to the HS region(inset of Fig.1c and Supplementary Fig.2).In addition,X I has not been observed in WSe2–WSe2homo-structures constructed from exfoliated or physical vapor deposi-tion(PVD)grown monolayers(Supplementary Fig.3).All these facts suggest that X I is not a defect-related exciton.Instead,the experimental results support the observation of an interlayer exciton.Due to the type-II band alignment of the MoSe2–WSe2HS18–20,as shown in Fig.2c,photoexcited electrons and holes will relax(dashed lines)to the conduction band edge of MoSe2and the valence band edge of WSe2,respectively.The Coulomb attraction between electrons in the MoSe2and holes in the WSe2gives rise to an interlayer exciton,X I,analogous to spatially indirect excitons in coupled quantum wells.The interlayer coupling yields the lowest energy bright exciton in the HS,which is consistent with the temperature dependence of X I PL,that is,it increases as temperature decreases (Supplementary Fig.4).From the intralayer and interlayer exciton spectral positions,we can infer the band offsets between the WSe 2and MoSe 2monolayers (Fig.2c).The energy difference between X W and X I at room temperature is 310meV.Considering the smaller binding energy of interlayer than intralayer excitons,this sets a lower bound on the conduction band offset.The energy difference between X M and X I then provides a lower bound on the valence band offset of 230meV.This value is consistent with the valence band offset of 228meV found in MoS 2–WSe 2HSs by micro X-ray photoelectron spectroscopy and scanning tunnelling spectro-scopy measurements 21.This experimental evidence strongly corroborates X I as an interlayer exciton.The observation of bright interlayer excitons in monolayer semiconducting HSs is of central importance,and the remainder of this paper will focus on their physical properties resulting from their spatially indirect nature and the underlying type-II band alignment.WSe 2HSMoSe 2W M SeIn te n s i t y (a .u .)1.31.51.7Energy (eV)MoSe 2HeterostructureWSe 2W0WX X X X −0MoMo−e hehe h1.3 1.41.51.6 1.7I n t e n s i t y (a .u .)Energy (eV)5μm 0123×104Y (μm )246X (μm)0246Figure 1|Intralayer and interlayer excitons of a monolayer MoSe 2–WSe 2vertical heterostructure.(a )Cartoon depiction of a MoSe 2–WSe 2heterostructure (HS).(b )Microscope image of a MoSe 2–WSe 2HS (device 1)with a white dashed line outlining the HS region.(c )Room-temperature photoluminescence of the heterostructure under 20m W laser excitation at 2.33eV.Inset:spatial map of integrated PL intensity from the low-energy peak (1.273–1.400eV),which is only appreciable in the heterostructure area,outlined by the dashed black line.(d )Photoluminescence of individual monolayers and the HS at 20K under 20m W excitation at 1.88eV (plotted on the samescale).Energy (eV)WSe MoSe PL energy (eV)E x c i t a t i o n e n e r g y (e V )1.28 1.3 1.32 1.34 1.36 1.381.61.651.71.754,0006,0008,00010,000IntensityFigure 2|Photoluminescence excitation spectroscopy of the interlayer exciton at 20K.(a )PLE intensity plot of the heterostructure region with an excitation power of 30m W and 5s charge-coupled device CCD integration time.(b )Spectrally integrated PLE response (red dots)overlaid on PL (black line)with 100m W excitation at 1.88eV.(c )Type-II semiconductor band alignment diagram for the 2D MoSe 2–WSe 2heterojunction.interlayer exciton .Applying vertical energy of Figure 3a contact stacked insu-Electrostatic contact shows the 100to about analogue of reversed,varied expected for from reduces device 2,conduction 3b,c.of the in the on top band-offset at X I PL energy of basis of would should have X I PL This effect,intensity.further Power dependence and lifetime of interlayer exciton PL .The interlayer exciton PLE spectrum as a function of laser power with excitation energy in resonance with X W o reveals several properties of the X I .Inspection of the normalized PLE intensity (Fig.4a)shows the evolution of a doublet in the interlayer excitonspectrum,highlighted by the red and Both peaks of the doublet display a consistent increased laser intensity,shown by the dashed which are included as a guide to the eye.intensity of X I also exhibits a strong saturation laser power,as shown in Fig.4b (absolute Supplementary Fig.6).The sublinear power excitation powers above 0.5m W is distinctly the intralayer excitons in isolated monolayers,saturation power threshold of about Fig.7).The low power saturation of X I PL lifetime than that of intralayer excitons.the intralayer exciton is substantially reduced interlayer charge hopping 23,which is quenching of intralayer exciton PL (Fig.Fig.8).Moreover,the lifetime of the interlayer because it is the lowest energy conﬁguration indirect nature leads to a reduced optical long lifetime is conﬁrmed by time-resolved Fig.4c.A ﬁt to a single exponential decay exciton lifetime of 1.8±0.3ns.This timescale the intralayer exciton lifetime,which is ps 24–27.By modelling the saturation behaviour three-level diagram,the calculated saturation interlayer exciton is about 180times (Supplementary Fig.7;Supplementary with our observation of low saturation intensity DiscussionWe attribute the observed doublet feature splitting of the monolayer MoSe 2conduction assignment is mainly based on the fact difference between the doublet is B 25with MoSe 2conduction band splitting predicted calculations 28.This explanation is also supported by the evolution of the relative strength of the two peaks with increasing excitation power,as shown in Fig.4a (similar results in device 1with 1.88eV excitation shown in Supplementary Fig.9).At low power,the lowest energy conﬁguration of interlayer excitons,with the electron in the lower spin-split band of MoSe 2,is populated ﬁrst.Due to phase space ﬁlling effects,the interlayer excitonSiO 2n + Si2MoSe 2e –h +e –h +P Ee –h +V g < 0WSe 2MoSe 2WSe 2MoSe 2h ωV g = 0Photon energy (eV)1.321.361.41.444080e –h +h +PL intensity (a.u.) -hω’-the interlayer exciton and band alignment.(a )Device 2geometry.The interlayer exciton has a out-of-plane electric polarization.(b )Electrostatic control of the band alignment and the interlayer exciton photoluminescence as a function of applied gate voltage under 70m W excitation at 1.744eV,1s integrationconﬁguration with the electron in the higher energy spin-split band starts to be ﬁlled at higher laser power.Consequently,the higher energy peak of the doublet becomes more prominent at higher excitation powers.The observed blue shift of X I as the excitation power increases,indicated by the dashed arrows in Fig.4a,is a signature of the repulsive interaction between the dipole-aligned interlayer excitons (cf.Fig.3a).This is a hallmark of spatially indirect excitons in gallium arsenide (GaAs)coupled quantum wells,which have been intensely studied for exciton Bose-Einstein condensation (BEC)phenomena 29.The observation of spatially indirect interlayer excitons in a type-II semiconducting 2D HS provides an intriguing platform to explore exciton BEC,where the observed extended lifetimes and repulsive interactions are two key ingredients towards the realization of this exotic state of matter.Moreover,the extraordinarily high binding energy for excitons in this truly 2D system may provide for degenerate exciton gases at elevated temperatures compared with other material systems 30.The long-lived interlayer exciton may also lead to new optoelectronic applications,such as photovoltaics 31–34and 2D HS nanolasers.MethodsDevice fabrication .Monolayers of MoSe 2are mechanically exfoliated onto 300nm SiO 2on heavily doped Si wafers and monolayers of WSe 2onto a layer of PMMA atop polyvinyl alcohol on Si.Both monolayers are identiﬁed with an opticalmicroscope and conﬁrmed by their PL spectra.Polyvinyl alcohol is dissolved in H 2O and the PMMA layer is then placed on a transfer loop or thin layer of poly-dimethylsiloxane (PDMS).The top monolayer is then placed in contact with the bottom monolayer with the aid of an optical microscope and micromanipulators.The substrate is then heated to cause the PMMA layer to release from the transfer media.The PMMA is subsequently dissolved in acetone for B 30min and then rinsed with isopropyl alcohol.Low-temperature PL measurements .Low-temperature measurements are con-ducted in a temperature-controlled Janis cold ﬁnger cryostat (sample in vacuum)with a diffraction-limited excitation beam diameter of B 1m m.PL is spectrally ﬁltered through a 0.5-m monochromator (Andor–Shamrock)and detected on a charge-coupled device (Andor—Newton).Spatial PL mapping is performed using a Mad City Labs Nano-T555nanopositioning system.For PLE measurements,a continuous wave Ti:sapphire laser (MSquared—SolsTiS)is used for excitation and ﬁltered from the PL signal using an 815-nm-long pass optical ﬁlter (Semrock).Electrostatic doping is accomplished with an indium drain contact deposited onto the monolayer WSe 2region of device 2and using the heavily doped Si as a tunable backgate.Time-resolved PL measurements .For interlayer lifetime measurements,we excite the sample with a o 200-fs pulsed Ti:sapphire laser (Coherent—MIRA).Interlayer PL is spectrally ﬁltered through a 0.5-m monochromator (Princeton—Acton 2500),and detected with a fast time-correlated single-photon counting system composed of a fast (o 30ps full width at half maximum)single-photon avalanche detector (Micro Photon Devices—PDM series)and a picosecond event timer (PicoQuant—PicoHarp 300).References1.Geim,A.K.&Grigorieva,I.V.Van der Waals heterostructures.Nature 499,419–425(2013).2.Dean,C.R.et al.Boron nitride substrates for high-quality graphene electronics.Nat.Nanotechnol.5,722–726(2010).3.Hunt,B.et al.Massive Dirac fermions and Hofstadter butterﬂy in a van derWaals heterostructure.Science 340,1427–1430(2013).4.Dean,C.R.et al.Hofstadter’s butterﬂy and the fractal quantum Hall effect inmoire superlattices.Nature 497,598–602(2013).5.Ponomarenko,L.A.et al.Cloning of Dirac fermions in graphene superlattices.Nature 497,594–597(2013).6.Novoselov,K.S.et al.Two-dimensional atomic crystals.Proc.Natl Acad.SciUSA 102,10451–10453(2005).7.Mak,K.F.,Lee,C.,Hone,J.,Shan,J.&Heinz,T.F.Atomically thin MoS 2:anew direct-gap semiconductor.Phys.Rev.Lett.105,136805(2010).8.Splendiani,A.et al.Emerging photoluminescence in monolayer MoS 2.NanoLett.10,1271–1275(2010).MoSe2WSe 2dPhoton energy (eV)Laser power (μW)Time (ns)I n t e n s i t y (a .u .)N o r m a l i z e d P L E i n t e n s i t y (c o u n t s (μW s )–1)Power (μW)I n t e g r a t e d P L E i n t e n s i t y (C o u n t s (μW s )–1)2060404Figure 4|Power-dependent photoluminescence of interlayer exciton and its lifetime at 20K.(a )Power dependence of the interlayer exciton for 1.722eV excitation with a bi-Lorentzian ﬁt to the 5and 100m W plots,normalized for power and charge-coupled device (CCD)integration time.(b )Spectrally integrated intensity of the interlayer exciton emission as a function of excitation power shows the saturation effect.(c )Time-resolvedphotoluminescence of the interlayer exciton (1.35eV)shows a lifetime of about 1.8ns.The dashed curve is the instrument response to the excitation laser pulse.(d )Illustration of the heterojunction band diagram,including the spin levels of the MoSe 2conduction band.The X I doublet has energy splitting equal to (o 0I Ào I )E 25meV.9.Mak,K.F.et al.Tightly bound trions in monolayer MoS2.Nat.Mater.12,207–211(2013).10.Ross,J.S.et al.Electrical control of neutral and charged excitons in amonolayer mun.4,1474(2013).11.Xiao,D.,Liu,G.-B.,Feng,W.,Xu,X.&Yao,W.Coupled spin and valleyphysics in monolayers of MoS2and other group-VI dichalcogenides.Phys.Rev.Lett.108,196802(2012).12.Cao,T.et al.Valley-selective circular dichroism of monolayer 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materialswith tunable direct band gaps by sandwiching different metal disulﬁdes and diselenides.Sci.Rep.3,1549(2013).21.Chiu,M.-H.et al.Determination of band alignment in transition metaldichalcogenide heterojunctions,Preprint at http://arXiv:1406.5137(2014). 22.Su,J.-J.&MacDonald,A.H.How to make a bilayer exciton condensateﬂow.Nat.Phys.4,799–802(2008).23.Hong,X.P.et al.Ultrafast charge transfer in atomically thin MoS2/WS2heterostructures.Nat.Nanotechnol.9,682–686(2014).24.Wang,G.et al.Valley dynamics probed through charged and neutral excitonemission in monolayer WSe2.Phys.Rev.B90,075413(2014).garde,D.et al.Carrier and polarization dynamics in monolayer MoS2.Phys.Rev.Lett.112,047401(2014).26.Mai,C.et al.Many-body effects in valleytronics:direct measurement of valleylifetimes in single-layer MoS2.Nano Lett.14,202–206(2013).27.Shi,H.et al.Exciton dynamics in suspended monolayer and few-layer MoS22Dcrystals.ACS Nano7,1072–1080(2012).28.Korma´nyos,A.,Zo´lyomi,V.,Drummond,N.D.&Burkard,G.Spin-orbitcoupling,quantum dots,and qubits in monolayer transition metaldichalcogenides.Phys.Rev.X4,011034(2014).29.Butov,L.V.,Lai,C.W.,Ivanov,A.L.,Gossard,A.C.&Chemla,D.S.TowardsBose-Einstein condensation of excitons in potential traps.Nature417,47–52 (2002).30.Fogler,M.M.,Butov,L.V.&Novoselov,K.S.High-temperature superﬂuiditywith indirect excitons in van der Waals mun.5,4555 (2014).31.Lee,C.H.et al.Atomically thin p-n junctions with van der Waalsheterointerfaces.Nat.Nanotechnol.9,676–681(2014).32.Furchi,M.M.,Pospischil,A.,Libisch,F.,Burgdorfer,J.&Mueller,T.Photovoltaic effect in an electrically tunable van der Waals heterojunction.Nano Lett.14,4785–4791(2014).33.Cheng,R.et al.Electroluminescence and photocurrent generation fromatomically sharp WSe2/MoS2heterojunction p-n diodes.Nano Lett.14,5590–5597(2014).34.Fang,H.et al.Strong interlayer coupling in van der Waals heterostructuresbuilt from single-layer chalcogenides.Proc.Natl A111,6198–6202 (2014).AcknowledgementsThis work is mainly supported by the US DoE,BES,Materials Sciences and Engineering Division(DE-SC0008145).N.J.G.,J.Y.and D.G.M.are supported by US DoE,BES, Materials Sciences and Engineering Division.W.Y.is supported by the Research Grant Council of Hong Kong(HKU17305914P,HKU9/CRF/13G),and the Croucher Foun-dation under the Croucher Innovation Award.X.X.thanks the support of the Cottrell Scholar Award.P.R.thanks the UW GO-MAP program for their support.A.M.J.is partially supported by the NSF(DGE-0718124).J.S.R.is partially supported by the NSF (DGE-1256082).S.W.and G.C.are partially supported by the State of Washington through the UW Clean Energy Institute.Device fabrication was performed at the Washington Nanofabrication Facility and NSF-funded Nanotech User Facility. Author contributionsX.X.and P.R.conceived the experiments.P.R.and P.K.fabricated the devices,assisted by J.S.R.P.R.performed the measurements,assisted by J.R.S.,A.M.J.,J.S.R.,S.W.and G.A. P.R.and X.X.performed data analysis,with input from W.Y.N.J.G.,J.Y.and D.G.M. synthesized and characterized the bulk WSe2crystals.X.X.,P.R.,J.R.S.and W.Y.wrote the paper.All authors discussed the results.Additional informationSupplementary Information accompanies this paper at / naturecommunicationsCompetingﬁnancial interests:The authors declare no competingﬁnancial interests. Reprints and permission information is available online at / reprintsandpermissions/How to cite this article:Rivera,P.et al.Observation of long-lived interlayer excitons in monolayer MoSe2–mun.6:6242doi:10.1038/ncomms7242(2015).。

2001考研英语真题及答案

2001年‎全国硕士研‎究生入学统‎一考试英语‎试题Secti‎o n I Struc‎tur e and Vocab‎ulary‎Part ADirec‎t ions‎:Benea‎th each of the follo‎wing sente‎n ces, there‎are four choic‎e s marke‎d [A], [B], [C] and [D]. Choos‎e the one that best compl‎e tes the sente‎n ce. Mark your answe‎r on ANSWE‎R SHEET‎1 by black‎ening‎the corre‎s pond‎i ng lette‎r in the brack‎e ts with a penci‎l. (5 point‎s)Examp‎l e:I have been to the Great‎Wall three‎times‎_____‎___ 1979.[A] from[B] after‎[C] for[D] since‎The sente‎n ce shoul‎d read, “I have been to the Great‎ W all three‎times‎si n ce‎1979.” There‎f ore, you shoul‎d choos‎e [D].Sampl‎e Answe‎r[A] [B] [C] [■]1. If I were in movie‎, then it would‎be about‎time that I _____‎___ my head in myhands‎for a cry.[A] bury[B] am buryi‎n g[C] burie‎d[D] would‎bury2. Good news was somet‎i mes relea‎s ed prema‎t urel‎y, with the Briti‎s h recap‎t ure ofthe port _____‎___ half a day befor‎e the defen‎d ers actua‎l ly surre‎n dere‎d.[A] to annou‎n ce[B] annou‎n ced[C] annou‎n cing‎[D] was annou‎n ced3. Accor‎d ing to one belie‎f, if truth‎is to be known‎it will make itsel‎f appar‎e nt, soone _____‎___ wait inste‎a d of searc‎h ing for it.[A] would‎rathe‎r[B] had to[C] canno‎t but[D] had best4. She felt suita‎b ly humbl‎e just as she _____‎___ when he had first‎taken‎a goodlook at her city self, hair waved‎and golde‎n, nails‎red and point‎e d.[A] had[B] had had[C] would‎have and[D] has had5. There‎was no sign that Mr. Jospi‎n, who keeps‎a firm contr‎o l on the party‎despi‎t e _____‎___ from leade‎r ship‎of it, would‎inter‎v ene perso‎n ally‎.[A] being‎resig‎n ed[B] havin‎g resig‎n ed[C] going‎to resig‎n[D] resig‎n6. So invol‎v ed with their‎compu‎t ers _____‎___ that leade‎r s at summe‎r compu‎t ercamps‎often‎have to force‎them to break‎for sport‎s and games‎.[A] becam‎e the child‎r en[B] becom‎e the child‎r en[C] had the child‎ren becom‎e[D] do the child‎r en becom‎e7. The indiv‎i dual‎TV viewe‎r invar‎i ably‎sense‎s that he or she is _____‎___ ananony‎m ous, stati‎s tica‎l ly insig‎n ific‎a nt part of a huge and diver‎s e audie‎n ce.[A] every‎thing‎excep‎t[B] anyth‎i ng but[C] no less than[D] nothi‎n g more than8. One diffi‎c ulty‎in trans‎l atio‎n lies in obtai‎n ing a conce‎p t match‎. _____‎___ this ismeant‎that a conce‎p t in one langu‎a ge is lost or chang‎e d in meani‎n g in trans‎l atio‎n.[A] By[B] In[C] For[D] With9. Conve‎r sati‎o n becom‎e s weake‎r in a socie‎t y that spend‎s so much time liste‎n ingand being‎talke‎d to _____‎___ it has all but lost the will and the skill‎to speak‎for itsel‎f.[A] as[B] which‎[C] that[D] what10. Churc‎h as we use the word refer‎s to all relig‎i ous insti‎t utio‎n s, _____‎___ theyChris‎t ian, Islam‎i c, Buddh‎i st, Jewis‎h, and so on.[A] be[B] being‎[C] were[D] arePart BDirec‎t ions‎:Benea‎th each of the follo‎wing sente‎n ces, there‎are four choic‎e s marke‎d [A], [B], [C] and [D]. Choos‎e the one that best compl‎e tes the sente‎n ce. Mark your answe‎r on ANSWE‎R SHEET‎1 by black‎ening‎the corre‎s pond‎i ng lette‎r in the racke‎t s with a penci‎l. (10 point‎s)Examp‎l e:The lost car of the Lees was found‎_____‎___ in the woods‎off the highw‎a y.[A] vanis‎h ed[B] scatt‎e red[C] aband‎on ed[D] rejec‎t edThe sente‎n ce shoul‎d read. “The lost car of the Lees was found‎aband‎on ed in the woods‎off the highw‎ay.” There‎f ore, you shoul‎d choos‎e [C].Sampl‎e Answe‎r[A] [B] [■][D]11. He is too young‎to be able to _____‎___ betwe‎e n right‎and wrong‎.[A] disca‎r d[B] disce‎rn[C] dispe‎r se[D] disre‎g ard12. It was no _____‎___ that his car was seen near the bank at the time of therobbe‎r y.[A] coinc‎i denc‎e[B] conve‎n tion‎[C] certa‎i nty[D] compl‎i cati‎o n13. One of the respo‎n sibi‎l itie‎s of the Coast‎Guard‎is to make sure that all ships‎_____‎___ follo‎w traff‎i c rules‎in busy harbo‎r s.[A] cauti‎o u sly‎[B] dutif‎u lly[C] faith‎f ully‎[D] skill‎f ully‎14. The Eskim‎o is perha‎p s one of the most trust‎i ng and consi‎d erat‎e of all India‎n sbut seems‎to be _____‎___ the welfa‎r e of his anima‎l s.[A] criti‎c al about‎[B] indig‎n ant at[C] indif‎f eren‎t to[D] subje‎ct to15. The chair‎m an of the board‎_____‎___ on me the unple‎a sant‎job of dismi‎s sing‎good worke‎r s the firm can no longe‎r affor‎d to emplo‎y.[A] compe‎lled[B] posed‎[C] press‎e d[D] tempt‎e d16. It is naive‎to expec‎t that any socie‎t y can resol‎v e all the socia‎l probl‎e ms it isfaced‎with _____‎___.[A] for long[B] in and out[C] once for all[D] by natur‎e17. Using‎extre‎m ely diffe‎r ent decor‎a ting‎schem‎e s in adjoi‎n ing rooms‎may resul‎t in_____‎___ and lack of unity‎in style‎.[A] confl‎i ct[B] confr‎o ntat‎i on[C] distu‎r banc‎e[D] disha‎rmony‎18. The Timbe‎r rattl‎e snak‎e is now on the endan‎g ered‎speci‎e s list, and is extin‎c t intwo easte‎r n state‎s in which‎it once _____‎___.[A] thriv‎e d[B] swell‎e d[C] prosp‎e red[D] flour‎i shed‎19. Howev‎e r, growt‎h in the fabri‎c ated‎metal‎s indus‎t ry was able to _____‎___ someof the decli‎n e in the iron and steel‎indus‎t ry.[A] overt‎u rn[B] overt‎a ke[C] offse‎t[D] oppre‎s s20. Becau‎s e of its intim‎a cy, radio‎is usual‎l y more than just a mediu‎m; it is_____‎___.[A] firm[B] compa‎n y[C] corpo‎r atio‎n[D] enter‎p rise‎21. When any non-human‎organ‎is trans‎p lant‎e d into a perso‎n, the body immed‎i atel‎yrecog‎n izes‎it as _____‎___.[A] novel‎[B] remot‎e[C] dista‎n t[D] forei‎g n22. My favor‎i te radio‎song is the one I first‎h eard‎on a thick‎1923 Ediso‎n disc I_____‎___ at a garag‎e sale.[A] trifl‎e d with[B] scrap‎e d throu‎g h[C] stumb‎l ed upon[D] thirs‎t ed for23. Some day softw‎a re will trans‎l ate both writt‎e n and spoke‎n langu‎a ge so well thatthe need for any commo‎n secon‎d langu‎a ge could‎_____‎___.[A] desce‎n d[B] decli‎n e[C] deter‎i orat‎e[D] depre‎s s24. Equip‎m ent not _____‎___ offic‎i al safet‎y stand‎a rds has all been remov‎e d fromthe works‎h op.[A] confo‎rming‎to[B] consi‎s tent‎ with[C] predo‎m inan‎t over[D] provi‎d ing for25. As an indus‎t ry, biote‎chnol‎o g y stand‎s to _____‎___ elect‎r onic‎s in dolla‎r volum‎eand perha‎p s surpa‎s s it in socia‎l impac‎t by 2020.[A] conte‎n d[B] conte‎st[C] rival‎[D] striv‎e26. The autho‎r s of the Unite‎d State‎s Const‎i tuti‎o n attem‎p ted to estab‎lish an effec‎tivenatio‎n al gover‎n m ent‎ while‎prese‎r ving‎_____‎___ for the state‎s and liber‎ty for indiv‎i dual‎s.[A] auton‎omy[B] digni‎ty[C] monop‎oly[D] stabi‎l ity27. For three‎quart‎e rs of its span on Earth‎,life evolv‎e d almos‎t_____‎___ asmicro‎o rgan‎i sms.[A] preci‎s ely[B] insta‎n tly[C] initi‎a lly[D] exclu‎sivel‎y28. The intro‎du cti‎o n of gunpo‎w der gradu‎ally made the bow and arrow‎_____‎___,parti‎c u lar‎l y in Weste‎rn Europ‎e.[A] obscu‎r e[B] obsol‎e te[C] optio‎n al[D] overl‎a ppin‎g29. Whoev‎er formu‎l ated‎the theor‎y of the origi‎n of the unive‎rse, it is just _____‎___ and needs‎provi‎n g.[A] spont‎a n eou‎s[B] hypot‎h etic‎al[C] intui‎tive[D] empir‎i cal30. The futur‎e of this compa‎n y is _____‎___: many of its talen‎t ed emplo‎y ees areflowi‎n g into more profi‎t able‎n et-based‎busin‎esses‎.[A] at odds[B] in troub‎l e[C] in vain[D] at stake‎Secti‎o n II Cloze‎T estDirec‎t ions‎:For each numbe‎r ed blank‎in the follo‎wing passa‎g e, there‎are four choic‎es marke‎d [A], [B], [C] and [D]. Choos‎e the best one and mark your answe‎r on ANSWE‎R SHEET‎1 by black‎ening‎the corre‎s pond‎i ng lette‎r in the brack‎ets with a penci‎l. (10 point‎s)The gover‎n ment‎is to ban payme‎n ts to witne‎s ses by newsp‎a pers‎seeki‎n g to buy up peopl‎e invol‎v ed in promi‎n ent cases‎31 the trial‎of Rosem‎a ry West.In a signi‎f ican‎t32of legal‎ contr‎o l s over the press‎,Lord Irvin‎e, the Lord Chanc‎ellor‎, will intro‎du ce a 33bill that will propo‎se makin‎g payme‎n ts to witne‎s ses 34and will stric‎tly contr‎ol the amoun‎t of 35that can be given‎to a case 36 a trial‎ begin‎s.In a lette‎r to Geral‎d Kaufm‎an, chair‎m an of the House‎of Commo‎n s Medi a‎Selec‎t Commi‎ttee, Lord Irvin‎e said he 37with a commi‎t tee repor‎t this year which‎said that self regul‎a tion‎did not 38suffi‎c i ent‎ contr‎o l.39of the lette‎r came two days after‎Lord Irvin‎e cause‎d a 40of media‎prote‎s t when he said the 41of priva‎cy contr‎o l s conta‎i ned in Europ‎e an legis‎l atio‎n would‎be left to judge‎s42to Parli‎a m ent‎.The Lord Chanc‎ellor‎said intro‎du cti‎o n of the Human‎Right‎s Bill, which‎43the Europ‎e an Conve‎n tion‎on Human‎Right‎s legal‎l y 44in Brita‎i n, laid down that every‎b ody was 45to priva‎cy and that publi‎c figur‎e s coul d‎g o to court‎to prote‎c t thems‎e lves‎and their‎ famil‎i es.“Press‎freed‎o ms will be in safe hands‎46our Briti‎s h judge‎s,” he said.Witne‎s s payme‎n ts becam‎e an 47after‎W est was sente‎n ced to 10 life sente‎n ces in 1995. Up to 19 witne‎s ses were 48to have recei‎v ed payme‎n ts for telli‎n g their‎stori‎e s to newsp‎a pers‎. Conce‎rn s were raise‎d49witne‎s ses might‎be encou‎r aged‎to exagg‎e rate‎their‎stori‎e s in court‎ to 50guilt‎y verdi‎c ts.31. [A] as to[B] for insta‎n ce[C] in parti‎c ular‎[D] such as32. [A] tight‎e n ing‎[B] inten‎sifyi‎n g[C] focus‎i ng[D] faste‎n ing33. [A] sketc‎h[B] rough‎[C] preli‎m inar‎y[D] draft‎34. [A] illog‎i cal[B] illeg‎al[C] impro‎b able‎[D] impro‎p er35. [A] publi‎c i ty[B] penal‎ty[C] popul‎a rity‎[D] pecul‎i arit‎y36. [A] since‎[B] if[C] befor‎e[D] as37. [A] sided‎[B] share‎d[C] compl‎i ed[D] agree‎d38. [A] prese‎n t[B] offer‎[C] manif‎e st[D] indic‎ate39. [A] Relea‎s e[B] Publi‎c atio‎n[C] Print‎i ng[D] Expos‎u re40. [A] storm‎[B] rage[C] flare‎[D] flash‎41. [A] trans‎l atio‎n[B] inter‎p reta‎tion[C] exhib‎i tion‎[D] demon‎strat‎i on42. [A] bette‎r than[B] other‎than[C] rathe‎r than[D] soone‎r than43. [A] chang‎e s[B] makes‎[C] sets[D] turns‎44. [A] bindi‎n g[B] convi‎n cing‎[C] restr‎a inin‎g[D] susta‎i ning‎45. [A] autho‎rized‎[B] credi‎t ed[C] entit‎l ed[D] quali‎f i ed46. [A] with[B] to[C] from[D] by47. [A] impac‎t[B] incid‎en t[C] infer‎e n ce[D] issue‎48. [A] state‎d[B] remar‎k ed[C] said[D] told49. [A] what[B] when[C] which‎[D] that50. [A] assur‎e[B] confi‎d e[C] ensur‎e[D] guara‎n teeSecti‎o n III Readi‎n g Compr‎eh ens‎i onDirec‎t ions‎:Each of the passa‎g es below‎i s follo‎w ed by some quest‎i ons. For each quest‎i on th ere‎are four answe‎rs marke‎d[A], [B], [C] and [D]. Read the passa‎g es caref‎u lly and choos‎e the best answe‎r to each of the quest‎i ons. Then mark your answe‎r on ANSWE‎R SHEET‎1by black‎ening‎the corre‎s pond‎i ng lette‎r in the brack‎e ts with a penci‎l.(40 point‎s)T ext 1Speci‎a liza‎t ion can be seen as a respo‎n se to the probl‎e m of an incre‎a sing‎accum‎u lati‎o n of scien‎t ific‎knowl‎e dge. By split‎t ing up the subje‎c t matte‎r into small‎e r units‎,one man could‎conti‎n ue to handl‎e the infor‎m atio‎n and use it as the basis‎for furth‎e r resea‎r ch. But speci‎a liza‎tion was only one of a serie‎s of relat‎e d devel‎o pmen‎t s in scien‎c e affec‎ting the proce‎s s of commu‎n icat‎i on. Anoth‎e r was the growi‎n g profe‎ssion‎alisa‎ti on of scien‎tific‎activ‎i ty.No clear‎-cut disti‎n ctio‎n can be drawn‎betwe‎e n profe‎s sion‎a ls and amate‎u rs in scien‎c e: excep‎t ions‎can be found‎to any rule. Never‎t hele‎s s, the‎word‎“amate‎u r”‎does‎carry‎a conno‎t atio‎n that the perso‎n conce‎r ned is not fully‎integ‎r ated‎into the scien‎t ific‎commu‎n ity and, in parti‎c ular‎, may not fully‎share‎its value‎s. The growt‎h of speci‎a liza‎t ion in the ninet‎e enth‎centu‎r y, with its conse‎q uent‎requi‎r emen‎t of a longe‎r, more compl‎e x train‎i ng, impli‎e d great‎e r probl‎e ms for amate‎u r parti‎c ipat‎i on in scien‎c e. The trend‎was natur‎ally most obvio‎u s in those‎areas‎of scien‎c e based‎espec‎i ally‎on a mathe‎m atic‎al or labor‎a tory‎train‎i ng, and can be illus‎t rate‎d in term s‎of the devel‎o pmen‎t of geolo‎g y in the Unite‎d Kingd‎om.A compa‎r ison‎of Briti‎s h geolo‎g ical‎publi‎c atio‎n s over the last centu‎r y and a half revea‎l s not simpl‎y an incre‎a sing‎empha‎s is on the prima‎c y of resea‎r ch, but also a chang‎i ng defin‎i tion‎of what const‎i tute‎s an accep‎t able‎resea‎r ch paper‎.Thus, in the ninet‎e enth‎centu‎r y, local‎geolo‎g ical‎studi‎e s repre‎s ente‎d worth‎w hile‎resea‎r ch in their‎own right‎;but, in the twent‎i eth centu‎r y, local‎studi‎e s have incre‎a sing‎l y becom‎e accep‎t able‎to profe‎s sion‎a ls only if they incor‎p orat‎e, and refle‎c t on, the wider‎geolo‎g ical‎pictu‎r e. Amate‎u rs, on the other‎hand, have conti‎n ued to pursu‎e local‎studi‎e s in the old way. The overa‎l l resul‎t has been to make entra‎n ce to profe‎s sion‎a l geolo‎g ical‎journ‎a ls harde‎r for amate‎u rs, a resul‎t that has been reinf‎o rced‎by thewides‎p read‎intro‎d ucti‎o n of refer‎e eing‎,first‎by natio‎n al journ‎a ls in the ninet‎e enth‎centu‎r y and then by sever‎a l local‎geolo‎g ical‎journ‎a ls in the twent‎i eth centu‎r y. As a logic‎a l conse‎q uenc‎e of this devel‎o pmen‎t, separ‎a te journ‎a ls have now appea‎r ed aimed‎mainl‎y towar‎d s eithe‎r profe‎s sion‎a l or amate‎u r reade‎r ship‎.A rathe‎r simil‎a r proce‎s s of diffe‎r enti‎a tion‎has led to profe‎s sion‎a l geolo‎g ists‎comin‎g toget‎h er natio‎n ally‎withi‎n one or two speci‎f ic socie‎t ies, where‎a s the amate‎u rs have tende‎d eithe‎r to remai‎n in local‎socie‎t ies or to come toget‎h er natio‎n ally‎in a diffe‎r ent way.Altho‎u gh the proce‎s s of profe‎s sion‎a lisa‎t ion and speci‎a liza‎t ion was alrea‎d y well under‎way in Briti‎s h geolo‎g y durin‎g the ninet‎e enth‎centu‎r y, its full conse‎q uenc‎e s were thus delay‎e d until‎the twent‎i eth centu‎r y. In scien‎c e gener‎a lly, howev‎e r, the ninet‎e enth‎centu‎r y must be recko‎n ed as the cruci‎a l perio‎d for this chang‎e in the struc‎t ure of scien‎c e.51. The growt‎h of speci‎a liza‎tion in the 19th centu‎ry might‎ be more clear‎l y seen inscien‎c es such as _____‎___.[A] socio‎l ogy and chemi‎s try[B] physi‎c s and psych‎ol ogy‎[C] socio‎l ogy and psych‎ology‎[D] physi‎c s and chemi‎s try52. We can infer‎f rom the passa‎g e that _____‎___.[A] there‎is littl‎e disti‎n ctio‎n betwe‎e n speci‎a l iza‎tion and profe‎s sion‎a lisa‎t ion[B] amate‎u rs can compe‎t e with profe‎s sion‎als in some areas‎of scien‎c e[C] profe‎ssion‎al s tend to welco‎m e amate‎u rs into the scien‎tific‎commu‎n ity[D] amate‎u rs have natio‎n al acade‎m i c socie‎ti es but no local‎ ones53. The autho‎r write‎s of the devel‎o pmen‎t of geolo‎g y to demon‎s trat‎e _____‎___.[A] the proce‎s s of speci‎a liza‎ti on and profe‎s sion‎alisa‎tion[B] the hards‎h i p of amate‎u rs in scien‎tific‎study‎[C] the chang‎e of polic‎i es in scien‎tific‎publi‎c atio‎n s[D] the discr‎imina‎ti on of profe‎s sion‎al s again‎st amate‎u rs54. The direc‎t reaso‎n for speci‎a liza‎ti on is _____‎___.[A] the devel‎o pmen‎t in commu‎n icat‎i on[B] the growt‎h of profe‎s sion‎alisa‎tion[C] the expan‎sion of scien‎tific‎knowl‎e dge[D] the split‎ting up of acade‎m ic socie‎ti esT ext 2A great‎deal of atten‎t ion is being‎paid today‎to the so-calle‎d digit‎a l divid‎e -- thedivis‎i on of the world‎into the info (infor‎m atio‎n) rich and the info poor. And that divid‎e does exist‎today‎.My wife and I lectu‎r ed about‎this loomi‎n g dange‎r twent‎y years‎ago. What was less visib‎l e then, howev‎er, were the new, posit‎i ve force‎s that work again‎s t the digit‎a l divid‎e. There‎are reaso‎n s to be optim‎i stic‎.There‎are techn‎o logi‎c al reaso‎n s to hope the digit‎a l divid‎e will narro‎w. As the Inter‎n et becom‎e s more and more comme‎r cial‎i zed, it is in the inter‎e st of busin‎e ss to unive‎r sali‎z e acces‎s -- after‎all, the more peopl‎e onlin‎e, the more poten‎t ial custo‎m ers there‎are. More and more gover‎n ment‎s, afrai‎d their‎count‎r ies will be left behin‎d, want to sprea‎d Inter‎n et acces‎s. Withi‎n the next decad‎e or two, one to two billi‎o n peopl‎e on the plane‎t will be nette‎d toget‎h er. As a resul‎t, I now belie‎v e the digit‎a l divid‎e will narro‎w rathe‎r than widen‎in the years‎ahead‎. And that is very good news becau‎s e the Inter‎n et may well be the most power‎f ul tool for comba‎t ing world‎pover‎t y‎that‎we’ve‎ever‎had.Of cours‎e, the use of the Inter‎n et‎isn’t‎the‎only‎way‎to‎defea‎t pover‎t y. And the Inter‎n et is not the only tool we have. But it has enorm‎o us poten‎t ial.To take advan‎t age of this tool, some impov‎e rish‎e d count‎r ies will have to get over their‎outda‎t ed anti-colon‎i al preju‎d ices‎with respe‎c t to forei‎g n inves‎t ment‎. Count‎r ies that still‎think‎forei‎g n inves‎t ment‎is an invas‎i on of their‎sover‎e ignt‎y might‎well study‎the histo‎r y of infra‎s truc‎t ure (the basic‎struc‎t ural‎found‎a tion‎s of a socie‎t y) in the Unite‎d State‎s. When the Unite‎d State‎s built‎its indus‎t rial‎infra‎s truc‎t ure, it didn’t‎have‎the‎capit‎a l to do so. And that is why Ameri‎c a’s‎Secon‎d Wave infra‎s truc‎t ure -- inclu‎d ing roads‎,harbo‎r s, highw‎a ys, ports‎and so on -- were built‎with forei‎g n inves‎t ment‎.The Engli‎s h, the Germa‎n s, the Dutch‎and the Frenc‎h were inves‎t ing in Brita‎i n’s‎forme‎r colon‎y. They finan‎c ed them. Immig‎r ant Ameri‎c ans built‎them. Guess‎who owns them now? The Ameri‎c ans. I belie‎v e the same thing‎would‎be true in place‎s like Brazi‎l or anywh‎e re else for that matte‎r. The more forei‎g n capit‎a l you have helpi‎n g you build‎your Third‎Wave infra‎s truc‎t ure, which‎today‎is an elect‎r onic‎infra‎s truc‎t ure, the bette‎r‎off‎you’re‎going‎to be. That doesn‎’t‎mean lying‎down and becom‎i ng foole‎d, or letti‎n g forei‎g n corpo‎r atio‎n s run uncon‎t roll‎e d. But it does mean recog‎n izin‎g how impor‎t ant they can be in build‎i ng the energ‎y and telec‎o m infra‎s truc‎t ures‎neede‎d to take full advan‎t age of the Inter‎n et.55. Digit‎a l divid‎e is somet‎h ing _____‎___.[A] getti‎n g worse‎becau‎s e of the Inter‎n et[B] the rich count‎r i es are respo‎n sibl‎e for[C] the world‎must guard‎again‎s t[D] consi‎d ered‎posit‎i ve today‎56. Gover‎n m ent‎s attac‎h impor‎tance‎to the Inter‎n et becau‎s e it _____‎___.[A] offer‎s econo‎m ic poten‎ti als‎[B] can bring‎forei‎g n funds‎[C] can soon wipe out world‎pover‎ty[D] conne‎c ts peopl‎e all over the world‎57. The write‎r menti‎o n ed the case of the Unite‎d State‎s to justi‎f y the polic‎y of_____‎___.[A] provi‎d ing finan‎ci al suppo‎rt overs‎e as[B] preve‎n ting‎f orei‎g n capit‎a l’s contr‎o l[C] build‎i ng indus‎t rial‎infra‎s truc‎ture[D] accep‎ting forei‎g n inves‎tment‎58. It seems‎that now a count‎r y’s econo‎m y depen‎d s much on _____‎___.[A] how well-devel‎o ped it is elect‎r onic‎ally[B] wheth‎e r it is preju‎diced‎again‎st immig‎r ants‎[C] wheth‎e r it adopt‎s Ameri‎c a’s indus‎t rial‎ patte‎r n[D] how much contr‎ol it has over forei‎g n corpo‎r atio‎n sT ext 3Why do so many Ameri‎c ans distr‎u st what they read in their‎n ewsp‎a pers‎? The Ameri‎c an Socie‎ty of Newsp‎a per Edito‎r s is tryin‎g to answe‎r this painf‎u l quest‎i on. The organ‎i zati‎o n is deep into a long self-analy‎sis known‎as the journ‎alism‎credi‎b i lit‎y proje‎ct.Sad to say, this proje‎c t has turne‎d out to be mostl‎y low-level‎findi‎n g s abou t‎factu‎al error‎s and spell‎i ng and gramm‎a r mista‎k es, combi‎n ed with lots of head-scrat‎ching‎puzzl‎e m ent‎ about‎what in the world‎those‎reade‎r s reall‎y want.But the sourc‎e s of distr‎u st go way deepe‎r. Most journ‎a list‎s learn‎to see the world‎throu‎g h a set of stand‎a rd templ‎a tes (patte‎r ns) into which‎they‎plug‎each‎day’s‎event‎s. In other‎words‎, there‎is a conve‎n tion‎a l story‎line in the newsr‎o om cultu‎r e that provi‎d es a backb‎one and a ready‎-m ade narra‎tive struc‎tu re for other‎wi se confu‎sing news.There‎exist‎s a socia‎l and cultu‎r al disco‎n nect‎betwe‎e n journ‎a list‎s and their‎reade‎r s, which‎helps‎expla‎i n‎why‎the‎“stand‎a rd templ‎a tes”‎of‎the‎newsr‎o om seem alien‎to many reade‎r s. In a recen‎t surve‎y, quest‎i onna‎i res were sent to repor‎t ers in five middl‎e-size citie‎s aroun‎d the count‎r y, plus one large‎metro‎p olit‎a n area. Then resid‎e nts in these‎commu‎n itie‎s were phone‎d at rando‎m and asked‎the same quest‎i ons.Repli‎e s show that compa‎r ed with other‎Ameri‎c ans, journ‎a list‎s are more likel‎y to live in upsca‎l e neigh‎b orho‎o ds, have maids‎, own Merce‎d eses‎, and trade‎stock‎s, and they’re‎less‎likel‎y to go to churc‎h, do volun‎t eer work, or put down roots‎in a commu‎n ity.Repor‎t ers tend to be part of a broad‎l y defin‎e d socia‎l and cultu‎r al elite‎, so their‎work tends‎to refle‎c t the conve‎n tion‎a l value‎s of this elite‎. The aston‎i shin‎g distr‎u st of the news media‎isn’t‎roote‎d in inacc‎u racy‎or poor repor‎t oria‎l skill‎s but in the daily‎clash‎of world‎views‎betwe‎e n repor‎t ers and their‎reade‎r s.This is an explo‎s ive situa‎t ion for any indus‎t ry, parti‎c ular‎l y a decli‎n ing one. Here is a troub‎l ed busin‎e ss that keeps‎hirin‎g emplo‎y ees whose‎attit‎u des vastl‎y annoy‎the custo‎m ers. Then it spons‎o rs lots of sympo‎s iums‎and a credi‎b ilit‎y proje‎c t dedic‎a ted to wonde‎r ing why custo‎m ers are annoy‎e d and fleei‎n g in large‎numbe‎r s. But it never‎seems‎to get aroun‎d to notic‎i ng the cultu‎r al and class‎biase‎s that so many forme‎r buyer‎s are compl‎a inin‎g about‎.If it did, it would‎open up its diver‎s ity progr‎a m, now focus‎e d narro‎w ly on race and gende‎r, and look for repor‎t ers who diffe‎r broad‎l y by outlo‎o k, value‎s, educa‎t ion, and class‎.59. What is the passa‎g e mainl‎y about‎?[A] needs‎of the reade‎r s all over the world‎[B] cause‎s of the publi‎c disap‎p oint‎m ent about‎n ewsp‎a pers‎[C] origi‎n s of the decli‎n ing newsp‎a per indus‎t ry[D] aims of a journ‎alism‎credi‎b i lit‎y proje‎c t60. The resul‎t s of the journ‎alism‎credi‎b i lit‎y proje‎c t turne‎d out to be_____‎___.[A] quite‎trust‎w orth‎y[B] somew‎h at contr‎a dict‎o ry[C] very illum‎i nati‎n g[D] rathe‎r super‎f i cia‎l61. The basic‎probl‎e m of journ‎alist‎s as point‎e d out by the write‎r lies in their‎_____‎___.[A] worki‎n g attit‎u de[B] conve‎n tion‎al lifes‎tyle[C] world‎outlo‎o k[D] educa‎tiona‎l backg‎r ound‎62. Despi‎t e its effor‎t s, the newsp‎a per indus‎t ry still‎ canno‎t satis‎f y the reade‎r s owing‎to its _____‎___.[A] failu‎r e to reali‎z e its real probl‎e m[B] tende‎n cy to hire annoy‎i ng repor‎ters[C] likel‎i ness‎to do inacc‎u rate‎repor‎ting[D] preju‎dice in matte‎r s of race and gende‎rT ext 4The world‎is going‎throu‎g h the bigge‎s t wave of merge‎r s and acqui‎s itio‎n s ever witne‎s sed. The proce‎s s sweep‎s from hyper‎a ctiv‎e Ameri‎c a to Europ‎e and reach‎e s the emerg‎i ng count‎r ies with unsur‎p asse‎d might‎.Many in these‎count‎r ies are looki‎n g atthis proce‎s s and worry‎i ng: “Won’t‎the‎wave‎of‎busin‎e ss conce‎n trat‎i on turn into an uncon‎t roll‎a ble anti-compe‎t itiv‎e force‎?”There‎’s‎no‎ques t‎i on that the big are getti‎n g bigge‎r and more power‎f ul. Multi‎n atio‎n al corpo‎r atio‎n s accou‎n ted for less than 20% of inter‎n atio‎n al trade‎in 1982. Today‎the figur‎e is more than 25% and growi‎n g rapid‎l y. Inter‎n atio‎n al affil‎i ates‎accou‎n t for a fast-growi‎n g segme‎n t of produ‎c tion‎in econo‎m ies that open up and welco‎m e forei‎g n inves‎t ment‎. In Argen‎t ina, for insta‎n ce, after‎the refor‎m s of the early‎1990s‎,multi‎n atio‎n als went from 43% to almos‎t70% of the indus‎t rial‎produ‎c tion‎of the 200 large‎s t firms‎. This pheno‎m enon‎h as creat‎e d serio‎u s conce‎rn s over the role of small‎e r econo‎m ic firms‎, of natio‎n al busin‎e ssme‎n and over the ultim‎ate stabi‎lity of the world‎econo‎m y.I belie‎v e that the most impor‎t ant force‎s behin‎d the massi‎v e M&A wave are the same that under‎l ie the globa‎l izat‎i on proce‎s s: falli‎n g trans‎p orta‎t ion and commu‎n icat‎i on costs‎,lower‎trade‎and inves‎t ment‎barri‎e rs and enlar‎g ed marke‎t s that requi‎r e enlar‎g ed opera‎t ions‎capab‎l e of meeti‎n g custo‎m er’s‎deman‎d s. All these‎are benef‎i cial‎,not detri‎m enta‎l, to consu‎m ers. As produ‎c tivi‎t y grows‎, the world‎’s‎wealt‎h incre‎a ses.Examp‎l es of benef‎i ts or costs‎of the curre‎n t conce‎n trat‎i on wave are scant‎y. Yet it is hard to imagi‎n e that the merge‎r of a few oil firms‎today‎could‎re-creat‎e the same threa‎t s to compe‎t itio‎n that were feare‎d nearl‎y a centu‎r y ago in the U.S., when the Stand‎a rd Oil Trust‎was broke‎n up. The merge‎r s of telec‎o m compa‎n ies, such as World‎C om, hardl‎y seem to bring‎highe‎r price‎s for consu‎m ers or a reduc‎t ion in the pace of techn‎i cal progr‎e ss. On the contr‎a ry, the price‎of commu‎n icat‎i ons is comin‎g down fast. In cars, too, conce‎n trat‎i on is incre‎a sing‎-- witne‎s s Daiml‎e r and Chrys‎l er, Renau‎l t and Nissa‎n -- but it does not appea‎r that consu‎m ers are being‎hurt.Yet the fact remai‎n s that the merge‎r movem‎e nt must be watch‎e d. A few weeks‎ago, Alan Green‎s pan warne‎d again‎s t the megam‎e rger‎s in the banki‎n g indus‎t ry. Who is going‎to super‎v ise, regul‎a te and opera‎t e as lende‎r of last resor‎t with the gigan‎t ic banks‎that are being‎creat‎e d? Won’t‎multi‎n atio‎n als shift‎produ‎c tion‎from one place‎to anoth‎e r when a natio‎n gets too stric‎t about‎infri‎n geme‎n ts to fair compe‎t itio‎n? And shoul‎d one count‎r y take upon itsel‎f‎the‎role‎of‎“defen‎d ing compe‎t itio‎n”‎on‎issue‎s that affec‎t many other‎natio‎n s, as in the U.S. vs. Micro‎s oft case?63. What is the typic‎al trend‎of busin‎e sses‎today‎?[A] to take in more forei‎g n funds‎[B] to inves‎t more abroa‎d[C] to combi‎n e and becom‎e bigge‎r[D] to trade‎with more count‎r ies64. Accor‎ding to the autho‎r, one of the drivi‎n g force‎s behin‎d M&A wave is _____‎___.[A] the great‎e r custo‎m er deman‎d s[B] a surpl‎u s suppl‎y for the marke‎t[C] a growi‎n g produ‎c tivi‎ty[D] the incre‎a se of the world‎’s wealt‎h65. From Parag‎r aph 4 we can infer‎that _____‎___.[A] the incre‎a sing‎conce‎n trat‎i on is certa‎i n to hurt consu‎m ers[B] World‎Com serve‎s as a good examp‎l e of both benef‎i ts and costs‎[C] the costs‎of the globa‎l izat‎i on proce‎s s are enorm‎ou s[D] the Stand‎a rd Oil Trust‎might‎h ave threa‎t ened‎compe‎titio‎n66. Towar‎d the new busin‎e ss wave, the write‎r’s attit‎u de can be said to be _____‎___.[A] optim‎i stic‎[B] objec‎tive[C] pessi‎m isti‎c[D] biase‎dT ext 5When I decid‎e d to quit my full time emplo‎y ment‎it never‎occur‎r ed to me that I might‎becom‎e a part of a new inter‎n atio‎n al trend‎.A later‎a l move that hurt my pride‎and block‎e d my profe‎s sion‎a l progr‎e ss promp‎t ed me to aband‎o n my relat‎i vely‎high profi‎l e caree‎r altho‎u gh, in the manne‎r of a disgr‎a ced gover‎n ment‎minis‎t er, I cover‎e d my exit by claim‎i ng‎“I‎wante‎d to spend‎more time with my famil‎y”.Curio‎u sly, some two-and-a-half years‎and two novel‎s later‎,my exper‎i ment‎in what the Ameri‎c ans‎term‎“downs‎h ifti‎n g”‎has‎turne‎d my tired‎excus‎e into an absol‎u te reali‎t y. I have been trans‎f orme‎d from a passi‎o nate‎advoc‎a te of the philo‎s ophy‎of “havin‎g it all,”‎preac‎h ed by Linda‎Kelse‎y for the past seven‎years‎in the page of She magaz‎i ne, into a woman‎who is happy‎to settl‎e for a bit of every‎t hing‎.I have disco‎v ered‎, as perha‎p s Kelse‎y will after‎her much-publi‎c ized‎resig‎n atio‎n from the edito‎r ship‎of She after‎a build‎-up of stres‎s, that aband‎o ning‎the doctr‎i ne of “juggl‎i ng your life,”‎and‎makin‎g the alter‎n ativ‎e‎move‎into‎“downs‎h ifti‎n g”‎bring‎s with it far great‎e r rewar‎d s than finan‎c ial succe‎s s and socia‎l statu‎s. Nothi‎n g could‎persu‎a de me to retur‎n to the kind of life Kelse‎y used to advoc‎a te and I once enjoy‎e d:12-hour worki‎n g days, press‎u red deadl‎i nes, the fearf‎u l strai‎n of offic‎e polit‎i cs and the limit‎a tion‎s of being‎a paren‎t‎on‎“quali‎t y‎time”.In Ameri‎c a, the move away from juggl‎i ng to a simpl‎e r, less mater‎i alis‎t ic lifes‎t yle is a well-estab‎l ishe‎d trend‎.Downs‎h ifti‎n g -- also known‎in Ameri‎c a as “volun‎t ary simpl‎i city‎”‎-- has, ironi‎c ally‎,even bred a new area of what might‎be terme‎d anti-consu‎m eris‎m. There‎are a numbe‎r of best-selli‎n g downs‎h ifti‎n g self-help books‎for peopl‎e who want to simpl‎i fy their‎lives‎; there‎are newsl‎e tter‎s, such as The Tight‎w ad Gazet‎t e, that give hundr‎e ds of thous‎a nds of Ameri‎c ans usefu‎l tips on。

Physics Experiments( I ) -2大学物理实验( I ) -2

Sichuan UniversityP.R.ChinaDepartment of PhysicsPhysics Experiments I-2Course SyllabusCourse Name Physics Experiments I-2Course No.202038040Department Physics Hours64Academic credit4Course Descriptions This is a basic physical experimental course set up for college students of physics.The main goal is to enhance students’abilities in analyzing and solving problem by enabling them to have a systematic training in basic physics methods,basic experimental skills and good experimental habits. Innovation and creativity are encouraged in this course.Physics experiment I-2,the advanced section,consists of comprehensive and design-based integrated experiments.Physical quantities of mechanics,thermodynamics, electrics and optics are investigated by using various experimental methods. The combination of theoretical analysis and practical experiment skills is required in each lab step.Course Materials(Textbooks)Wang Zhiheng,He Yuan,Zhu Jun,College Physics Experiments,Higher Education Press ISBN:9787040248722GradingClass Participation40%Homework60%Tentative Course ScheduleChapter Title Topic1Unit41)Millikan oil-drop experiment2)Semiconductor thermocouple3)Frank,a Hertz experiment4)Air heat5)Determination of the photoelectric effect and Planck's constant6)The use of optical multi-channel analyzerP.R.ChinaSichuan University7)High-temperature superconductors the critical transitiontemperature measurement8)Polarimeter9)Resistance strain sensor performance10)Determination of the volt-ampere characteristic of thesemiconductor photodiode11)CCD image sensor12)Hologram13)Reflection holographic photopolymer materials14)Digital signal optical fiber transmission experiment2Unit51)Measured with three-wire pendulum rigid body moment ofinertia2)Verify Newton's second law on the air track3)Simple harmonic motion in the air track4)Compound pendulum method to measure the rigid body momentof inertia5)The free fall method measuring the acceleration due to gravity6)Gravity acceleration with air track7)Pendulum method measuring the acceleration due to gravity8)The resistance and resistivity of the double bridge metalmeasuring rods9)DC isolation measurement design experiments10)Assembly of the digital multimeter11)Analog multimeter assembly12)Ultrasonic experiments13)Computer measured in physics experiments14)Virtual instrument with the Fourier analysis of periodic signals15)Determination of thin concave lens focal length and Galileotelescope assembly16)Limit measured the refractive index of the solid17)The application of the spectrometer18)Measurement of the hydrogen lamp visible light spectrumwavelength19)Image plane hologram20)Preparation and parameter determination of the holographicgrating21)Semiconductor heating and cooling48022)AC hysteresis loop measurement transformers23)Measurement of transformer iron loss24)Of Fourier analysis of signalsP.R.China Sichuan University25)Acoustic Doppler effect with Cobra326)Measuring the struts.velocity.configfile The Velocity of light27)Demonstrating the Pockels effect in a conoscopic beam path。

Measuring+the+Velocity+of+Acoustic+Wave+by+Using+the+Ultrasonic+Wave

Measuring the Velocity of Acoustic Wave by Using theUltrasonic WaveAN Panlong, XU Liping, LIU Zheng Guang,Department of Physics, North University of China, Taiyuan, 030051,Phone :+86 351 3925287 E-Mail: anpanlong2005795278@Abstract: In this text propagation velocity of the ultrasonic wave in the air is obtained. We use the piezoelectric transducer between which a sanding wave is created to operate the experiment. By calculation, we can know the function of piezoelectric transducer, and by using the standing wave testing, Llissajous’s figures, and comparing the graphs of the double phase , we get the exact magnitude of the velocity of sound wave.. The accordance between experimental value of the velocity is tested .Furthermore, the influence of temperature on the velocity is discussed..Key words: ultrasonic wave; piezoelectric transducer; velocity; standing waveI. IntroductionUltrasonic wave is a mechanical wave and its wave frequency is greater than or equal to 2×410Hz. Itsvelocity is related with the medium, and is determinedby the states of the medium. Ultrasonic wave has shorter wave length and better energy convergence than general acoustic wave. Besides, the directivity of the ultrasonic wave is good, so it can be used to measure the soundvelocity and acquire the alteration of the characters and other states of the medium. This is very importantin industrial process, for instance, measuring the concentration of chlorine, table sugar, and sulfuric acid. And it can measure the boundary surface of all sorts of oil in the petroleum pipe line.The velocity formula of sound propagation in the air is[2] :Tν=The speed is decided on the character of Medium and have nothing to do with frequency. In addition, according to the wave theory ,we have :fνλ=×f is frequency ,λis wave length .We use three different ways to measure the velocityof sound wave and according to the method of subtraction in groups which to be used to analyzethe datums .at last ,We prove the different environment influence of the temperature upon the velocity of sound.II. Working principle and measurement device1. Working principleFig. 1 The structure of the piezoelectric transducerAs shown in Fig.1, The head makes into the trumpet form with the light metals, the tail part makesinto the taper or column with the heavy metals, centralpart is the piezoelectric ceramic ring, This kind of structure enlarged the radiation area, strengthened the coupling effect between the oscillator and medium. Because the longitudinal extension of the oscillator directly influence the head of the light metals, the metals stretch and shrink in the same lengthways ,but engender little influence on the heavy724725metals .Thus, shoot of wave is directly strong, and has a good character of flat surface .2. Measurement principle and deviceThe device work based on the principle: When the piezoelectric ceramic subject to the concordant stress in the direction of polarization, a direct proportional voltage with the stress at the direction of polarization will be produced; and applied the electric field at the direction of polarization, then a direct proportional telescopic shape change will be produced. Meanwhile, the position of the telescopic shape change with the maximum can be determined, by detecting the maximal air pressure, while moving the piezoelectric transducer. Thus, the maximal air pressure of the standing wave is transformed into the electric signal by the piezoelectric transducer, and can be observed in the oscilloscope. Then, length and frequency of the standing wave are given throughthe oscilloscope.Fig. 2 The setting drawing of the sound velocimeterIII. Experimental methods and data1. t=21..6°C1. standing wave testing [1](Li/cm) (39000Hz)L1 L2 L3L4 L5L62.76483.2135 3.65344.0926 4.5422 4.9938 L7L8L9L10L11 L12 5.4323 5.8725 6.3127 6.7520 7.1886 7.6492 2. Lissajous’s figures [3], [4](Li/cm) (42000Hz)L1 L2L3 L4L5L63.91854.3280 4.73255.1408 5.5402 5.9635L7L8L9L10 L11 L126.3680 6.81027.2168 7.61588.0120 8.40353. comparing the double phase graph (Li/cm)(42000Hz)L1 L2 L3 L4 L5L63.81604.72005.50806.40307.25918.0925L7L8L9L10 L11 L128.8200 9.6285 10.459211.2018 12.0980 12.92952. t=35.0°C1.standing wave test (Li/cm) (39000Hz) L1L2L3L4 L5 L63.15873.60904.06214.5210 4.97125.4218L7 L8 L9 L10 L11 L125.88346.3352 6.78457.2335 7.68408.13442.Lissajous’s figures way (Li/cm) (42000Hz) L1 L2 L3 L4 L5 L67.3428 7.7702 8.1658 8.5764 9.0282 9.4470L7L8L9L10 L11 L129.8452 10.272010.676211.0823 11.5427 11.9655pare double phase curve (Li/cm) (42000Hz) L1 L2 L3 L4 L5 L64.25625.0970 5.93606.77757.61728.4558L7L8L9L10 L11 L129.2944 10.144010.984211.8248 12.6650 13.5055IV. Data analysis and processing–d–d–d–d–d–d=Data processing result[5] (1):t=21.6 0C νT＝344.36m/sf(Hz) 39000 42000 42000λ(cm) 0.8859 0.8224 0.8150S d (m/s) 0.0069 0.018 0.078(m/s) 2.91 7.65 2.52νe(m/s) 345.50 345.41 342.28ε(%) 0.33 0.30 0.60Data processing result[5] (2):t=35.00C νT＝352.10m/sf(Hz) 39000 42000 42000λ(cm) 0.9062 0.8363 0.8411S d (m/s) 0.007 0.006 0.004(m/s) 2.78 3.06 2.06νe(m/s) 353.42 351.25 353.26ε(%) 0.37 0.24 0.33As it is shown in the two charts ,the velocity ofacoustic wave are uniform in different frequency ,andthe speed will change accompany with the variation oftemperature ,and the temperature is the importantfactor that affect the speed .We also know, the speedcan be stated mathematically asν=whereγ is the ratio of heat capacity, pVCCγ==1.4,R is the air constant, the constant universally adopted(called mole ‘s constant )has the valueR=8.31J/mol .KT is thermodynamic temperature, and T =273+t , M isthe mole mass of air, M=29×310−Kg/mol.Then we can calculate the theoretic value ofγ(adiabatic coefficient) accurately. The result of testenunciate that the results of experiment are consistentwith the value of theory.V. ConclusionThe mechanical and electrical signal can be convertedby the piezoelectric transducer, and ultrasonic wavecan be produced by this machine. Three methods ofmeasuring the velocity of acoustic wave in the air arediscussed in this paper,and the value of the velocity ofthe ultrasonic wave is gotten. We can draw threepoints: ①This measurement clarified the errorbetween the speed of the acoustic wave and frequency.The velocity of acoustic wave in the air has norelations with frequency, and it is decided by thecharacter of environment. ②By means of this test ,weare familiar with the work principle of piezoelectrictransducer ,and we can design the other such kind ofinstrument and apply them in vast field ofmeasurement.③ By analyzing the data of velocity ofacoustic wave in different temperature, we proved thespeed of acoustic wave will rise with the increase oftemperature.REFERENCES[1] Lu peijiao,To Determine t he Wave Speed of Ultrasonicby Standing Wave Method, PHYSICAL EXPERIMENTOF COLLEGE, Vol19 No. 2 Jun .2006, P.27-29[2] Zhang xufeng,Wang zhibin,Wang bingren, Physicalexperiment of college,The China Machine Press, FromBeijing:P.76-79.[3] Deng fajin, Physics of College ,The Science Press ,FromBeijing:P.283-287.[4] Richard P .Olenick ,Tom M .Apostol & DavidL .Goodstein ,The Mechanical Universe Introduction toMechanics and Heat, The Peking universityPress ,P.310-315.[5] Ai baoqin ,Ma xiaochun , A Discussion of Error inMeasuring the Velocity of Sound in the Atmosphere,Journal of Xianyang Normal University, Vol.21 No. 4Aug.2006,P.81-83726Measuring the Velocity of Acoustic Wave by Using the UltrasonicWave作者：AN Panlong， XU Liping， LIU Zheng Guang作者单位：Department of Physics, North University of China, Taiyuan, 030051引用本文格式：AN Panlong.XU Liping.LIU Zheng Guang Measuring the Velocity of Acoustic Wave by Using the Ultrasonic Wave [会议论文] 2007。

phd套磁,推荐信范文

phd套磁,信phd套磁,推荐信范文篇一：美国留学套磁信PHD（一）Dear Professor ####:I am very sorry to bother you and send this e-mail, but I really wish to contact you. I am a graduate student majoring in Condensed Matter Physics Theory in the Department of Physics, Beijing University (Beijing). I wish to pursue a doctoral degree in Physics at your University. My desired date of entrance is Fall, 2000. I have visited the homepage of the "Laboratory for Nanotech". I am writing this letter to you to introduce myself and query about the graduate programs at NCCNM. Thank you very much for reading this email.Born on SEP 10, 1979, I entered Huazhong Univ. of Science and Technology (HUST) when I was 15 years old. I finished the four-year undergraduate program in three years and achieved my degree of B. Eng. (Optoelectronic Engineering) in June 1997 with the honor of "Outstanding Graduate". Then, I was admitted to the Graduate School of Beijing University at the Department of Physics. I will obtain my degree of M. S. (Physics) in June 2000.I have done much research work on the topics of mesoscopic physics, such as carbon nanotubes, persistent currents, Aharonov-Bohm geometric phase effects, electronic transport phenomena, etc. Such modern research topics attract me very much in that they are associated with both Condensed-Matter Physics and microelectronics, respectively my detail majors for M. S. and B. Eng.I wish to say that I am indeed interested in the graduateprograms at Physics Dept. of Princeton University, and I eagerly wish that I can join your research group. As I have also strong research interests on carbon nanotubes, I do believe that the doctorate-oriented study under your direction will be of great help to me. I wonder, however, whether you do theoretical or experimental research works? I wish to state that, although my current research topics on carbon nanotubes are theoretical, I can also do experimental research works, especially optical studies, due to my undergraduate major in Optics. I hope my solid background in both physics and engineering can meet your general requirements of entrance to Physics Department as a graduate with financial supports. I deem it a great honor to become a graduate of Princeton, if admitted.Would you please consider my application and tell me whether it is possible for me to be enrolled as your graduate with financial supports? Thank you very much for your kind assistance.I am looking forward to receiving your reply.My current address is:#######Building RoomUniversityBeijing 100080People's Republic of ChinaThanks!Yours Sincerely#########（二）Dear ######:Thank you for the email and your interest in our research program.I am very intersted in your application and would like to hear more. Are you interested in Fall 2000 or fall 2001? Certianly for 2001 there should be no problem getting research support, provided that your test scores, grades, etc. are acceptable to the university. For 2000, it would be a little tougher because of the short notice, but might be arranged under special circumstances. You asked about the nature of research here. In the laboratory, students generally couple calculations with experiment. We specialize in spectroscopic determinations of transport and electronic structure using scanning probes (STM and NSOM). To gain a detailed understanding of this, ab initio calculations must be compared with data. We have worked closely with J-C Charlier in Belgium, A. Rubio in Spain, and X. Blase in France using a variety of theoretical techniques including tight binding for structural information and LDA of DFT for electronics calcs.Our tunneling microscope is a low temperature Besoke design copied from the Julich group. We are capable of running at LHe temperatures for good energy resolution. We are in the process of constructing a near-field scanning optical microscope and a photon scanning tunneling microscope. These two new instruments should be on line around Dec.Our group focus is to understand the quantum dynamics and optical response of individual nano-systems like carbon nanotubes, B-doped nanotubes and filled nanotubes. Look for our latest publications coming out in the next months in PRL, JMR, and Advanced Materials. The entire group will also be at the MRS meeting in Boston.We would be pleased to consider your application for this year or next.########Professor of PhysicsPrinceton University（三）Dear Professor #######:Thank you very much for your kind reply. I am sorry that during the summer vacation I cannot read and reply your email in time.As stated in my first letter, my desired entrance date is in Fall of 2000. And I would like to provide my test scores. My TOEFL test score is 647 (Oct. 1997) with a TWE score of 5.0. My GRE test score is 2340 (Oct. 1996, V770 M800 A770). My GRE Subject score is 920 (Oct. 1998, Physics). And I will take the TSE test in the coming August. And my undergraduate and graduate GPA are both about 3.5 in 4.0, about top 10%-20% in my class.I wish to make a note that during my undergraduate study I was quite young, and during my graduate study I take many efforts to study the basic courses in Physics by myself, which may be the reason my GPAs are not in the top 5%. But now I believe that I have been quite familiar in the knowledges of Physics, both the courses and the researches. So I hope that my test scores and grades are acceptable to Priceton with financial supports.As to the research, I am very glad to learn the research background you provided in your letter. I am quite familiar with the works of X. Blase published in PRL and APL. I also know that J-C Charlier is a famous specialist in this field. So perhaps I could do theoretical research works in your group. Also, I am very glad to know that you have the needed main instruments for carbon nanotubes in your group, so that both theoretical and experimental works can be done.I am puzzled at the "MRS meeting in Boston" you mentionedin your letter. What is the full-name of MRS? Is it a meeting specialized in nano-systems? I do research works on carbon nanotubes almost totally by myself, and perhaps are not familiar with such fixed terms. Would you please explain the contents of this meeting? Thanks. And you mentioned that your latest publications will come out in next months in PRL. Would you please send me the page number of this paper in PRL, and if possible, the full text of this paper? The journal PRL reaches to China very late, usually several months to half a year after published, and I don't have the account to find the full-texts of PRL on-line.I am looking forward to receiving your warmhearted reply.Thanks.Yours sincerely###########（四）Dear #######:Thank you again for your email. From the sounds of your scores and grades, you should have no problem entering Princeton. I am quite familiar with the program that you are in and have had several close friends that have been there at Beijing University. In fact, ####### was in graduate school with me and she was in the accelerated program. She has done extremely well in the U.S. and after graduation went on to do some first rate science at a university in California. Since 2001 is your target date, I can begin to arrange funding for a research assistantship for you. These are nicer than teaching assistantships because they allow you to focus only on your research. Naturally, you will not be obligated to accept should you find other options. However, I believe that you will be most welcomed here in my group.You had asked about some of my publications, if you send me your address I can send preprints. They may take some time to get to China. You can find some of our work listed on our web site under my cv. This is an incomplete list but the PRL of last year is there and the latest hasnt yet been released from the publishers.We have been doing some interesting things lately with topological defects on tube manifolds that you might like. We have recently imaged nanotubes which exhibit a change in chirality along the tube! Tunneling spectra show that this produces subtle changes in the LDOS as predicted in some of X. Blase's work. We have also begun optical studies on individual nanotubes using near-field scanning optical microscopy and spectroscopy. We are particualry interested in how the surface plasmon resonances (governed by tube topology) effects the third order nonlinear susceptability in these objects.Thank you again for your interest in our group. May I suggest that we keep in contact over the year. Let me know your progress and I will try to help with the application procedures should you decide to join us.SincerelyXXX（五）Dear Professor，My name is XXX, a PH.D candidate of XXX(university).I got your email address from the web and I am very interested in your research field. This message is to ask for the information of the PH.D and Postdoctoral program of your group.I have published 5 papers(see the attachment for my paper list written by Latex convention) since 1996, including differenttopics: Controlling Chaos, Dynamics, and Bio-membranes. The two papers of Bio-membrane were both finished in this year, one is to discuss the pattern formation of periodic square texture(egg-arton) in Lipid bilayers; the other is to discuss the Complex vesicle under the framework of the spontaneous rvature energy model.I am also interested in Polymer dynamics, DNA structure transition and have read many related papers. In fact, I have started to do some calculations in this field.Would you please to consider my application to join your group, especially as a graduate student under your guidance? The reasons I want to obtain my PH.D there are: 1). A PH.D obtained in such a famous University will be helpful to get a good research position when I come back;2). I want to be educated at a high level since my dream is to be a successful researcher in the future.Looking to your message.Best regards!Yours, XXXRe: You paper XXX（六）Dear Prof.I've read your paper XXX in magazine XXX, some part of it seems hard for me to fully understand:1. XXX theroem ... I have some questions: 1.XXX2.XXXAlso I'm very interesting in your research, and want to join your crew and ...If you're interested, I'll send my CV in next mail.Regards.Re: I'm a student who is interested in Research of XXXDear Prof.篇二：【美梦网】套磁之博士学位PHD申请解析套磁之博士学位PHD申请解析在整个申请美国留学套磁的过程中，不仅要了解美国留学套磁的流程，还注意美国套磁时的事项，不要心急，稳中求胜，以便顺利完成美国套磁。

ApplPhysLett

Photoluminescence from colloids containing aluminum hydroxide nanocrystals with uniform sizeT.H.Li,1,2L.Z.Liu,1X.L.Wu,1,a͒J.C.Shen,1F.Gao,1and Paul K.Chu3,a͒1Department of Physics,Nanjing National Laboratory of Microstructures,Nanjing University,Nanjing210093,People’s Republic of China2College of Electronic Engineering,Guangxi Normal University,Guilin541004,People’s Republic of China 3Department of Physics and Materials Science,City University of Hong Kong,Tat Chee Avenue,Kowloon,Hong Kong͑Received28June2010;accepted31August2010;published online20September2010͒Aluminum hydroxide nanocrystals consisting of an amorphous shell and crystalline core are fabricated by pulsed laser ablation of an aluminum target in water.The colloid consisting of nanocrystals with a uniform size exhibits a size-independent photoluminescence͑PL͒band at ϳ383nm.According to the PL excitation spectra and time-resolved PL decay analysis,this PL band originates from oxygen vacancies in the amorphous shell and Förster energy transfer occurs between the oxygen vacancy levels in the crystalline core and amorphous shell.These phenomena are found to alter the PL excitation spectra.©2010American Institute of Physics.͓doi:10.1063/1.3491161͔Aluminum hydroxide͓Al͑OH͒3,AHO͔is an intermedi-ate compound in the Bayer process which is commercially adopted by the aluminum and alumina industry.1AHO has many applications such as micro-organism and phosphate absorbents for waste treatment,alumina catalyst precursors, antacid drugs,andﬂame retardant materials.2–6Various poly-morphs of AHO such as bayerite,gibbsite,nordstrandite,and doyleite have been produced7–10and these polymorphs have a double layer structure consisting of OH ions held together by hydrogen bonding.11Since defects or color centers in alu-mina such as oxygen vacancies͑OVs͒can substantially af-fect the electrical,optical,and thermal properties of the materials,12many other physical properties of AHO may also be impacted by defects or impurity centers.Hence,it is im-portant to investigate these phenomena in order to widen its applications,particularly those pertaining to optoelectronic devices.Investigations on AHO have hitherto mainly fo-cused on the formation of the different nanostructures and conversion to alumina nanocrystals͑NCs͒͑Refs.7,9,and 10͒and there have been relatively few studies on the photo-luminescence͑PL͒properties of boehmite whiskers,nano-rods,and nanoﬂake.8,13In particular,speciﬁc analyses of the PL property and mechanism of AHO NCs with a composite nanostructure have not been performed systematically.In this work,we fabricate a homogeneous AHO NC colloid by pulsed laser ablation of a crystalline Al sheet in water and study the PL characteristics.Our results suggest that ultra-violet laser irradiation with different energies alters the amounts of different OVs consequently modifying the PL spectra.The experimental setup for the AHO colloid solution fabrication is shown in Fig.1͑a͒.A high purity Al target ͑99.99wt%͒with dimensions of30ϫ30ϫ1mm3was electrochemically polished to remove the surface oxide and then placed into a500ml cylindrical glass containing8mm deep deionized water.The glass was placed on a movableﬂat so that the position could be adjusted.A248mm laser beam with a10Hz repetition rate irradiated the Al target via a90°prism and a lens was also added to enhance the laser inten-sity.After10min of laser irradiation at a power of200͑sample A͒,300͑sample B͒,or451mJ/pulse͑sample C͒, milky colloidal solutions containing AHO NCs were pro-duced.In order to obtain AHO NCs with a uniform size,thesolution was centrifuged at8000rpm for10min and theupper colloidal solution was used to measure optical proper-ties.The materials were characterized by transmission elec-tron microscopy͑TEM͒,Raman spectroscopy,PL,PL exci-tation͑PLE͒,and x-ray diffraction͑XRD͒and the analyticaldetails can be found elsewhere.14,15All the measurementswere conducted at room temperature.The typical TEM images of the synthesized AHO NCs insamples B and C are depicted in Figs.1͑b͒and1͑c͒,respec-a͒Authors to whom correspondence should be addressed.Electronic ad-dresses:hkxlwu@ andpaul.chu@.hk.FIG.1.͑Color online͒͑a͒Schematic of the experimental setup used in pulsed laser ablation in water.͓͑b͒and͑c͔͒TEM images of samples B and C.The insets show the SAED images of individual NCs.͑d͒HR-TEM image of sample B.APPLIED PHYSICS LETTERS97,121901͑2010͒0003-6951/2010/97͑12͒/121901/3/$30.00©2010American Institute of Physics97,121901-1tively.The nanoparticles have an almost spherical shape due to minimization of surface free energy.16The NCs in sample B has a uniform diameter of about 6.8nm and those in sample C are approximately 7.4nm.The NCs in sample A are slightly smaller.To disclose the crystalline structure of these NCs,a representative high-resolution TEM image ac-quired from sample B is depicted in Fig.1͑d ͒.The lattice fringe spacings of 0.198and 0.185nm are in good agreement with those of the ͑4¯11͒and ͑105͒planes of AHO ͑bayerite ͒.To further determine their structure and components,the anti-Stokes Raman spectrum and XRD pattern of sample B are presented in Figs.2͑a ͒and 2͑b ͒,respectively.There are three strong Raman peaks at Ϫ295.8,Ϫ321.8,and −356.7cm −1and their positions coincide with those of AHO bayerite.17The XRD spectrum shows many diffraction peaks that can be indexed to the ͑4¯11͒and ͑105͒planes of bayerite.In addition,there is a broad background in these spectra indicating the presence of an amorphous component.This amorphous component is more substantial in the NCs fabri-cated with a higher laser power,as illustrated by the two selected-area electron diffraction ͑SAED ͒images obtained from individual NCs in samples B and C in the insets of Figs.1͑b ͒and 1͑c ͒,respectively.Obviously,the NCs in sample B have better crystallinity than those in sample C.This is understandable because a high laser power produces NCs with a larger size.Some of them do not have enough time to crystallize and thus the NCs contain a larger amor-phous component.The results indicate that they are crystal-line AHO NCs with different orientations and the surfaces of these NCs contain an amorphous component.Since the SAED image is taken from an NC,the inference is reason-able.Figure 3͑a ͒shows the PL spectra acquired from the three AHO NC colloids ͑A,B,and C ͒excited by the 250nm line of a Xe lamp.All the spectra show the same PL band cen-tered at ϳ383nm with a linewidth of about 90nm.Its in-tensity decreases with increasing irradiation power during sample fabrication.The violet PL is obviously not associatedwith quantum conﬁnement because there is no NC size de-pendence.To investigate the PL origin,the PLE spectra are presented in Fig.3͑b ͒.Interestingly,the PLE spectra are quite different.For example,sample A shows only a PLE peak at 299nm with a linewidth of about 45nm.The 299nm peak becomes a small shoulder in sample C and another strong PLE peak appears at 254nm.The 254nm peak has a linewidth of about 22nm which is narrower than that of the 299nm peak.The PLE spectrum acquired from sample B exhibits two salient peaks at 254and 299nm.The PLE re-sults clearly show that the 383nm PL peak does not originate from the transition of electrons in some defects.According to the linewidths of the PL and PLE peaks,it can be inferred that at least three luminescent centers are involved in the excitation and recombination processes.Since not much is known about the luminescent proper-ties of AHO nanomaterials,the assignment of the PL peak is difﬁcult.Yu et al.8have investigated the PL characteristics of crystalline AHO whiskers and proposed that the 383nm PL band is associated with both F +͑an OV occupied by one electron ͒and F centers ͑an OV occupied by two electrons ͒with energy levels of 4.1͑302nm ͒and 4.8eV ͑258nm ͒.8,18The F centers have higher energy than F +.19Since the AHO NCs in this study also inevitably contain a large number of OVs,the above assignment appears to be reasonable.How-ever,the luminescent center should give rise to a 3.2eV PL band.By taking into account the amorphous component in our samples,the observed 3.2eV peak can be attributed to the OVs in the amorphous AHO that has a lower energy than those in the crystalline structure.12The large linewidth of the PL peak supports the assignment.Generally,the amorphous component is located on the surface of the NCs and conse-quently,the ratio of the crystalline to amorphous structure determines the PL intensity.This implies that the PL inten-sity from sample C should be the greatest because it has the biggest NC size and amorphous component.However,the PL spectra do not indicate this trend and in fact,sample A shows the largest intensity.This indicates that there must be other reasons for the PL intensity.To further explore this phenomenon,we refer to the PLE spectra.The 383nm PL peak from sample C mainly comes from the 253nm PLE band ͑F center ͒and the 382nmPLFIG.2.͑Color online ͒͑a ͒Anti-Stokes Raman and ͑b ͒XRD spectra of sample B.͑c ͒Time-resolved PL spectrum acquired from sample B by moni-toring the 383nmemission.FIG.3.͑Color online ͒͑a ͒PL spectra of samples A,B,and C,taken under excitation with the 250nm line of a Xe lamp.͑b ͒PLE spectra of samples A,B,and C by monitoring the 383nm emission.band from sample A is associated with the 299nm PLE band ͑F +center ͒.Since the three levels of 253,299,and 382nm are from different defect states,a Förster energy transfer pro-cess is believed to take place in the samples,20,21as illus-trated in Fig.4which depicts the energy band diagram of a AHO NC with an amorphous shell.Accordingly,we can de-scribe the electron excitation and recombination processes as follows.Photoexcitation of electrons occurs in the OVs ͑F +and F centers ͒in the crystalline AHO core,whereas ra-diative recombination takes place in the OV level with an energy of 3.2eV ͑383nm ͒in the amorphous shell via reso-nance energy transfer.This model can explain the following experimental results very well.The 253and 299nm PLE bands related with the F and F +centers in samples C and A,respectively,have large intensities.Under large power laser irradiation,a large number of self-trapped excitons are produced.22The OVs can easily trap two electrons and thus sample C has a high density of F centers.21When the irradi-ating laser power is low,the OVs can readily trap one elec-tron and thus the density of the F +centers is higher.For an intermediate irradiation power of 300͑300mJ/pulse ͒,the F and F +centers exist simultaneously and their densities de-pend on the irradiation laser power.To provide evidence of resonant energy transfer,the time-resolved PL decay curves are acquired on an Edinburgh FLS920ﬂuorescence spectrophotometer equipped with a 450W Xe lamp as the excitation source.The typical PL decay curves at 383nm obtained from sample C is presented in Fig.2͑c ͒.By subtracting the spectrometer background sig-nal and ﬁtting the experimental decay transients with two stretched exponential functions,the lifetimes of the 383nm band are derived to be 0.24and 3.83ns.The decay time values determined from samples A and B are similar and have the same order of magnitude as that observed from sample C.The one order of magnitude difference in the two lifetimes indicates that the electron transition occurs between two different defect levels and it is consistent with our model.In our samples,the OVs in the crystalline core and amorphous shell serve as the donors and acceptors,respec-tively.Overlapping of the acceptor and donor electronic lev-els,the small separation,and relatively long lifetime of the electronic excitation of the donor play important roles in the efﬁcient energy transfer.It is well known that the smaller the size of the NCs,the higher is the efﬁciency to produce pho-toexcited carriers.23Samples A and C have the smallest and largest NC sizes,and hence,their PL intensities are the larg-est and lowest,respectively.In summary,we have produced a colloid containing AHO NCs with a uniform size by pulsed laser ablation in water and observe that the emission band at ϳ383nm is independent of NC size.The PL band can be attributed to OVs in the amorphous shell on the NCs.The PLE spectral result and time-resolved PL decay analysis disclose Förster energy transfer between the OV levels in the amorphous shell layer and crystalline core of the AHO NCs as a result of different laser irradiation power.The results provide insight into the inﬂuence of laser irradiation on the preparation of AHO nanomaterials.This work was jointly supported by Grants ͑Grant Nos.60976063,60721063,and BK2008020͒from the National and Jiangsu Natural Science Foundations.Partial support was also from Hong Kong Research Grants Council ͑RGC ͒General Research Fund ͑GRF ͒under Grant No.CityU 112608.1K.J.Bayer,Patent No.DE-PS 43977͑01August,1887͒.2J.F.Chen,L.Shao,F.Guo,and X.M.Wang,Chem.Eng.Sci.58,569͑2003͒.3H.Liu,J.S.Tse,J.Hu,Z.Liu,L.Wang,J.Chen,D.J.Weidner,Y .Meng,D.Hausemann,and H.K.Mao,J.Phys.Chem.B 109,8857͑2005͒.4R.Demichelis,Y .Noel,B.Civalleri,C.Roetti,M.Ferrero,and R.Dovesi,J.Phys.Chem.B 111,9337͑2007͒.5S.Goldberg,J.A.Davis,and J.D.Hem,in The Environmental Chemistry of Aluminum ,edited by G.Sposito ͑Lewis,New York,1996͒.6J.Chen,S.Truesdail,F.Lu,G.Zhang,C.Belvin,B.Koopman,S.Farrah,and D.Shah,Water Res.32,2171͑1998͒.7Y .I.Seo,Y .J.Lee,D.G.Kim,K.H.Lee,and Y .D.Kim,Appl.Surf.Sci.256,4434͑2010͒.8Z.Q.Yu,C.X.Wang,X.T.Gu,and C.Li,J.Lumin.106,153͑2004͒.9Y .P.Lee,Y .H.Liu,and C.S.Yeh,Phys.Chem.Chem.Phys.1,4681͑1999͒.10Y .Liu,D.Ma,R.A.Blackley,W.Z.Zhou,X.W.Han,and X.H.Bao,J.Phys.Chem.C 112,4124͑2008͒.11R.Demichelis,B.Civalleri,Y .Noel,A.Meyer,and R.Dovesi,Chem.Phys.Lett.465,220͑2008͒.12S.F.Liu,L.G.Zhang,Y .Fan,and J.S.Luo,Appl.Phys.Lett.89,051911͑2006͒.13X.Y .Chen,Z.J.Zhang,X.L.Li,and S.W.Lee,Solid State Commun.145,368͑2008͒.14X.L.Wu,S.J.Xiong,J.Zhu,J.Wang,J.C.Shen,and Paul K.Chu,Nano Lett.9,4053͑2009͒.15L.Z.Liu,X.L.Wu,F.Gao,Y .M.Yang,T.H.Li,and Paul K.Chu,Opt.Lett.35,1022͑2010͒.16L.Z.Liu,X.L.Wu,T.H.Li,and Paul K.Chu,Appl.Phys.Lett.96,173111͑2010͒.17H.D.Ruan,R.L.Frost,and J.T.Kloprogge,J.Raman Spectrosc.32,745͑2001͒.18K.H.Lee and J.H.Crawford,Phys.Rev.B 15,4065͑1977͒.19H.T.Chen,X.L.Wu,S.J.Xiong,W.C.Zhang,and J.Zhu,Appl.Phys.A:Mater.Sci.Process.97,365͑2009͒.20B.D.Evans,J.Nucl.Mater.219,202͑1995͒.21T.Förster,in Modern Quantum Chemistry,Part 2,edited by O.Sinanoglu ͑McGraw-Hill,New York,1965͒.22X.L.Wu,X.L.Guo,Z.G.Liu,G.G.Siu,S.S.Jiang,and D.Feng,Appl.Phys.Lett.69,3963͑1996͒.23X.L.Wu,G.G.Siu,S.Tong,X.N.Liu,X.M.Bao,S.S.Jiang,and D.Feng,Phys.Rev.B 57,9945͑1998͒.FIG.4.͑Color online ͒Schematic of Förster energy transfer between the OV levels in amorphous shell and crystalline core in an AHO NC.。

物理社团英语作文

物理社团英语作文As the universe of academia expands, the Physics Club at our school stands as a beacon of intellectual curiosity and scientific exploration. This essay delves into the heart of the club, showcasing its significance in fostering a love for physics among students and its role in the educational tapestry of our institution.The club, a vibrant assembly of physics enthusiasts, meets regularly to discuss the intricacies of the physical world. It is a platform where the laws of motion, the mysteries of quantum mechanics, and the vastness of astrophysics are not just subjects in a textbook but living, breathing entities that inspire awe and wonder.Our meetings are a blend of structured learning and free-flowing conversation. Led by our dedicated faculty advisor, Mr. Thompson, we embark on a journey through the cosmos, where each session is a step into the unknown, a quest for understanding the universe's most profound questions. Mr. Thompson, with his passion for physics, ignites a spark in us that fuels our desire to learn more.The club is not just about theoretical knowledge; it is a practical haven for experimentation. We have access to state-of-the-art equipment that allows us to conduct experiments that bring the abstract concepts of physics to life. From building simple circuits to observing the behavior of lightthrough a prism, our hands-on experiences solidify our grasp on the subject.One of the highlights of the Physics Club is the annual science fair. It is an event where members showcase their projects, ranging from model rockets to solar-powered devices. This year, I worked on a project that demonstrated the principles of energy conservation using a Rube Goldberg machine. It was a challenging yet rewarding experience thatnot only honed my problem-solving skills but also allowed meto present complex ideas in a fun and engaging way.Beyond the classroom, the club also organizes field trips to places like the local planetarium and the physics departmentof a nearby university. These trips are eye-opening andprovide us with a broader perspective on how physics isapplied in the real world.Being a part of the Physics Club has been an enriching experience. It has not only deepened my understanding of physics but also taught me the value of collaboration and the joy of discovery. The club is a testament to the fact that learning can be a communal activity, where each member contributes to a collective pool of knowledge and enthusiasm.In conclusion, the Physics Club is a cornerstone of ourschool's commitment to academic excellence and scientific inquiry. It is a place where the boundaries of knowledge are pushed, where the future scientists and engineers of tomorrow are nurtured, and where the spirit of inquiry is as vibrantas the subject matter itself.。

单电子辐射跃迁选择定则的讨论

单电子辐射跃迁选择定则的讨论（理学院物理系物理学）摘要原子辐射跃迁选择定则是原子物理学中的一个重要原则。

本文主要采用两种方法对单电子辐射跃迁选择定则进行讨论。

第一种方法，利用量子方法讨论；第二种方法，利用半经典方法讨论；两种方法分别对电子的轨道和自旋有无耦合的情况下进行了推导。

用两种不同的方法，得到了一致的结果。

关键词：电偶极辐射；跃迁几率；角动量守恒；量子数；选择定则Discussion of Single Elect ron’s transition Selection Rule(Department of Physics, College of science, Physics )AbstractSelection rule of atom transition is one of the important principles in the atom physics. This paper adopts two methods to discuss the selection rule of the single electron transition.In the first method, quantum method is used to analyze the problem.In the second method, semiclassical method is used to discuss the thesis. Two cases that the electric orbit and spin have coupling and no coupling are respectively discussed in both methods. By two different methods, the same result is conclued.Keywords：Electric dipole radiation；Transition probability；Conservation of angular momentum；Quantum number；Selection rule目录1 引言 (1)2 量子方法讨论选择定则 (1)2.1 电子的轨道和自旋无耦合的情况 (1)2.2 电子的轨道和自旋有耦合的情况 (3)3 半经典方法讨论选择定则 (7)3.1 角动量的矢量合成法则 (7)3.2 电子组态变动定则 (7)3.3 L S-无耦合的跃迁选择定则 (9)3.4 L S-有耦合的跃迁选择定则 (10)4 结论 (11)参考文献 (12)致谢 (13)1引言微观粒子（分子、原子、原子核、基本粒子等）的运动规律，是本世纪二十年代在总结大量实验事实和旧量子论的基础上建立起来的。

2001年全国硕士研究生入学统一考试英语附答案

绝密★启用前2001年全国硕士研究生入学统一考试英语（科目代码：201）考生注意事项1. 答题前，考生须在试题册指定位置上填写考生姓名和考生编号；在答题卡指定位置上填写报考单位、考生姓名和考生编号，并涂写考生编号信息点。

2. 考生须把试题册上的“试卷条形码”粘贴条取下，粘贴在答题卡的“试卷条形码粘贴位置”框中。

不按规定粘贴条形码而影响评卷结果的，责任由考生自负。

3. 选择题的答案必须涂写在答题卡相应题号的选项上，非选择题的答案必须书写在答题卡指定位置的边框区域内。

超出答题区域书写的答案无效；在草稿纸、试题册上答题无效。

4. 填（书）写部分必须使用黑色字迹签字笔或者钢笔书写，字迹工整、笔迹清楚；涂写部分必须使用2B铅笔填涂。

5. 考试结束，将答题卡和试题册按规定交回。

Section I Structure and Vocabulary（略）Section II Use of EnglishDirections:For each numbered blank in the following passage, there are four choices marked [A], [B], [C] and [D]. Choose the best one and mark your answer on ANSWER SHEET 1 by blackening the corresponding letter in the brackets with a pencil. (10 points)The government is to ban payments to witnesses by newspapers seeking to buy up people involved in prominent cases 31 the trial of Rosemary West.In a significant 32 of legal controls over the press, Lord Irvine, the Lord Chancellor, will introduce a 33 bill that will propose making payments to witnesses 34 and will strictly control the amount of 35 that can be given to a case 36 a trial begins.In a letter to Gerald Kaufman, chairman of the House of Commons Media Select Committee, Lord Irvine said he 37 with a committee report this year which said that self regulation did not 38 sufficient control.39 of the letter came two days after Lord Irvine caused a 40 of media protest when he said the 41 of privacy controls contained in European legislation would be left to judges 42 to Parliament.The Lord Chancellor said introduction of the Human Rights Bill, which 43 the European Convention on Human Rights legally 44 in Britain, laid down that everybody was 45 to privacy and that public figures could go to court to protect themselves and their families.“Press freedoms will be in safe hands 46 our British judges,” he said.Witness payments became an 47 after West was sentenced to 10 life sentences in 1995. Up to 19 witnesses were 48 to have received payments for telling their stories to newspapers. Concerns were raised 49 witnesses might be encouraged to exaggerate their2001年英语试题第2页（共14页）stories in court to 50 guilty verdicts.31. [A] as to [B] for instance [C] in particular [D] such as32. [A] tightening [B] intensifying [C] focusing [D] fastening33. [A] sketch [B] rough [C] preliminary [D] draft34. [A] illogical [B] illegal [C] improbable [D] improper35. [A] publicity [B] penalty [C] popularity [D] peculiarity36. [A] since [B] if [C] before [D] as37. [A] sided [B] shared [C] complied [D] agreed38. [A] present [B] offer [C] manifest [D] indicate39. [A] Release [B] Publication [C] Printing [D] Exposure40. [A] storm [B] rage [C] flare [D] flash41. [A] translation [B] interpretation [C] exhibition [D] demonstration42. [A] better than [B] other than [C] rather than [D] sooner than43. [A] changes [B] makes [C] sets [D] turns44. [A] binding [B] convincing [C] restraining [D] sustaining45. [A] authorized [B] credited [C] entitled [D] qualified46. [A] with [B] to [C] from [D] by47. [A] impact [B] incident [C] inference [D] issue48. [A] stated [B] remarked [C] said [D] told49. [A] what [B] when [C] which [D] that50. [A] assure [B] confide [C] ensure [D] guaranteeSection III Reading ComprehensionDirections:Each of the passages below is followed by some questions. For each question there are four answers marked [A], [B], [C] and [D]. Read the passages carefully and choose the best answer to each of the questions. Then mark your answer on ANSWER SHEET 1 by blackening the corresponding letter in the brackets with a pencil. (40 points)2001年英语试题第3页（共14页）Text 1Specialization can be seen as a response to the problem of an increasing accumulation of scientific knowledge. By splitting up the subject matter into smaller units, one man could continue to handle the information and use it as the basis for further research. But specialization was only one of a series of related developments in science affecting the process of communication. Another was the growing professionalisation of scientific activity.No clear-cut distinction can be drawn between professionals and amateurs in science: exceptions can be found to any rule. Nevertheless, the word “amateur” does carry a connotation that the person concerned is not fully integrated into the scientific community and, in particular, may not fully share its values. The growth of specialization in the nineteenth century, with its consequent requirement of a longer, more complex training, implied greater problems for amateur participation in science. The trend was naturally most obvious in those areas of science based especially on a mathematical or laboratory training, and can be illustrated in terms of the development of geology in the United Kingdom.A comparison of British geological publications over the last century and a half reveals not simply an increasing emphasis on the primacy of research, but also a changing definition of what constitutes an acceptable research paper. Thus, in the nineteenth century, local geological studies represented worthwhile research in their own right; but, in the twentieth century, local studies have increasingly become acceptable to professionals only if they incorporate, and reflect on, the wider geological picture. Amateurs, on the other hand, have continued to pursue local studies in the old way. The overall result has been to make entrance to professional geological journals harder for amateurs, a result that has been reinforced by the widespread introduction of refereeing, first by national journals in the nineteenth century and then by several local geological journals in the twentieth century. As a logical consequence of this development, separate journals have now appeared aimed mainly towards either professional or amateur readership. A rather similar process of differentiation has led to professional geologists coming together nationally within one or two specific societies, whereas the amateurs have tended either to remain in local societies or to come together nationally in a different way.2001年英语试题第4页（共14页）Although the process of professionalisation and specialization was already well under way in British geology during the nineteenth century, its full consequences were thus delayed until the twentieth century. In science generally, however, the nineteenth century must be reckoned as the crucial period for this change in the structure of science.51. The growth of specialization in the 19th century might be more clearly seen in sciencessuch as ________.[A] sociology and chemistry [B] physics and psychology[C] sociology and psychology [D] physics and chemistry52. We can infer from the passage that ________.[A] there is little distinction between specialization and professionalisation[B] amateurs can compete with professionals in some areas of science[C] professionals tend to welcome amateurs into the scientific community[D] amateurs have national academic societies but no local ones53. The author writes of the development of geology to demonstrate ________.[A] the process of specialization and professionalisation[B] the hardship of amateurs in scientific study[C] the change of policies in scientific publications[D] the discrimination of professionals against amateurs54. The direct reason for specialization is ________.[A] the development in communication[B] the growth of professionalisation[C] the expansion of scientific knowledge[D] the splitting up of academic societiesText 2A great deal of attention is being paid today to the so-called digital divide -- the division of the world into the info (information) rich and the info poor. And that divide does exist today. My wife and I lectured about this looming danger twenty years ago. What was less visible then,2001年英语试题第5页（共14页）however, were the new, positive forces that work against the digital divide. There are reasons to be optimistic.There are technological reasons to hope the digital divide will narrow. As the Internet becomes more and more commercialized, it is in the interest of business to universalize access — after all, the more people online, the more potential customers there are. More and more governments, afraid their countries will be left behind, want to spread Internet access. Within the next decade or two, one to two billion people on the planet will be netted together. As a result, I now believe the digital divide will narrow rather than widen in the years ahead. And that is very good news because the Internet may well be the most powerful tool for combating world poverty that we’ve ever had.Of course, the use of the Internet isn’t the only way to defeat poverty. And the Internet is not the only tool we have. But it has enormous potential.To take advantage of this tool, some impoverished countries will have to get over their outdated anti-colonial prejudices with respect to foreign investment. Countries that still think foreign investment is an invasion of their sovereignty might well study the history of infrastructure (the basic structural foundations of a society) in the United States. When the United States built its industrial infrastructure, it didn’t have the capital to do so. And that is why America’s Second Wave infrastructure -- including roads, harbors, highways, ports and so on — were built with foreign investment. The English, the Germans, the Dutch and the French were investing in Britain’s former colony. They financed them. Immigrant Americans built them. Guess who owns them now? The Americans. I believe the same thing would be true in places like Brazil or anywhere else for that matter. The more foreign capital you have helping you build your Third Wave infrastructure, which today is an electronic infrastructure, the better off you’re going to be. That doesn’t mean lying down and becoming fooled, or letting foreign corporations run uncontrolled. But it does mean recognizing how important they can be in building the energy and telecom infrastructures needed to take full advantage of the Internet.55. Digital divide is something ________.[A] getting worse because of the Internet2001年英语试题第6页（共14页）[B] the rich countries are responsible for[C] the world must guard against[D] considered positive today56. Governments attach importance to the Internet because it ________.[A] offers economic potentials[B] can bring foreign funds[C] can soon wipe out world poverty[D] connects people all over the world57. The writer mentioned the case of the United States to justify the policy of ________.[A] providing financial support overseas[B] preventing foreign capital’s control[C] building industrial infrastructure[D] accepting foreign investment58. It seems that now a country’s economy depends much on ________.[A] how well-developed it is electronically[B] whether it is prejudiced against immigrants[C] whether it adopts America’s industrial pattern[D] how much control it has over foreign corporationsText 3Why do so many Americans distrust what they read in their newspapers? The American Society of Newspaper Editors is trying to answer this painful question. The organization is deep into a long self-analysis known as the journalism credibility project.Sad to say, this project has turned out to be mostly low-level findings about factual errors and spelling and grammar mistakes, combined with lots of head-scratching puzzlement about what in the world those readers really want.But the sources of distrust go way deeper. Most journalists learn to see the world through a set of standard templates (patterns) into which they plug each day’s events. In other words, there is a conventional story line in the newsroom culture that provides a backbone and a ready-made2001年英语试题第7页（共14页）narrative structure for otherwise confusing news.There exists a social and cultural disconnect between journalists and their readers, which helps explain why the “standard templates” of the newsroom seem alien to many readers. In a recent survey, questionnaires were sent to reporters in five middle-size cities around the country, plus one large metropolitan area. Then residents in these communities were phoned at random and asked the same questions.Replies show that compared with other Americans, journalists are more likely to live in upscale neighborhoods, have maids, own Mercedeses, and trade stocks, and they’re less likely to go to church, do volunteer work, or put down roots in a community.Reporters tend to be part of a broadly defined social and cultural elite, so their work tends to reflect the conventional values of this elite. The astonishing distrust of the news media isn’t rooted in inaccuracy or poor reportorial skills but in the daily clash of world views between reporters and their readers.This is an explosive situation for any industry, particularly a declining one. Here is a troubled business that keeps hiring employees whose attitudes vastly annoy the customers. Then it sponsors lots of symposiums and a credibility project dedicated to wondering why customers are annoyed and fleeing in large numbers. But it never seems to get around to noticing the cultural and class biases that so many former buyers are complaining about. If it did, it would open up its diversity program, now focused narrowly on race and gender, and look for reporters who differ broadly by outlook, values, education, and class.59. What is the passage mainly about?[A] needs of the readers all over the world[B] causes of the public disappointment about newspapers[C] origins of the declining newspaper industry[D] aims of a journalism credibility project60. The results of the journalism credibility project turned out to be ________.[A] quite trustworthy [B] somewhat contradictory[C] very illuminating [D] rather superficial2001年英语试题第8页（共14页）61. The basic problem of journalists as pointed out by the writer lies in their ________.[A] working attitude [B] conventional lifestyle[D] educational background[C] world outlook62. Despite its efforts, the newspaper industry still cannot satisfy the readers owing to its________.[A] failure to realize its real problem[B] tendency to hire annoying reporters[C] likeliness to do inaccurate reporting[D] prejudice in matters of race and genderText 4The world is going through the biggest wave of mergers and acquisitions ever witnessed. The process sweeps from hyperactive America to Europe and reaches the emerging countries with unsurpassed might. Many in these countries are looking at this process and worrying: “Won’t the wave of business concentration turn into an uncontrollable anti-competitive force?”There’s no question that the big are getting bigger and more powerful. Multinational corporations accounted for less than 20% of international trade in 1982. Today the figure is more than 25% and growing rapidly. International affiliates account for a fast-growing segment of production in economies that open up and welcome foreign investment. In Argentina, for instance, after the reforms of the early 1990s, multinationals went from 43% to almost 70% of the industrial production of the 200 largest firms. This phenomenon has created serious concerns over the role of smaller economic firms, of national businessmen and over the ultimate stability of the world economy.I believe that the most important forces behind the massive M&A wave are the same that underlie the globalization process: falling transportation and communication costs, lower trade and investment barriers and enlarged markets that require enlarged operations capable of meeting customer’s demands. All these are beneficial, not detrimental, to consumers. As productivity grows, the world’s wealth increases.2001年英语试题第9页（共14页）Examples of benefits or costs of the current concentration wave are scanty. Yet it is hard to imagine that the merger of a few oil firms today could re-create the same threats to competition that were feared nearly a century ago in the U.S., when the Standard Oil Trust was broken up. The mergers of telecom companies, such as WorldCom, hardly seem to bring higher prices for consumers or a reduction in the pace of technical progress. On the contrary, the price of communications is coming down fast. In cars, too, concentration is increasing — witness Daimler and Chrysler, Renault and Nissan — but it does not appear that consumers are being hurt.Yet the fact remains that the merger movement must be watched. A few weeks ago, Alan Greenspan warned against the megamergers in the banking industry. Who is going to supervise, regulate and operate as lender of last resort with the gigantic banks that are being created? Won’t multinationals shift production from one place to another when a nation gets too strict about infringements to fair competition? And should one country take upon itself the role of “defending competition” on issues that affect many other nations, as in the U.S. vs. Microsoft case?63. What is the typical trend of businesses today?[A] to take in more foreign funds [B] to invest more abroad[C] to combine and become bigger [D] to trade with more countries64. According to the author, one of the driving forces behind M&A wave is ________.[A] the greater customer demands [B] a surplus supply for the market[C] a growing productivity [D] the increase of the world’s wealth65. From Paragraph 4 we can infer that ________.[A] the increasing concentration is certain to hurt consumers[B] WorldCom serves as a good example of both benefits and costs[C] the costs of the globalization process are enormous[D] the Standard Oil Trust might have threatened competition66. Toward the new business wave, the writer’s attitude can be said to be ________.optimistic [B]objective[A]biasedpessimistic [D][C]2001年英语试题第10页（共14页）Text 5When I decided to quit my full time employment it never occurred to me that I might become a part of a new international trend. A lateral move that hurt my pride and blocked my professional progress prompted me to abandon my relatively high profile career although, in the manner of a disgraced government minister, I covered my exit by claiming “I wanted to spend more time with my family”.Curiously, some two-and-a-half years and two novels later, my experiment in what the Americans term “downshifting” has turned my tired excuse into an absolute reality. I have been transformed from a passionate advocate of the philosophy of “having it all,” preached by Linda Kelsey for the past seven years in the page of She magazine, into a woman who is happy to settle for a bit of everything.I have discovered, as perhaps Kelsey will after her much-publicized resignation from the editorship of She after a build-up of stress, that abandoning the doctrine of “juggling your life,” and making the alternative move into “downshifting” brings with it far greater rewards than financial success and social status. Nothing could persuade me to return to the kind of life Kelsey used to advocate and I once enjoyed: 12-hour working days, pressured deadlines, the fearful strain of office politics and the limitations of being a parent on “quality time”.In America, the move away from juggling to a simpler, less materialistic lifestyle is a well-established trend. Downshifting — also known in America as “voluntary simplicity” — has, ironically, even bred a new area of what might be termed anti-consumerism. There are a number of best-selling downshifting self-help books for people who want to simplify their lives; there are newsletters, such as The Tightwad Gazette, that give hundreds of thousands of Americans useful tips on anything from recycling their cling-film to making their own soap; there are even support groups for those who want to achieve the mid-’90s equivalent of dropping out.While in America the trend started as a reaction to the economic decline — after the mass redundancies caused by downsizing in the late ’80s — and is still linked to the politics of thrift, in Britain, at least among the middle-class downshifters of my acquaintance, we have different reasons for seeking to simplify our lives.2001年英语试题第11页（共14页）For the women of my generation who were urged to keep juggling through the ’80s, downshifting in the mid-’90s is not so much a search for the mythical good life — growing your own organic vegetables, and risking turning into one — as a personal recognition of your limitations.67. Which of the following is true according to Paragraph 1?[A] Full-time employment is a new international trend.[B] The writer was compelled by circumstances to leave her job.[C] “A lateral move” means stepping out of full-time employment.[D] The writer was only too eager to spend more time with her family.68. The writer’s experiment shows that downshifting ________.[A] enables her to realize her dream[B] helps her mold a new philosophy of life[C] prompts her to abandon her high social status[D] leads her to accept the doctrine of She magazine69. “Juggling one’s life” probably means living a life characterized by ________.[A] non-materialistic lifestyle[B] a bit of everything[C] extreme stress[D] anti-consumerism70. According to the passage, downshifting emerged in the U.S. as a result of ________.[A] the quick pace of modern life[B] man’s adventurous spirit[C] man’s search for mythical experiences[D] the economic situation2001年英语试题第12页（共14页）Section IV English-Chinese TranslationDirections:Read the following passage carefully and then translate the underlined sentences into Chinese. Your translation must be written neatly on ANSWER SHEET 2. (15 points) In less than 30 years’ time the Star Trek holodeck will be a reality. Direct links between the brain’s nervous system and a computer will also create full sensory virtual environments, allowing virtual vacations like those in the film Total Recall.71) There will be television chat shows hosted by robots, and cars with pollution monitors that will disable them when they offend. 72) Children will play with dolls equipped with personality chips, computers with in-built personalities will be regarded as workmates rather than tools, relaxation will be in front of smell-television, and digital age will have arrived.According to BT’s futurologist, Ian Pearson, these are among the developments scheduled for the first few decades of the new millennium (a period of 1,000 years), when supercomputers will dramatically accelerate progress in all areas of life.73) Pearson has pieced together the work of hundreds of researchers around the world to produce a unique millennium technology calendar that gives the latest dates when we can expect hundreds of key breakthroughs and discoveries to take place. Some of the biggest developments will be in medicine, including an extended life expectancy and dozens of artificial organs coming into use between now and 2040.Pearson also predicts a breakthrough in computer-human links. “By linking directly to our nervous system, computers could pick up what we feel and, hopefully, simulate feeling too so that we can start to develop full sensory environments, rather like the holidays in Total Recall or the Star Trek holodeck,” he says. 74) But that, Pearson points out, is only the start of man-machine integration: “It will be the beginning of the long process of integration that will ultimately lead to a fully electronic human before the end of the next century.”2001年英语试题第13页（共14页）Through his research, Pearson is able to put dates to most of the breakthroughs that can be predicted. However, there are still no forecasts for when faster-than-light travel will be available, or when human cloning will be perfected, or when time travel will be possible. But he does expect social problems as a result of technological advances. A boom in neighborhood surveillance cameras will, for example, cause problems in 2010, while the arrival of synthetic lifelike robots will mean people may not be able to distinguish between their human friends and the droids. 75) And home appliances will also become so smart that controlling and operating them will result in the breakout of a new psychological disorder — kitchen rage.Section V Writing76.Directions:Among all the worthy feelings of mankind, love is probably the noblest, but everyone has his/her own understanding of it.There has been a discussion recently on the issue in a newspaper. Write an essay to the newspaper to1) show your understanding of the symbolic meaning of the picture below,2) give a specific example, and3) give your suggestion as to the best way to show love.You should write about 200 words on ANSWER SHEET 2. (20 points)2001年英语试题第14页（共14页）。