Parity-Violating Electron Scattering from 4He and the Strange Electric Form Factor of the N

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绝缘体与安德森绝缘体

Thouless’ picture
A very important step to modern theory
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Question: Is the presence of electronic band gap a necessary and/or sufficient condition for the formation of insulators?
Answer: It is neither necessary nor sufficient. Counter examples: Anderson insulators: insulating states without band gap (but with a mobility gap), so that it not necessary. Superconductors: there is a electronic band gap (superconducting gap) like insulators due to electron pairing, but current can flow without resistance. Since the systems support collective excitations beside single-electronic excitations. The super-current in the system is a collective excitations.

英文原文

Liang Guo Stephen L.Hodson Timothy S.FisherXianfan Xu1e-mail:xxu@ School of Mechanical Engineering and Birck Nanotechnology Center,Purdue University,West Lafayette,IN47907Heat Transfer AcrossMetal-Dielectric Interfaces During Ultrafast-Laser Heating Heat transfer across metal-dielectric interfaces involves transport of electrons and pho-nons accomplished either by coupling between phonons in metal and dielectric or by cou-pling between electrons in metal and phonons in dielectric.In this work,we investigate heat transfer across metal-dielectric interfaces during ultrafast-laser heating of thin metalﬁlms coated on dielectric substrates.By employing ultrafast-laser heating that cre-ates strong thermal nonequilibrium between electrons and phonons in metal,it is possible to isolate the effect of the direct electron–phonon coupling across the interface and thus facilitate its study.Transient thermo-reﬂectance measurements using femtosecond laser pulses are performed on Au–Si samples while the simulation results based on a two-temperature model are compared with the measured data.A contact resistance between electrons in Au and phonons in Si represents the coupling strength of the direct electron–phonon interactions at the interface.Our results reveal that this contact resist-ance can be sufﬁciently small to indicate strong direct coupling between electrons in metal and phonons in dielectric.[DOI:10.1115/1.4005255]Keywords:interface thermal resistance,ultrafast laser,thermo-reﬂectance,two-temper-ature model,electron–phonon coupling1IntroductionInterface heat transfer is one of the major concerns in the design of microscale and nanoscale devices.In metal,electrons,and pho-nons are both energy carriers while in dielectric phonons are the main energy carrier.Therefore,for metal-dielectric composite structures,heat can transfer across the interface by coupling between phonons in metal and dielectric or by coupling between electrons in metal and phonons in dielectric through electron-interface scattering.Phonon–phonon coupling has been simulated mainly by the acoustic mismatch model and the diffuse mismatch model[1].As for electron–phonon coupling,there are different viewpoints.Some studies have assumed that electron–phonon coupling across a metal-dielectric interface is negligible and heat transfer occurs as electron–phonon coupling within metal and then phonon–phonon coupling across the interface[2].Electron–phonon coupling between metal(Cr,Ti,Al,Ni,and Pt)and SiO2 has exhibited negligible apparent thermal resistance using a parallel-strip technique[3].On the other hand,comparison between simulations and transient thermal reﬂectance(TTR) measurements for Au-dielectric interfaces reveals that energy could be lost to the substrate by electron-interface scattering dur-ing ultrafast-laser heating,and this effect depends on electron temperature and substrate thermal properties[4–6].In this study,we employ TTR techniques to investigate inter-face heat transfer for thin goldﬁlms of varying thicknesses on sili-con substrates.(Here,we consider silicon as a dielectric since heat is carried by phonons in silicon.)Similar work has been reported [5].In our model,we consider two temperatures in metal and also the temperature in the dielectric substrate.This allows us to inves-tigate the effect of both the coupling between electrons in metal and phonons in the dielectric substrate,and the coupling between phonons in metal and phonons in the dielectric substrate,and allows us to isolate the effect of the electron–phonon coupling across the interface that can be determined from the TTR mea-surement.Experimentally,we employ pulse stretching to mini-mize the effect of nonequilibrium among the electrons.As a result,the experimental data can be well-explained using the com-putational model.The thermal resistance between electrons in Au and phonons in Si,which quantiﬁes the direct electron–phonon coupling strength,is calculated from the measured data.The results reveal that in the thermal nonequilibrium state,this electron–phonon coupling at the interface is strong enough to dominate the overall interface heat transfer.2TTR MeasurementAu–Si samples of varying Au thicknesses were prepared by thermal evaporation at a pressure of the order of10À7Torr.The thicknesses of the goldﬁlms are39,46,60,77,and250nm,meas-ured using an atomic force microscope.The pump-and-probe technique is used in a collinear scheme to measure the thermo-reﬂectance signal.The laser pulses are generated by a Spectra Physics Ti:Sapphire ampliﬁed femtosecond system with a central wavelength of800nm and a repetition rate of5kHz.The wave-length of the pump beam is then converted to400nm with a sec-ond harmonic crystal.The pump pulse has a pulse width(full width at half maximum-FWHM)of390fs measured by the sum-frequency cross-correlation method and is focused onto the sam-ple with a spot radius of20.3l m.The probe beam has a central wavelength of800nm and a pulse width of205fs measured by autocorrelation and is focused with a spot radius of16.9l m.This pump pulse width is intentionally stretched from the original pulse width of50fs to minimize the inﬂuence of thermal nonequili-brium among electrons since the electron thermalization time in Au can be of the order of100fs[7].This thermalization time is pump wavelength and pumpﬂuence dependent,and can be of the order of10fs if higher laserﬂuence is used[8,9].Our experiments did show the importance of pulse stretching.Figure1shows the TTR measurement results for the sample of thickness77nm with different pumpﬂuences before and after stretching the pulse.The plots show the normalized relative reﬂectance change(ÀD R/R)1Corresponding author.Contributed by the Heat Transfer Division of ASME for publication in the J OURNAL OF H EAT T RANSFER.Manuscript received May18,2011;ﬁnal manuscript received September30,2011;published online February13,2012.Assoc.Editor: Robert D.Tzou.with the delay time between the pump and the probe pulses to show the contrast in cooling rates.With a shorter pulse (Fig.1(a )),a steep initial drop is seen in the signal,which is attributed to the behavior of nonequilibrium among electrons.Since the TTM to be used for simulation assumes a well-deﬁned tempera-ture for electrons,i.e.,the electrons in gold have reached thermal equilibrium (not necessarily a uniform temperature),the model cannot predict the fast initial drop in the signals in Fig.1(a ).As will be shown later,the signals obtained by stretching the pulse can be predicted well using the TTM.3Two-Temperature Model for Thermal Reflectance MeasurementsUltrafast-laser heating induces thermal nonequilibrium between electrons and phonons in metal,which can be described by the TTM [10–13].We note that the heterogeneous interface consid-ered here involves three primary temperature variables (two in the metal and one in the dielectric).The “two-temperature”model is applied to the metal side.For investigating electron–phonon and phonon–phonon coupling at the interface,two thermal resistances are deﬁned:R es (its reciprocal)indicates the coupling strength between electrons in metal and phonons in dielectric,while R ps indicates the coupling strength between phonons in metal and phonons in dielectric.(Large thermal resistance corresponds to weak coupling.)The resulting governing equations,initial,and interface conditions areC e @T e @t ¼k e @2T e@x2ÀG ðT e ÀT p ÞþS (1a )C p @T p @t ¼k p @2T p @x 2þG ðT e ÀT p Þ(1b )C s @T s @t ¼k s @2T s@x(1c )T e ðt ¼0Þ¼T p ðt ¼0Þ¼T s ðt ¼0Þ¼T 0(2)Àk e@T e @xx ¼L ¼T e ÀT s R es x ¼L(3a )Àk p @T px ¼L ¼T p ÀT s ps x ¼L(3b )Àk s@T sx ¼L ¼T e ÀT s es x ¼L þT p ÀT s ps x ¼L(3c )The subscripts e ,p ,and s denote electrons in metal,phonons in metal,and phonons in the dielectric substrate,respectively.C is the volumetric heat capacity,k is the thermal conductivity,G is the electron–phonon coupling factor governing the rate of energy transfer from electrons to phonons in metal,and L is the thickness of the metal layer.At the front surface of the metal layer insula-tion boundary condition is used due to the much larger heat ﬂux caused by laser heating relative to the heat loss to air.At the rear surface of the substrate,since the thickness of the substrate used is large enough (1l m)so that there is no temperature rise during the time period of consideration,the insulation boundary condition is also applied.Thermal properties of phonons in both metal and dielectric are taken as temperature-independent due to the weak temperature dependence.The thermal conductivity of phonons in metal is much smaller than that of the electrons and is taken in this work as 0.001times the bulk thermal conductivity of gold (311W/(mK)).The volumetric heat capacity of the metal phonon is taken as that of the bulk gold.C e is taken as proportional to T e [14]with the proportion coefﬁcient being 70J/(m 3K 2)[15],and k e is calculated by the model and the data used in Ref.[13]which is valid from the room temperature to the Fermi temperature (6.39Â104K in Au,[14]).G can be obtained using the model derived in Ref.[16].In this work,the value of G at the room tem-perature is taken as 4.6Â1016W/(m 3K)[17],and its dependence on electron and phonon temperatures follows [16].The laser heat-ing source term S is represented by the model used in [13]asS ¼0:94ð1ÀR ÞJ t p ðd þd b Þ1Àexp ÀL d þd bexp Àx d þd b À2:77t t p2"#(4)which assumes all the absorbed laser energy is deposited in the metal layer.J is the ﬂuence of the pump laser,R is the surface re-ﬂectance to the pump,t p is the pulse width (FWHM),d is the opti-cal penetration depth,and d b is the electron ballistic length (around 100nm in Au,[18]).R es and R ps are treated as free pa-rameters for ﬁtting the experimental data.The wavelength of the probe laser in the experiment is centered at 800nm.For this wavelength,the incident photon energy is below the interband transition threshold in Au,which is around 2.47eV [18],and the Drude model can be used to relate the tem-peratures of electrons and phonons to the dielectric function and then the index of refraction,which is expressed as [19]e ¼e 1Àx 2px ðx þi x s Þ(5)x is the frequency of the probe laser and x p is the plasma fre-quency (1.37Â1016rad/s in Au evaluated using the data in Ref.[14]).x s is the electron collisional frequency,the inverse of the electron relaxation time.The temperature dependence ofelectricalFig.1TTR measurement results for the Au–Si sample of Authickness 77nm with different ﬂuences.(a )Results before pulse stretching;(b )results after pulse stretching.resistivity indicates that x s is approximately proportional to pho-non temperature at high temperature [14]and from the Fermi liq-uid theory,its variation with electron temperature is quadratic (T e 2)[20].Therefore,x s is related to T e and T p approximately asx s ¼A ee T 2e þB ep T p(6)A ee is estimated from the low-temperature measurement [21]andB ep is usually estimated from the thermal or electrical resistivity near the room temperature [14].In this work,A ee is taken as the lit-erature value 1.2Â107s À1K À2[6]while e 1and B ep are evaluated by ﬁtting the room-temperature value of the complex dielectric con-stant at 800nm wavelength provided in Ref.[22],which are found to be 9.7and 3.6Â1011s À1K À1,respectively.The complex index of refraction n 0þin 00is the square root of the dielectric ing Eqs.(5)and (6),n 0and n 00are evaluated as 0.16and 4.90,respectively,which agree with the empirical values [23].The re-ﬂectance is then calculated from n 0and n 00by the method of transfer matrix [24],which considers multiple reﬂections in thin ﬁlms.4Results and DiscussionThe results of TTR measurements with a pump ﬂuence of 147J/m 2are plotted in Fig.2.The fast decrease of the reﬂectance indicates that energy transfer between electrons and phonons in metal,followed by a relatively slow decrease after several ps which indicates electrons and phonons have reached thermal equi-librium.The initial cooling rates are smaller for samples with thicknesses less than the electron ballistic length since the electron temperature is almost uniform across the thin ﬁlm,and coupling with phonons within the metal ﬁlm and the dielectric substrate is the only cooling mechanism.For a thicker sample of thickness 250nm,the initial decrease is much faster due to thermal diffu-sion in the gold ﬁlm caused by a gradient of the electron tempera-ture in the ﬁlm.We investigate the effect of R es and R ps using the thermo-reﬂectance signal.Two values of R ps ,1Â10À10m 2K/W and 1Â10À7m 2K/W,are used,each with a parameterized range of values for R es .Figure 3shows the calculated results for the sample with a 39nm-thick gold ﬁlm.Little difference can be seen between Figs.3(a )and 3(b )while different cooling rates are obtained with varying R es in either plot,indicating that the cooling rate is not sensitive to the coupling strength between phonons in metal and dielectric.Note that an interface resistance of 1Â10À10m 2K/W is lower than any reported value,indicating a very high coupling strength between the phonons in metal and dielectric.Conversely,the results vary greatly with the coupling strength between electrons in metal and phonons in dielectric at the interface.This is because the lattice (phonon)temperature rise in metal is much smaller than the elec-tron temperature that the interface coupling between phonons in metal and dielectric does not inﬂuence the surface temperature,which directly determines the measured reﬂectance.On the other hand,the temperature rise of electrons is much higher,and conse-quently,the cooling rate is sensitive to R es .The relatively high sensitivity of R es to that of R ps demonstrates that the former can be isolated for the study of the coupling between electrons in metal and phonons in dielectric.We now use the measured TTR data to estimate R es ,the thermal resistance between electrons in metal and phonons in dielectric.R es is adjusted by the least square method to ﬁt the simulation results with the measured data,and the results are shown in Fig.4.We note that it is impossible to ﬁt the measured results using insu-lation interface condition (i.e.,no coupling or extremely large thermal resistance between electrons in metal and phonons in the dielectric substrate),which will signiﬁcantly underestimate the cooling rate.For thin samples,we ﬁnd that the value of R es is of the order of 10À10to 10À9m 2K/W.This value is below the ther-mal resistances of representative solid–solid interfaces measured in thermal equilibrium [25].This indicates that the direct coupling between electrons in metal and phonons in dielectric is strong.It is also noted that the resistance values increases with the thickness of the gold ﬁlm,indicating a decrease in the coupling strength between electrons in metal and the dielectric substrate.This could be due to the lower electron temperature obtained in thicker ﬁlms,and a decrease of the coupling strength with a decrease in the electron temperature [5].For the sample of thickness 250nm,R es has little effect on the simulation result since the interface is too far from the absorbing surface to inﬂuence the surface tempera-ture,and therefore it is not presented here.The agreement between the ﬁtted results and the measured data is generally good.The small discrepancy between the measured and the ﬁtted results can result from inaccuracy in computingtheFig.2TTR measurement results on Au–Si samples of varying AuthicknessesFig.3Simulation results with varying R es for the Au–Si sample of Au thickness 39nm.(a )R ps 51310210m 2K/W;(b )R ps 5131027m 2K/W.absorption or the temperature.Figure 1(b)shows the normalized TTR measurement results on the sample of thickness 77nm with three laser ﬂuences.It is seen that small variations in the shape of the TTR signals can be caused by different laser ﬂuences and thus the maximum temperature reached in the ﬁlm.Absorption in metal,multiple reﬂections between the metal surface and the Au–Si inter-face,and possible deviations of the properties of thin ﬁlms from those of bulk can all contribute to uncertainties in the temperature simulation;therefore affecting the calculated reﬂectance.With the values of R es shown in Fig.4,the calculation shows that the highest electron temperature,which is at the surface of 39nm–thick gold ﬁlm,is about 6700K.The highest temperature of electrons is roughly inversely proportional to the thickness of the ﬁlms for the four thinner ﬁlms.The highest temperature of elec-trons is much less than the Fermi temperature and thus ensures the validity of the linear dependence of C e on T e [14].The highest temperature for the lattice in metal is about 780K,also in the 39nm-thick gold ﬁlm.This large temperature difference between electrons and lattice indicates that the interface heat transfer is dominated by the coupling between electrons in metal and the phonons in the dielectric substrate.As shown in Fig.4,the meas-ured R es is very low,of the order of 10À10to 10À9m 2K/W.Even if R ps ,which is not determined in this study,is also that low (note that 10À10to 10À9m 2K/W is lower than any reported values),because of the large difference in temperatures between electrons and the phonons in metal,the interface heat transfer rate (Eqs.(3a )–(3c ))due to the coupling between electrons in metal and the substrate is much larger than that due to the coupling between phonons in metal and the substrate.5ConclusionsIn conclusion,TTR measurements using femtosecond laser pulses are performed on Au–Si samples and the results are analyzed using the TTM model.It is shown that due to the strong nonequilibrium between electrons and phonons during ultrafast-laser heating,it is possible to isolate the effect of the direct electron–phonon coupling across the interface,allowing investiga-tion of its ing stretched femtosecond pulses is shown to be able to minimize the nonequilibrium effect among electrons,and is thus more suitable for this study.The TTR measurement data can be well-represented using the TTM parison between the TTR data and the TTM results indicates that the direct coupling due to electron-interface scattering dominates the interface heat transfer during ultrafast-laser heating of thin ﬁlms.AcknowledgmentThis paper is based upon work supported by the Defense Advanced Research Projects Agency and SPAWAR Systems Cen-ter,Paciﬁc under Contract No.N66001-09-C-2013.The authors also thank C.Liebig,Y.Wang,and W.Wu for helpful discussions.NomenclatureA ee ¼coefﬁcient in Eq.(6),s À1K À2B ep ¼coefﬁcient in Eq.(6),s À1K À1C ¼volumetric heat capacity,J/(m 3K)G ¼electron–phonon coupling factor,W/(m 3K)i ¼unit of the imaginary number J ¼ﬂuence of the pump,J/m 2k ¼thermal conductivity,W/(mK)L ¼metal ﬁlm thickness,mn 0¼real part of the complex index of refractionn 00¼imaginary part of the complex index of refraction R ¼interface thermal resistance,m 2K/W;reﬂectance S ¼laser source term,W/m3Fig.4Comparison between the measurement and the simulation results for Au–Si samples of different Au thicknesses.The open circle represents the meas-ured data and the solid line represents the simulation results.(a )39nm ﬁtted by R es 55310210m 2K/W;(b )46nm ﬁtted by R es 56310210m 2K/W;(c )60nm ﬁtted by R es 51.231029m 2K/W;and (d )77nm ﬁtted by R es 51.831029m 2K/W.T¼temperature,Kt¼time,st p¼pulse width of the pump(FWHM),sx¼spatial coordinate,me¼complex dielectric constante1¼constant in the Drude modeld¼radiation penetration depth,md b¼electron ballistic depth,mx¼angular frequency of the probe,rad/sx p¼plasma frequency,rad/sx s¼electron collisional frequency,rad/sSubscripts0¼initial statee¼electron in metales¼electron in metal and phonon in dielectricp¼phonon in metalps¼phonon in metal and phonon in dielectrics¼phonon in dielectricReferences[1]Cahill,D.G.,Ford,W.K.,Goodson,K.E.,Mahan,G.D.,Majumdar,A.,Maris,H.J.,Merlin,R.,and 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激光熔覆马氏体

第27卷第2期粉末冶金材料科学与工程2022年4月V ol.27 No.2 Materials Science and Engineering of Powder Metallurgy Apr. 2022DOI:10.19976/ki.43-1448/TF.2021090激光熔覆马氏体/铁素体涂层的组织与抗磨耐蚀性能张磊1, 2，陈小明1, 2，霍嘉翔1，张凯1, 2，曹文菁1, 2，程新闯3(1. 水利部产品质量标准研究所浙江省水利水电装备表面工程技术研究重点实验室，杭州 310012；2. 水利部杭州机械设计研究所水利机械及其再制造技术浙江省工程实验室，杭州 310012；3. 绍兴市曹娥江大闸管理局，绍兴 312000)摘要：为提高液压活塞杆的耐腐蚀和抗磨损性能，在45号钢表面采用激光熔覆技术在不同激光功率下制备具有马氏体/铁素体组织的Fe基合金熔覆层。

利用X射线衍射仪、扫描电镜、X射线能谱仪等手段表征涂层的物相组成、微观形貌和元素分布，采用维氏硬度计和干滑动摩擦试验机对涂层的显微硬度和抗磨损性能进行测试，并通过电化学工作站研究熔覆层的耐腐蚀性能。

结果表明：Fe基合金熔覆层的主要物相为α-Fe、Ni-Cr-Fe、γ-(Fe,C)和Fe9.7Mo0.3等，主要组织为马氏体、铁素体和少量残余奥氏体。

熔覆层的枝晶态组织均匀致密，无裂纹和孔隙缺陷，涂层与基体呈冶金结合。

涂层的硬度与耐磨性能随激光功率增大而提高，当功率为2.4 kW时，涂层的平均显微硬度(HV)为647.64，耐磨性能为45号钢的9.37倍，磨损机制为磨粒磨损。

随激光功率提高，Fe基合金熔覆层的耐腐蚀性能先升高后降低，当激光功率为2.0 kW时涂层具有最佳耐腐蚀性能，显著高于活塞杆常用碳钢、不锈钢以及电镀硬铬等材料，可在相关领域替代电镀铬。

关键词：激光熔覆；Fe基合金；组织；磨损；腐蚀；活塞杆中图分类号：TG174.44文献标志码：A 文章编号：1673-0224(2022)02-196-09All Rights Reserved.Microstructure and wear-corrosion resistance performance oflaser cladding martensite/ferrite coatingZHANG Lei1, 2, CHEN Xiaoming1, 2, HUO Jiaxiang1, ZHANG Kai1, 2, CAO Wenjing1, 2, CHENG Xinchuang3(1. Key Laboratory of Surface Engineering of Equipment for Hydraulic Engineering of Zhejiang Province, Standard &Quality Control Research Institute, Ministry of Water Resources, Hangzhou 310012, China;2. Water Machinery and Remanufacturing Technology Engineering Laboratory of Zhejiang Province, HangzhouMechanical Research Institute, Ministry of Water Resources, Hangzhou 310012, China;3. Shaoxing Municipal Cao’e River Floodgate Construction Administration Committee, Shaoxing 312000, China)Abstract: To improve the corrosion resistance and wear resistance of piston rod, Fe-based coatings with martensite andferrite structure were prepared on 45# steel by laser cladding. The phase compositions, microstructure and elementsdistribution of the coatings were characterized by X-ray diffractometer, scanning electron microscope and X-ray energydispersive spectrometer. The microhardness and wear resistance of the coatings were tested by Vickers hardness testerand dry sliding friction wear tester. Furthermore, the corrosion resistance of laser cladding Fe-based coatings was studiedby electrochemical workstation. The results show that the phase of laser cladding Fe-based alloy coating is mainlycomposed of α-Fe, Ni-Cr-Fe, γ-(Fe,C), Fe9.7Mo0.3. The main microstructure is martensite, ferrite and a small amount ofresidual austenite. The dendritic structure of coating is uniform, compact, without cracks or pores. The coating and thesubstrate are bonded metallurgically. The hardness and wear resistance of the coatings increase with increasing基金项目：浙江省“一带一路”国际科技合作项目(2019C04019)；浙江省公益性技术应用研究计划资助项目(GC22E017317，LGC19E090001，2018C37029)收稿日期：2021−11−02；修订日期：2021−12−23通信作者：张磊，工程师，硕士。

阴虚阳亢的道理

湍流图
加速演化的宇宙
湍流的研究方法
非线性动力学பைடு நூலகம்沌探索发展了二项有普遍意
义的重要研究方法：符号动力学方法与相空间重构方法。相空间重构方法应用于人体在某一时间段的动态状态作为观察对象，这就是金络康宝对人体实时状态描述的结果。动力系统重建，通过一维时间系列反向构造出原来系统相空间的结构；这就是手段调理及调理结果。
对称破却与阴虚阳亢
现已发现，Alzheimer病患者脑内的L氨基酸发生了
明显的D转化。换言之，这里发生了对称破缺的破坏，对称破缺本身就是一种序，是大脑左右功能分工协调的表现，而不是各行其是。如何促进这种协调应当是治疗健身的基本原则。因为精神状态的优化是最有效和最普遍的途径，免疫功能的改善只是其中的部分效应。精神状态的大部分特别是涉及内部生机系统与器官的调控又是意志所不能控制的。中医的经络系统给我们提供了连通大脑左右半球的信号通道。
阴虚阳亢是有机体的永恒规律
宇称不守恒
生物体手性分子不对称
金络康宝对人体测评的结论。
谢谢大家
n.
蛋白质三维与核小体基因表达
从生物学上揭示蛋白质合成及机理
阴阳与自旋矩阵
素问已说明四维与人体的关系。
狄拉克的四维自旋矩阵中有二个神秘因子：
自旋朝上与自旋朝下的电子。同时还具有二个分量具有负能量（1932证实为正电子），这就是反物质存在的证据。上面内容写成享誉世界的【量子力学原理】。这理论告诉我们，世界是由正反物质构成的。这是中医阴与阳来源的最佳证据。
生物界的左右不对称介绍
手性与对称破却
手性破却与阴虚阳亢
手性破缺的理论和假说①
宇称不守恒弱相互作用对应于产生手性分子有两种假说。一种是以带电电流宇称不守恒（Charged Current－Parity Non conservation:CC－PNC）为基础的假说，认为由于 CC－PNC，β衰变产生径向极化电子，进入物质内部产生圆偏振光辐射，导致底物分子立体选择合成或分解，从而产生手性分子。另一种以弱中性流宇称不守恒（Neutral Current－Parity Non conservation:NC－PNC）为基础的假说，认为NC－ PNC造成对映体之间微小的能差叫宇称破缺能差（Parity Violating Energy Difference, PVED），PVED进一步在对映体分子的物理性质及反应速率上形成差别，经过放大可以产生均一的手性。

共振声子辅助的太赫兹量子级联激光器中杂质散射的蒙特卡洛模拟(英文)

共振声子辅助的太赫兹量子级联激光器中杂质散射的蒙特卡洛
模拟(英文)
曹俊诚;吕京涛
【期刊名称】《半导体学报：英文版》
【年(卷),期】2006(27)2
【摘要】通过蒙特卡洛方法研究了基于共振声子散射的太赫兹量子级联激光器中杂质散射对激光器性能的影响.使用单子带静态屏蔽模型来处理电子与杂质的散射过程.发现电子与杂质的散射为电子在有源区中的注入和抽取过程提供了另外一个通道.这一过程可以影响电子在不同子带的占据数以及器件的电流.所以,在考虑基于共振声子散射的太赫兹量子级联激光器中的电子输运过程时,需要包含电子与杂质的散射过程.
【总页数】5页(P304-308)
【关键词】太赫兹;量子级联激光器;蒙特卡洛方法
【作者】曹俊诚;吕京涛
【作者单位】中国科学院上海微系统与信息技术研究所信息功能材料国家重点实验室
【正文语种】中文
【中图分类】TN302
【相关文献】
1.共振声子弛豫的太赫兹量子级联激光器有源区结构设计 [J], 宋亚峰;朱勤生
2.共振声子太赫兹量子级联激光器研究 [J], 黎华;韩英军;谭智勇;张戎;郭旭光;曹俊诚
3.基于太赫兹量子阱探测器的太赫兹量子级联激光器发射谱研究 [J], 谭智勇;郭旭光;曹俊诚;黎华;韩英军
4.太赫兹Si/SiGe量子级联激光器波导模拟(英文) [J], 陈锐;林桂江;陈松岩;李成;赖虹凯;余金中
5.共振声子太赫兹量子级联激光器研究(英文) [J], 曹俊诚;黎华;吕京涛
因版权原因，仅展示原文概要，查看原文内容请购买。

常用材料分析方法中英文对照

1. Elemental Analysis 元素分析Atomic absorption spectroscopy 原子吸收光谱Auger electron spectroscopy (AES) 俄歇电子能谱Electron probe microanalysis (EPMA) 电子探针微分析Electron spectroscopy for chemical analysis (ESCA) 化学分析电子能谱Energy dispersive spectroscopy (EDS) 能量色散谱Flame photometry 火焰光度法Wavelength dispersive spectroscopy (WDS)X-ray fluorescence X射线荧光2. Molecular and Solid State Analysis 分子与固态分析Chromatography [gas chromatography (GC), size exclusion chromatography (SEC)]色谱[气相色谱，体积排除色谱]Electron diffraction 电子衍射Electron microscopy [scanning electron microscopy (SEM),transmission electron microscopy (TEM),scanning TEM (STEM)] 电子显微镜Electron spin resonance (ESR) 电子自旋共振Infrared spectroscopy (IR) 红外光谱Mass spectrometry 质谱Mercury porosimetry 压汞法Mossbauer spectroscopy 穆斯堡尔谱Nuclear magnetic resonance (NMR) 核磁共振Neutron diffraction 中子衍射Optical microscopy 光学显微镜Optical rotatory dispersion (ORD) 旋光色散Raman spectroscopy 拉曼光谱Rutherford back scattering (RBS) 卢瑟福背散射Small angle x-ray scattering (SAXS) 小角X射线散射Thermal analysis [differential scanning calorimetry (DSC),thermal gravimetric analysis (TGA),differential thermal analysis (DTA) temperature desorption spectroscopy (TDS),thermomechanical analysis (TMA)]热分析[差示扫描量热计法，热-重分析，微分热分析，升温脱附，热机械分析]UV spectroscopy 紫外光谱X-ray techniques [x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), x-ray emission,x-ray absorption] X射线技术[x射线光电子能谱，x射线衍射，x射线发射，x射线吸收]3. Surface Characterization Techniques 表面表征技术Electron energy loss spectroscopy (EELS) 电子能量损失谱Ellipsometry 椭圆偏振术Extended x-ray absorption fine structure (EXAFS) 扩展X射线吸收精细结构Helium (or atom) diffractionLateral (or frictional) force microscopy (LFM) 横向（摩擦）力显微镜Low-energy electron diffraction (LEED) 低能电子衍射Magnetic force microscopy (MFM) 磁力显微镜Near-edge x-ray adsorption fine structure (NEXAFS) 近边X射线吸收精细结构Near field scanning 近场扫描Reflection high-energy electron diffraction (RHEED) 反射高能电子衍射Scanning tunneling microscopy (STM) 扫描隧道显微镜Scanning force microscopy (SFM) 扫描力显微镜Secondary ion mass spectroscopy (SIMS) 二次离子质谱Surface enhanced raman spectroscopy (SERS) 表面增强拉曼光谱Surface extended x-ray adsorption fine structure (SEXAFS) 表面扩展X射线吸收精细结构Surface force apparatus 表面力仪器。

Supersymmetric QCD Parity Nonconservation in Top Quark Pairs at the Tevatron

a r X i v :h e p -p h /9709275v 2 12 F eb 1998PKU-TP-97-20MSUHEP-70825THU-TP-97-08hep-ph/9709275Supersymmetric QCD Parity Nonconservation inTop Quark Pairs at the TevatronChong Sheng Li (a ),C.–P.Yuan (b ),Hong-Yi Zhou (c )(a )Department of Physics,Peking University,Beijing 100871,China (b )Department of Physics and Astronomy,Michigan State University,East Lansing,Michigan 48824,USA(c )Institute of Modern Physics and Department of Physics,Tsinghua University,Beijing 100084,ChinaABSTRACTIn the supersymmetry (SUSY)models,because of the mass diﬀerence between the left-and right-top squarks,the supersymmetric QCD in-teractions can generate parity violating eﬀects in the production of t ¯t pairs.We show that SUSY QCD radiative corrections to the parity vi-olating asymmetry in the production rates of the left-and right-handedtop quarks via the q ¯q →t ¯tprocess can reach about 3%at the Fermilab Tevatron with√1IntroductionIn a recent paper[1],we studied the parity violating asymmetry induced from the supersymmetric electroweak(SUSY EW)and Yukawa(SUSY Yukawa)corrections at the one loop level.Two classes of supersymmetry(SUSY)models were considered:the mini-mal supergravity(mSUGRA)models[2]and the minimal supersymmetric models(MSSM) with scenarios motivated by current data[3,4].After sampling a range of values of SUSY parameters in the region that might give large contributions to the parity-violating asym-metry A,and which are also consistent with either of the above two classes of models, we found that the asymmetry A due to the one-loop SUSY EW(αm2t/m2W)and SUSY Yukawa corrections for the production process q¯q→g→t¯t at the upgraded Tevatron is generally small,less than a few percent.However,the sign can be either positive or negative depending on the values of the SUSY parameters.(The eﬀect from the Standard Model(SM)weak corrections to this asymmetry is typically less than a fraction of percent [5,6].)In the supersymmetric Standard Model,some superparticles experience not only the electroweak interaction but also the strong interaction.Although the SM QCD interaction respects the discrete symmetries of charge conjugation(C)and parity(P),the SUSY QCD interactions for superparticles,in their mass eigenstates,need not be C and P invariant. (Needless to say,in the strong interaction eigenstates,the SUSY QCD interaction is C-and P-invariant.)For either the mSUGRA or the MSSM models,the masses of the left-stop(the supersymmetric partner of the left-handed top quark)and the right-stop can be noticeably diﬀerent due to the large mass of the top quark.This is a general feature of the supersymmetry models in which the electroweak symmetry is broken spontaneously via radiative corrections.Since both the left-stop and the right-stop contribute to the loop corrections for the t¯t pair production process q¯q,gg→t¯t,the diﬀerent masses of the top-squarks will induce a parity violating asymmetry.It is this eﬀect that we shall study in this paper.Because the t¯t pairs are produced predominantly via the QCD process q¯q→t¯t√at the Tevatron(a p¯p collider with CM energyThis amounts to a signal at∼90%c.l.(conﬁdence level)with2fb−1,or99%c.l.with10 fb−1.Thus,a study of A at the Tevatron could yield information about the allowed range of SUSY model parameter space.2SUSY QCD Corrections and Parity ViolationI.Squark mixingsIn the MSSM the mass eigenstates˜q1and˜q2of the squarks are related to the(strong) current eigenstates˜q L and˜q R via the mixing angleθ˜q by˜q1=˜q L cosθ˜q+˜q R sinθ˜q,˜q2=−˜q L sinθ˜q+˜q R cosθ˜q.(1) For the top squarks,the mixing angleθ˜t and the masses m˜t1,2can be calculated by diago-nalizing the following mass matrix[3],M2˜t = M2˜t L m t m LRm t m LR M2˜t R,M2˜t L =m2˜t L+m2t+(13sin2θW)cos(2β)m2Z,M2˜t R =m2˜t R+m2t+2N R+N L=σR−σLSome of the one loop scattering amplitudes of q ¯q →t ¯twere already presented in Refs.[10,11]for calculating the total production rates of t ¯tpairs.To calculate the parity violating asymmetry A in the t ¯tsystem,additional renormalized amplitudes are needed.In terms of the tree-level amplitude,M 0,and the next-to-leading order SUSY QCD corrections,δM ,the renormalized amplitudes at the one-loop level can be writtenas M =M 0+δM .Denote the momenta of the initial and the ﬁnal state particles asq l (p 4)¯q m (p 3)→t i (p 2)¯tj (p 1),and the Dirac four-spinor as u i ≡u (p i )(v i ≡v (p i ))for particle (anti-particle)i .Then,M 0=ig 2s (T c ji T clm )J 1·J 2/ˆs ,where J µ1=¯v (p 3)γµu (p 4)and J µ2=¯u (p 2)γµv (p 1);ˆs is the invariant mass of the t ¯t pair;g s and T c ij are the gauge coupling andthe generator of the group SU (3)c ,respectively.To calculate the parity violating asymmetry induced by the SUSY QCD eﬀects,we fol-low the method presented in Ref.[12],in which the asymmetry was calculated numerically using the helicity amplitude method.To obtain the renormalized scattering amplitudes,we adopt the dimensional regularization scheme to regulate the ultraviolet divergences and the on-mass-shell renormalization scheme [13]to deﬁne the input parameters.The SUSY QCD corrections to the scattering amplitudes arise from the vertex diagram,the gluon self-energy and the box diagrams,as well as the crossed-box diagrams.The renormalized amplitudes can be written asδM =δM v 1+δM v 2+δM s +δM DB +δM CB ,(4)where δM v 1and δM v 2are vertex corrections,δM s is the self-energy correction,and δM DB and δM CB are the contributions from the box diagrams and crossed-box diagrams,respec-tively.The results for these separate contributions are,δM v 1=ig 2s(T c ji T c lm )¯u (p 2)[F v 10·J 1+F v 11/J 1+/J 1/F v 13+/F v 14/J 1+/F v 16·J 1+(F Av 11/J 1+/J 1/F Av 13+/F Av 14/J 1+/F Av 16·J 1)γ5]v (p 1)/ˆs ,(5)δM v 2=ig 2s (T c ji T c lm )¯v (p 3)(F v 21/J 2+/F v 26·J 2)u (p 4)/ˆs ,(6)δM s =F s0M 0,(7)δM DB =ig 2s7δM CB=ig2s1ˆs=M t¯t.1As discussed in the previous section,the SUSY parameters relevant to our study arem˜t1,m˜t2,θ˜t(or m˜tL,m˜tR,m LR),m˜b R,m˜qL,R,and m˜g.To simplify our discussion,we assumem˜qL,R =m˜b R=m˜tL,so that there are only four SUSY parameters to be considered,m˜t1,m˜t2,θ˜t and m˜g.(The SU(2)L gauge symmetry requires that m2˜b L=m2˜t L.)The mSUGRA models predict radiative breaking of the electroweak gauge symmetryinduced by the large top quark mass.Consequently,it is possible to have large splitting in the masses of the left-stop and the right-stop,while the masses of all the other(left-or right-)squarks are about the same[17].For the MSSM models with scenarios motivated by current data[4],a light˜t1is likely to be the right-stop(˜t R),with a mass at the order of m W;the other squarks are heavier than˜t1.Since heavy superparticles decouple in loopcontributions,we expect that a lighter˜t1would induce a larger asymmetry.Because theparity-violating eﬀects from the SUSY QCD interactions arise from the mass diﬀerencebetween˜t1and˜t2,it is obvious from Eq.(1)that the largest parity violating eﬀect occurswhenθ˜t is±π/2for m˜tR≤m˜t L.Whenθ˜t=±π/4,the parity asymmetry should be zero. This is evident from the results shown in the Appendix,which indicate that the amplitudesthat contribute to A are all proportional to Z i=∓cos(2θ˜t).In either the mSUGRA or the MSSM models,the gluinos are usually as heavy as thelight squarks,on the order of a few hundred GeV.However,Farrar has argued[18]that lightgluinos are still a possibility.If gluinos are light,then a heavy top quark can decay into astop and a light gluino for m˜t1<(m t−m˜g)such that the branching ratio of t→bW+could show a large diﬀerence from that(∼100%)predicted by the SM.The CDF collaborationhas measured the branching ratio of t→bW+to be0.87+0.13−0.30+0.13−0.11[19].At the1σlevel,this implies that a50(90)GeV˜t1requires the mass of the gluino to be larger than about 120(80)GeV forθ˜t=±π/2.However,at the2σlevel(i.e.95%c.l.),there is no useful limit on the mass of the gluino.2To represent diﬀerent classes of SUSY models in which the parity-violating asymmetry induced by the SUSY QCD interactions can be large,we show in Table1four represen-tative sets of models.They are labeled by the set of parameters(m˜t1,m˜t2,θ˜t),which areequal to(50,1033,−1.38),(90,1033,−1.38),(50,558,−1.25)and(90,558,−1.25),respec-tively.(All the masses are in units of GeV.)Based upon Eq.(2),one can also label thesemodels by(m˜tL ,m˜tR,m LR),which are(1000,90,1100),(1000,118,1100),(500,40,520)and(500,88,520),respectively,forβ=π/4.It is interesting to note that for all the models listed in Table1,the asymmetry A is negative(i.e.σR<σL)for m˜g<200GeV,and its magnitude can be as large as 3%for models with light˜t1.The maximal|A|occurs when m˜g is about equal to(m t−m˜t1)because of the mass threshold enhancement.For m˜g>200GeV,the asymmetry A becomes positive,with a few percent in magnitude,and monotonically decreases as m˜g paring these results with those induced by the SUSY EW and SUSY Yukawa corrections[1],it is clear that SUSY QCD interactions can generate a relatively larger parity-violating asymmetry.The diﬀerential asymmetry A(M t¯t)also exhibits an interesting behaviour as a function of the t¯t invariant mass M t¯t.This is illustrated in Table2for theﬁrst SUSY model inTable1((m˜t1,m˜t2,θ˜t)=(50,1033,-1.38)).As shown,|A(M t¯t)|increases as M t¯t increasesfor m˜g<200GeV,which is similar to the eﬀects from the SUSY EW and SUSY YukawaTable1:Parity violating asymmetry A in p¯p→t¯t+X,as a function of m˜g,for four sets of SUSYmodels labeled by(m˜t1,m˜t2,θ˜t).m˜g(GeV)(90,1033,−1.38)(90,558,−1.25)-1.10%-0.98%-1.53%-1.40%-2.34%-2.21%-2.86%-2.89%-3.16%-3.43%-2.58%-2.80%-1.18%-1.30%0.99%0.82%1.60% 1.40%1.53% 1.35%1.27% 1.16%1.04%0.95%3Without the cuts in(10),the values of A for theﬁrst model in Table1are−1.0%,−2.65%,and +0.94%for m˜g=2,120,200GeV,respectively.4These apparent problems in Ref.[22]were also pointed out in Ref.[21].6Table2:The diﬀerential asymmetry A(M t¯t)and cross section dσ/d M t¯t(in unit of fb/GeV)as a function of M t¯t for theﬁrst SUSY model in Table1with various m˜g values.M t¯t(GeV)m˜g=120GeV358-0.73%16.3-0.42%36.60.95%31.4 378-1.63%29.0-0.67%38.7 2.12%34.2 398-2.17%27.2-0.82%35.0 3.67%32.4 425-2.64%22.8-1.13%24.1 1.05%20.0 475-3.40%13.8-1.47%13.7-0.62%10.8 525-3.81%7.9-1.76%7.7-1.71% 5.9 575-4.34% 4.4-3.26%0.032-4.66%0.024Table3:The SUSY QCD corrections(∆σ)to the q¯q→t¯t production rates at the Tevatron with √2501001201351501752002252502753001.170.26-0.04-0.18-0.87-0.49-0.020.330.300.240.190.16m˜g=200GeV and m˜t=m˜q=75GeV,we obtain a39%,in contrast to33%,correction in the total cross section without cuts.Including cuts in(10)only slightly increases the correction to40%.For completeness,in Table3we show the SUSY QCD corrections∆σto the q¯q→t¯t√production rates at the Tevatron withUp to now,we have only considered the one loop SUSY QCD eﬀects on the parity violating asymmetry A in t¯t pair production.Amusingly,the parity-violating asymmetry induced by the SUSY QCD interactions can also occur at the Born level.If gluinos are very light,of the order of1GeV,this asymmetry can be generated by the tree level process ˜g˜g→t¯t.Unfortunately,its production rate is smaller than the gg→t¯t rate,which is only about one tenth of the q¯q→t¯t rate at the Tevatron.Hence,it cannot be measured at the Tevatron.However,at the CERN Large Hadron Collider(LHC),the production rate of˜g˜g→t¯t is large enough to allow the measurement of the parity-violating asymmetry induced by the SUSY QCD interactions.The asymmetry in the production rates of t L¯t and t R¯t,generated by the˜g˜g fusion process alone,can reach about10%for M t¯t larger than about500GeV.We shall present its details and include the eﬀect from the gg and q¯q fusion processes in a future publication[23].This work is supported in part by the National Natural Science Foundation of China, and by the U.S.NSF grant PHY-9507683.AppendixWe give here the form factors for the matrix elements appearing in Eqs.(8)-(12). They are written in terms of the conventional one-,two-,three-and four-point scalar loop integrals deﬁned in Ref.[24].F v1µ= i=1,23αs12π[m˜g Y i((p2−p1)µC0/2−Cµ](−p2,k,m˜g,m˜t i,m˜t i)F v11= i=1,23αs3π[B1X i−2m t m˜g B′0Y i+2m2t B′1X i](m2t,m˜g,m˜t i)F v1µ3= i=1,23αs8π(−2X i)Cµν(−p2,k,m˜ti,m˜g,m˜g)8+ i=1,2αs8πZ i(C20+(m2t−m2˜g)C0)(−p2,k,m˜t i,m˜g,m˜g) + i=1,2αs8πZ i m t Cµ(−p2,k,m˜ti,m˜g,m˜g)F Av1µ4=−F Av1µ3F Av1µν6= i=1,23αs12πZ i[((p2−p1)νCµ/2−Cµν)](−p2,k,m˜g,m˜t i,m˜t i)F v21=3αs3πB1(m2q,m˜g,m˜q)F v2µν6=3αs6π[−(p2−p1)νCµ/2−Cµν](p4,−k,m˜g,m˜q,m˜q)F s0=3αs6−(m2˜g(B0+1)−2B22)/k2)(k2,m˜g,m˜g)−(B21+B1+14π (2B22(k2,m˜q,m˜q)−A0(m˜q))/k2−2B′22(0,m˜q,m˜q)F DB 1= i=1,2αs4π(m t X i+m˜g Y i)Dµ(−p2,p4,p3,m˜ti,m˜g,m˜q,m˜g)F DBµν3= i=1,2αs4πZ i(m2t−m2˜g)D0(−p2,p4,p3,m˜t i,m˜g,m˜q,m˜g)F DBµ5= i=1,2αsF DBµν6= i=1,2αs√√√√∂p2,B′1=∂B1(p2,m1,m2)∂p2,C20=gµνCµν−1References[1]C.S.Li,R.J.Oakes,J.M.Yang,and C.-P.Yuan,Phys.Lett.B398(1997)298.[2]For reviews,see H.P.Nilles,Phys.Rep.110(1984)1;P.Nath,R.Arnowitt and A.Chamseddine,Applied N=1Supergravity,ICTP series in Theoretical Physics,(World Scientiﬁc,1984);L.E.Ib´a˜n ez and G.G.Ross,in Perspectives on Higgs Physics,ed.G.L.Kane,(World Scientiﬁc,1993).[3]H.E.Haber and C.L.Kane,Phys.Rep.117(1985)75;J.F.Gunion and H.E.Haber,Nucl.Phys.B272(1986)1.[4]S.Ambrosanio,G.L.Kane,G.D.Kribs,S.P.Martin and S.Mrenna,Phys.Rev.Lett.76(1996)3498;S.Dimopoulos,M.Dine,S.Raby and S.Thomas,Phys.Rev.Lett.76(1996)3494.[5]Kao,dinsky and C.–P.Yuan,FSU-HEP-930508,1993(unpublished);DPFConf.1994:pp.713-716;Int.J Mod.Phys.A12(1997)1341.[6]C.Kao,Phys.Lett.B348(1995)155.[7]enen,J.Smith and W.L.van Neerven,Phys.Lett.B321(1994)254.[8]W.Beenakker,A.Denner,W.Hollik,R.Mertig,T.Sack and D.Wackeroth,Nucl.Phys.B411(1994)343.[9]D.Amidei and R.Brock,“Report of the T eV2000Study Group on Future ElectroWeakPhysics at the Tevatron”,Fermilab-Pub-96/082,and references therein.[10]J.M.Yang and C.S.Li,Phys.Rev.D52(1995)1541;C.S.Li,B.Q.Hu,J.M.Yang,and C.G.Hu,Phys.Rev.D52(1995)5014;Erratum,Phys.Rev.D53(1996)4112;C.S.Li,H.Y.Zhou,Y.L.Zhu,and J.M.Yang,Phys.Lett.B379(1996)135;J.M.Yang and C.S.Li,Phys.Rev.D54(1996)4380.[11]J.Kim,J.L.Lopez,D.V.Nanopoulos,and R.Rangarajan,Phys.Rev.D54(1996)4364;S.Catani et al.,Phys.Lett.B378(1996)329;T.Gehrmann et al.,Phys.Lett.B381(1996)221;J.A.Coarasa et al.,hep-ph/9607485;S.Frixione,hep-ph/9702287,to be published in Heavy Flavours II,World Scientiﬁc, Singapore.[12]G.L.Kane,dinsky and C.–P.Yuan,Phys.Rev.D45(1992)124.[13]A.Denner,Fortschr.Phys.,41,307(1993).[14]CDF Collaboration,Phys.Rev.Lett.74(1995)2626;DØCollaboration,Phys.Rev.Lett.74(1995)2632;L.Roberts,in the Proceedings of the28th International Conference on High Energy Physics,Warsaw,Poland,1996.[15]A.D.Martin,W.J.Stirling and R.G.Roberts,Phys.Lett.B354(1995)155.[16]i,J.Huston,S.Kuhlmann,F.Olness,J.Owens,D.Soper,W.K.Tung,H.Weerts,Phys.Rev.D55(1997)1280.[17]For example,see G.L.Kane,C.Kolda,L.Roszkowski,J.D.Wells Phys.Rev.D49(1994)6173;and references therein.[18]G.R.Farrar,hep-ph/9707467;and references therein.[19]By CDF Collaboration(J.Incandela for the collaboration),Nuovo Cim.109A(1996)741.[20]L.Clavelli and G.R.Goldstein,hep-ph/9708405.[21]Z.Sullivan,Phys.Rev.D56,451(1997).[22]S.Alam,K.Hagiwara,and S.Matsumoto,Phys.Rev.D55(1997)1307.[23]C.S.Li,P.Nadolsky,C.–P.Yuan and H.Y.Zhou,in preparation.[24]G.Passarino and M.Veltman,Nucl.Phys.B160(1979)151;G.´t Hooft and M.Veltman,Nucl.Phys.B153(1979)365.。

Parity Violation in eP Scattering at JLab - Experimental Hall A宇称在JLab实验大厅散射EP-精品文档60页

-New results from HAPPEX -Photocathodes -Polarimetry -Targets -Diagnostics -Counting Electronics
P. A. Souder
The Standard Model
June 24, 2005
DIS Parity
Ebeam = 1.165 GeV Ibeam = 180 μA Polarization ~85% Target = 2.5 KW
P. A. Souder
(it just wont break!)
The Annoying Standard Model
Nuclear Physics Long Range Plan: What is the new standard model?
Low Q2 offers unique and complementary probes of new physics
Most experts see the precision data as
remarkably consistent
Are Leptonic and hadronic Z couplings seem inconsistent?
Perhaps there are bigger deviations lurking elsewhere???
New Contact Interactions
LEP200
Lee ~ 15 TeV
Does Supersymmetry (SUSY) provide a candidate for dark matter?
•Lightest SUSY particle (neutralino) is stable if baryon (B) and lepton (L) numbers are conserved

X射线衍射和小角X射线散射

晶体的X射线衍射特征
[Crystal Structure Analysis, 3rd Edition, p. 48]
晶体结构及其晶胞类型
[Methods of Experimental Physics Volume 16: Polymers, Part B Crystal Structure and Morphology, p. 5]
X射线衍射需要在广角范围内测定，因此又被称为广角X射线衍射（Wide-Angle X-ray Scattering, WAXS）。
小角X射线散射
如果被照射试样具有不同电子密度的非周期性结构，则次生X射线不会发生干涉现象，该现象被称为漫射X射线衍射（简称散射）。
X射线散射需要在小角度范围内测定，因此又被称为小角X射线散射（Small-Angle Xray Scattering, SAXS）。
晶面指数与晶胞参数
[Fundamentals of Powder Diffraction and Structural Characterization of Materials, 2nd Edition, p. 9]
Bragg方程
设晶体的晶面距为 d，X射线以与晶面间交
角为的方向照射，从晶面散射出来的X射
粉末衍射条纹摄制及处理
[Fundamentals of Powder Diffraction and Structural Characterization of Materials, 2nd Edition, p. 265]
粉末衍射平板图案摄制
[Fundamentals of Powder Diffraction and Structural Characterization of Materials, 2nd Edition, p. 153]

用vasp计算硅的能带结构

用vasp计算硅的能带结构在最此次仿真之前，因为从未用过vasp软件，所以必须得学习此软件及一些能带的知识。

vasp是使用赝势和平面波基组，进行从头量子力学分子动力学计算的软件包。

用vasp计算硅的能带结构首先要了解晶体硅的结构，它是两个嵌套在一起的FCC布拉菲晶格，相对的位置为(a/4,a/4,a/4), 其中a=5.4A是大的正方晶格的晶格常数。

在计算中，我们采用FCC的原胞，每个原胞里有两个硅原子。

VASP计算需要以下的四个文件：INCAR（控制参数）, KPOINTS（倒空间撒点）, POSCAR（原子坐标）, POTCAR（赝势文件）为了计算能带结构，我们首先要进行一次自洽计算，得到体系正确的基态电子密度。

然后固定此电荷分布，对于选定的特殊的K点进一步进行非自洽的能带计算。

有了需要的K点的能量本征值，也就得到了我们所需要的能带。

步骤一.—自洽计算产生正确的基态电子密度：以下是用到的各个文件样本:INCAR 文件：SYSTEM = SiStartparameter for this run:NWRITE = 2; LPETIM=F write-flag & timerPREC = medium medium, high lowISTART = 0 job : 0-new 1-cont 2-samecutICHARG = 2 charge: 1-file 2-atom 10-constISPIN = 1 spin polarized calculation?Electronic Relaxation 1NELM = 90; NELMIN= 8; NELMDL= 10 # of ELM stepsEDIFF = 0.1E-03 stopping-criterion for ELMLREAL = .FALSE. real-space projectionIonic relaxationEDIFFG = 0.1E-02 stopping-criterion for IOMNSW = 0 number of steps for IOMIBRION = 2 ionic relax: 0-MD 1-quasi-New 2-CGISIF = 2 stress and relaxationPOTIM = 0.10 time-step for ionic-motionTEIN = 0.0 initial temperatureTEBEG = 0.0; TEEND = 0.0 temperature during runDOS related values:ISMEAR = 0 ; SIGMA = 0.10 broadening in eV -4-tet -1-fermi 0-gausElectronic relaxation 2 (details)Write flagsLWAVE = T write WAVECARLCHARG = T write CHGCARVASP给INCAR文件中的很多参数都设置了默认值，所以如果你对参数不熟悉，可以直接用默认的参数值。

脉冲激光晶化非晶硅薄膜的有限差分模拟

微电子器件、晶显示等领域有着非常重要的应用价值。另外，液它还是薄膜太阳能电池的优质材料。目前，液
晶面板制造产业中主要采取两步结晶的方式制备多晶硅薄膜，即先用等离子增强化学气相沉积（ＥＶＤ）衬ＰＣ在底上低温（０生长非晶硅薄膜，＜６０ｏＣ）然后通过激光诱导晶化技术使非晶硅转化成多晶硅［。主要是利用瞬１间激光脉冲产生的高能量入射到非晶硅薄膜表面，在薄膜表层产生热能效应，仅使非晶硅薄膜在瞬间达到熔点
问题。
ｃ丁）（ｆａＴＤ
一
［（筹＋（ｔＫＴ］Ｅ，）ｚ）
（１）
式中：（、是非晶硅的比热容；是材料密度；丁）热传导系数；ｘ，）＝Ｅ（）ｃ，，）ｆＤＫ（是Ｅ（￡ｚＥ（）为激光热源项，表它示时刻非晶硅薄膜表面下３处吸收的功率密度，脉冲激光的能量密度和时间有关。处设其时间域内满７与此
文章编号：１０－３２２０）７ｌ０—４０１４２（０８０一１００
脉冲激光晶化非晶硅薄膜的有限差分模拟
袁志军，楼祺洪，周军，董景星，魏运荣，王之江
（国科学院上海光学精密机械研究所，上海２１０）中０８０
维普资讯
第２Ｏ卷第７பைடு நூலகம் ２００８年７月

High-pressure vibrational and optical study of Bi(2)Te(3)

PHYSICAL REVIEW B84,104112(2011)High-pressure vibrational and optical study of Bi2Te3R.Vilaplana,1,*O.Gomis,1F.J.Manj´o n,2A.Segura,3E.P´e rez-Gonz´a lez,4P.Rodr´ıguez-Hern´a ndez,4A.Mu˜n oz,4 J.Gonz´a lez,5,6V.Mar´ın-Borr´a s,3V.Mu˜n oz-Sanjos´e,3C.Drasar,7and V.Kucek71Centro de Tecnolog´ıas F´ısicas,MALTA Consolider Team,Universitat Polit`e cnica de Val`e ncia,46022Valencia,Spain 2Instituto de Dise˜n o para la Fabricaci´o n y Producci´o n Automatizada,MALTA Consolider Team,Universitat Polit`e cnica de Val`e ncia,46022Valencia,Spain3Instituto de Ciencia de Materiales de la Universidad de Valencia—MALTA Consolider Team—Departamento de F´ısica Aplicada,Universitatde Val`e ncia,46100Burjassot,Valencia,Spain4MALTA Consolider Team—Departamento de F´ısica Fundamental II and Instituto Universitario de Materiales y Nanotecnolog´ıa,Universidad de La Laguna,La Laguna,Tenerife,Spain5DCITIMAC,MALTA Consolider Team,Universidad de Cantabria,Avda.de Los Castros s/n,39005Santander,Spain6Centro de Estudios de Semiconductores,Universidad de los Andes,M´e rida5201,Venezuela7Faculty of Chemical Technology,University of Pardubice,Studentsk´a95,53210-Pardubice,Czech Republic(Received8June2011;revised manuscript received25July2011;published9September2011)We report an experimental and theoretical lattice dynamics study of bismuth telluride(Bi2Te3)up to23GPa together with an experimental and theoretical study of the optical absorption and reﬂection up to10GPa.The indirect bandgap of the low-pressure rhombohedral(R-3m)phase(α-Bi2Te3)was observed to decrease withpressure at a rate of−6meV/GPa.In regard to lattice dynamics,Raman-active modes ofα-Bi2Te3were observedup to7.4GPa.The pressure dependence of their frequency and width provides evidence of the presence of anelectronic-topological transition around4.0GPa.Above7.4GPa a phase transition is detected to the C2/mstructure.On further increasing pressure two additional phase transitions,attributed to the C2/c and disorderedbcc(Im-3m)phases,have been observed near15.5and21.6GPa in good agreement with the structures recentlyobserved by means of x-ray diffraction at high pressures in Bi2Te3.After release of pressure the sample reverts backto the original rhombohedral phase after considerable hysteresis.Raman-and IR-mode symmetries,frequencies,and pressure coefﬁcients in the different phases are reported and discussed.DOI:10.1103/PhysRevB.84.104112PACS number(s):61.50.Ks,62.50.−p,78.20.Ci,78.30.−jI.INTRODUCTIONBismuth telluride(Bi2Te3)is a layered chalcogenide with a tremendous impact for thermoelectric applications.1The thermoelectric properties of Bi2Te3and their alloys have been extensively studied due to their promising operation in the temperature range of300–400K.In fact Bi2Te3is the material with the best thermoelectric performance at ambient temperature.2,3Recently,it has been shown that Bi2Te3can be exfoliated like graphene and that a single layer exhibits high electrical conductivity and low thermal conductivity so that a new nanostructure route can be envisaged to improve dramatically the thermoelectrical properties of this compound by means of either charge-carrier conﬁnement or acoustic-phonon conﬁnement.4,5Bi2Te3is a narrow bandgap semiconductor with tetradymite crystal structure[R-3m,space group(S.G.)166,Z=3].6 This rhombohedral-layered structure is formed by layers, which containﬁve hexagonal close-packed atomic sublayers (Te-Bi-Te-Bi-Te)and is named a quintuple linked by van der Waals forces.The same layered structure is common to other narrow bandgap semiconductor chalcogenides,like Bi2Se3and Sb2Te3,and has been found in As2Te3at high pressures.7 Bi2Te3,as well as Bi2Se3and Sb2Te3,has been recently predicted to behave as a topological insulator8;i.e.,a new class of materials that behave as insulators in the bulk but conduct electrical current in the surface.The topological insulators are characterized by the presence of a strong spin-orbit(SO) coupling that leads to the opening of a narrow bandgap and causes certain topological invariants in the bulk to differ from their values in vacuum.The sudden change of invariants at the interface results in metallic,time-reversal invariant-surface states whose properties are useful for applications in spintronics and quantum computation.9,10Therefore,in recent years a number of papers have been devoted to the search of the 3D-topological insulators among Sb2Te3,Bi2Te3,and Bi2Se3, and different works observed the features of the topological nature of the band structure in the three compounds.11–13 High-pressure studies are very useful to understand mate-rials properties and design new materials because the increase in pressure allows us to reduce the interatomic distances and toﬁnely tune the materials properties.It has been veriﬁed that the thermoelectric properties of semiconductor chalcogenides improve with increasing pressure,and that the study of the properties of these materials could help in the design of better thermoelectric materials by substituting external pressure by chemical pressure.14–18Therefore,the electrical and thermoelectric properties of Sb2Te3,Bi2Te3,and Bi2Se3, as well as their electronic-band structure,have been studied at high pressures.19–27In fact a decrease of the bandgap energy with increasing pressure was found in Bi2Te3.19,20 Furthermore,recent high-pressure studies in these compounds have shown a pressure-induced superconductivity28,29that has further stimulated high-pressure studies.30However,the pressure dependence of many properties of these layered chalcogenides is still not known.In particular the determi-nation of the crystalline structures of these materials at high pressures has been a long puzzle15,23,31,32and the space groups of the high-pressure phases of Bi2Te3have been elucidatedR.VILAPLANA et al.PHYSICAL REVIEW B84,104112(2011)only recently by powder x-ray diffraction measurements at synchrotron-radiation sources33,34specially with the use of particle-swarm optimization algorithms for crystal-structure prediction.34Recent high-pressure powder x-ray diffraction measure-ments have evidenced a pressure-induced electronic topolog-ical transition(ETT)in Bi2Te3around3.2GPa as a changein compressibility.29,31,32,35,36An ETT or Lifshitz transitionoccurs when an extreme of the electronic-band structure,whichis associated to a Van Hove singularity in the density of states,crosses the Fermi-energy level.37This crossing,which canbe driven by pressure,temperature,doping,etc.,results in achange in the topology of the Fermi surface that changes theelectronic density of states near the Fermi energy.An ETTis a2.5transition in the Ehrenfest description of the phasetransitions so no discontinuity of the volume(ﬁrst derivativeof the Gibbs free energy)but a change in the compressibility(second derivative of the Gibbs free energy)is expected inthe vicinity of the ETT.Anomalies in the phonon spectrumare also expected for materials undergoing an ETT38,39andhave been observed in a number of materials40,41as well as inSb1.5Bi0.5Te3.31The lattice dynamics of Bi2Te3have been studied ex-perimentally at room pressure42–44and a recent study sug-gests that Raman spectroscopy can be used to monitorthe number of single quintuple layers in nanostructuredBi2Te3,as in graphene.45Theoretical studies of the lat-tice dynamics of Bi2Te3at room pressure have also beenperformed;46–49however,Raman measurements at high pres-sures have only been reported up to0.5GPa,50and to ourknowledge there is no theoretical study of the lattice dynamicsproperties of Bi2Te3under high pressure.As a part of oursystematic study of the structural stability and the vibrationalproperties of the semiconductor chalcogenide family,wereport in this work room-temperature Raman-scattering mea-surements in Bi2Te3up to23GPa together with total-energyand lattice-dynamical ab initio calculations at different pres-sures.We discuss the recent observation of a pressure-inducedETT in the rhombohedral phase ofα-Bi2Te3and study whetherthe Raman-scattering signal of the Bi2Te3at pressures above7.4GPa match with the proposed high-pressure phases recentlyreported for this compound33,34and which have also beenfound in Sb2Te3at high pressures.51II.EXPERIMENTAL DETAILSWe have used single crystals of p-type Bi2Te3that weregrown using a modiﬁed Bridgman technique.A polycrystallineingot was synthesized by the reaction of stoichiometricquantities of the constituting elements(5N).Afterward,thepolycrystalline material was annealed and submitted to thegrowth process in a vertical Bridgman furnace.Preliminaryroom-temperature measurements on single crystalline samples(15mm×4mm×0.3mm)yield in-plane electrical resistivityρ⊥c=1.7·10−5 m and Hall coefﬁcient R H(B c)= g0.52cm3C−1.Following the calculation presented in Ref.52,the latter gives hole concentration of7.2·1018cm−3andminority electron concentration of2.1·1017cm−3.A smallﬂake of the single crystal(100μm×100μm×5μm)was inserted in a membrane-type diamond anvil cell(DAC)with a4:1methanol-ethanol mixture as pressure-transmitting medium,which ensures hydrostatic con-ditions up to10GPa and quasihydrostatic conditions between 10and23GPa.53,54Pressure was determined by the ruby luminescence method.55Unpolarized room-temperature Raman-scattering measure-ments at high pressures were performed in backscattering geometry using two setups:(i)A Horiba Jobin Yvon LabRAM HR microspectrometer equipped with a TE-cooled multichan-nel CCD detector and a spectral resolution below2cm−1. HeNe laser(6328˚A line)was used for excitation.(ii)A Horiba Jobin Yvon T64000triple-axis spectrometer with resolution of1cm−1.In this case an Ar+laser(6470˚A line)was used for excitation.In order not to burn the sample,power levels below2mW were used inside the DAC.This power is higher than that used in Raman measurements at room pressure due to superior cooling of the sample in direct contact with the pressure-transmitting media and the diamonds.Optical transmission and reﬂection measurements under pressure were performed by putting the DAC in a home-built Fourier Transform infrared(FTIR)setup operating in the mid-IR region(400–4000cm−1).The pressure-transmitting medium was KBr.The setup consists of a commercial TEO-400FTIR interferometer by ScienceTech S.L.,which includes a Globar thermal-infrared source and a Michelson interferome-ter,and a liquid-nitrogen cooled Mercury-Cadmium-Telluride (MCT)detector with wavelength cutoff at25μm(400cm−1) from IR Associates Inc.A gold-coated parabolic mirror focuses the collimated IR beam onto a calibrated iris of1 to3mm diameter.A gold-coated X15Cassegrain microscope objective focuses the IR beam inside the DAC to a size of70–200μm.A second Cassegrain microscope objective collects the transmitted IR beam and sends it to the detector after being focused by another parabolic mirror.In the reﬂection conﬁguration,aﬂat gold mirror is placed at45◦before the focusing Cassegrain objective,blocking half of the IR beam. The half-beam let into the DAC is reﬂected by the sample, then by theﬂat gold mirror,andﬁnally focused on the MCT detector by another parabolic mirror.III.AB INITIO CALCULATIONSTwo recent works have reported the structures of the high-pressure phases of Bi2Te3up to52GPa.33,34The rhombohedral(R-3m)structure(α-Bi2Te3)is suggested to transform to the C2/m(β-Bi2Te3,S.G.12,Z=4)and the C2/c (γ-Bi2Te3,S.G.15,Z=4)structures above8.2and13.4GPa, respectively.34Furthermore,a fourth phase(δ-Bi2Te3)has been found above14.5GPa and assigned to a disordered bcc structure(Im-3m,S.G.229,Z=1).33,34In order to explore the relative stability of these phases in Bi2Te3we have performed ab initio total-energy calculations within the density functional theory(DFT)56using the plane-wave method and the pseudopotential theory with the Vienna ab initio simulation package(V ASP)57We have used the projector-augmented wave scheme(PAW)58implemented in this package.Ba-sis set,including plane waves up to an energy cutoff of 320eV,were used in order to achieve highly converged results and accurate descriptions of the electronic properties. We have used the generalized gradient approximation(GGA)HIGH-PRESSURE VIBRATIONAL AND OPTICAL STUDY...PHYSICAL REVIEW B84,104112(2011)for the description of the exchange-correlation energy with the PBEsol59exchange-correlation prescription.Dense special k-points sampling for the Brillouin zone(BZ)integration were performed in order to obtain very well-converged energies and forces.At each selected volume,the structures were fully relaxed to their equilibrium conﬁguration through the calculation of the forces on atoms and the stress tensor.In the relaxed equilibrium conﬁguration,the forces on the atoms are less than0.002eV/˚A and the deviation of the stress tensor from a diagonal hydrostatic form is less than1kbar(0.1GPa). Since the calculation of the disordered bcc phase was not possible to do,we have attempted to perform calculations for the bcc-like monoclinic C2/m structure proposed in Ref.34. The application of DFT-based total-energy calculations to the study of semiconductors properties under high pressure has been reviewed in Ref.60,showing that the phase stability, electronic and dynamical properties of compounds under pressure are well describe by DFT.Furthermore,since the calculation of the disordered bcc phase is not possible to do with the V ASP code,we have attempted to perform calculations for the bcc-like mono-clinic C2/m structure proposed in Ref.34.Also,because the thermodynamic-phase transition between two structures occurs when the Gibbs free energy(G)is the same for both phases,we have obtained the Gibbs free energy of the different phases using a quasiharmonic Debye model61that allows obtaining G at room temperature from calculations performed for T=0K in order to discuss the relative stability of the different phases proposed in the present work.In order to fully conﬁrm whether the experimentally mea-sured Raman scattering of the high-pressure phases of Bi2Te3 agree with theoretical estimates for these phases,we have also performed lattice-dynamics calculations of the phonon modes in the R-3m,C2/m,and C2/c phases at the zone center ( point)of the BZ.Our theoretical results enable us to assign the Raman modes observed for the different phases of Bi2Te3. Furthermore,the calculations also provide information about the symmetry of the modes and polarization vectors,which is not readily accessible in the present experiment.Highly converged results on forces are required for the calculation of the dynamical matrix.We use the direct-force constant approach(or supercell method).62Highly converged results on forces are required for the calculation of the dynamical matrix. The construction of the dynamical matrix at the point of the BZ is particularly simple and involves separate calculations of the forces in which aﬁxed displacement from the equilibrium conﬁguration of the atoms within the primitive unit cell is considered.Symmetry aids by reducing the number of such independent displacements,reducing the computational effort in the study of the analyzed structures considered in this work.Diagonalization of the dynamical matrix provides both the frequencies of the normal modes and their polarization vectors.It allows to us to identify the irreducible representation and the character of the phonon’s modes at the point.In this work we provide and discuss the calculated frequencies and pressure coefﬁcients of the Raman-active modes for the three calculated phases of Bi2Te3.The theoretical results obtained for infrared-active modes for the three calculated phases of Bi2Te3are given as supplementary material of this article.63Finally,we want to mention that we have also checked the effect of the SO coupling in the structural stability and the phonon frequencies of the different phases.We have found that the effect of the SO coupling is very small and did not affect our present results(small differences of1–3cm−1in the phonon frequencies at the point)but increased substantially the computer time so that the cost of the computation was very high for the more complex monoclinic high-pressure phases, as already discussed in Ref.34.Therefore,all the theoretical values corresponding to lattice-dynamics calculations in the present paper do not include the SO coupling.In order to test our calculations,we show in Table I the calculated lattice parameters in the different phases of Bi2Te3at different pressures.For the sake of comparison we show in Table I other theoretical calculations and experimental results available.As far as the R-3m phase is concerned,our calculated lattice parameters are in relatively good agreement with experimental values from Refs.6and36.Our calculations with GGA-PBEsol give values which are intermediate between those calculated with GGA-PBE and local density approximation (LDA),as it is generally known.Additionally,we give the calculated lattice parameters of Bi2Te3in the monoclinic C2/m and C2/c structures at7.7and15.5GPa,respectively,for comparison with experimental data.Note that in Table I the a and b lattice parameters of the C2/m and C2/c structures at7.7and15.5GPa are very similar to those reported by Zhu et al.;34however,the c lattice parameter andβangle for monoclinic C2/m and C2/c structures differ from those obtained by Zhu et al.34The reason is the results of our ab initio calculations are given in the standard setting for the monoclinic structures,in contrast with Ref.34,for a better comparison to future experiments since many experimentalists use the standard setting.IV.RESULTS AND DISCUSSIONA.Optical absorption ofα-Bi2Te3under pressureIt is known thatα-Bi2Te3has an indirect forbidden bandgap, E gap,between130and170meV.19,64–66Figure1shows the optical transmittance of ourα-Bi2Te3sample in the mid-IR region at room pressure outside the DAC.The spectrum near the fundamental absorption edge is dominated by large interferences.The large amplitude of the interference fringe pattern in the transparent region is a result of the high value of the refractive index,that is larger than9.42,65,66The sample transmittance and the interference-fringe amplitude decreases at low-photon energy due to the onset of free-carrier absorption and to high energies due to the fundamental absorption edge caused by band-to-band absorption.The absorption coefﬁcient can be accurately determined from the transmittance spectrum only in a small photon energy range between the end of the interference pattern and the photon energy at which the transmitted intensity merges into noise.In this interval the absorption coefﬁcient exhibits an exponential dependence on the photon energy.This prevents a detailed analysis of the absorption edge shape.Consequently,the optical bandgap has been determined byﬁtting a calculated transmittance to the experimental one.We calculate the transmittance by assumingR.VILAPLANA et al.PHYSICAL REVIEW B 84,104112(2011)TABLE I.Calculated (th.)and experimental (exp.)lattice parameters,bulk modulus (B 0),and its derivative (B 0 )of Bi 2Te 3in the R -3m structure at ambient pressure and calculated lattice parameters of Bi 2Te 3in the C 2/m and C 2/c structures at 8.4and 15.5GPa,respectively.a(˚A)b(˚A)c(˚A)β(∞)B 0(GPa)B 0 Ref.α-Bi 2Te 3(0GPa)th.(GGA-PBEsol)4.38029.98241.924.89This work th.(GGA-PBESol)a 4.37530.16741.614.68This workth.(GGA-PBE)4.4531.6349th.(GGA-PBE)a 4.4731.1249th.(LDA)a 4.3630.3847exp.4.38530.4976exp.4.38330.38032.5b 10.1b 3640.9c3.2c β-Bi 2Te 3(8.4GPa)th.(GGA-PBESol)14.8834.0669.12189.7341.254.06This workth.(GGA-PBE)d 14.8654.05617.468148.3934exp.d14.6454.09617.251148.4834γ-Bi 2Te 3(15.5GPa)th.(GGA-PBESol)9.8956.9627.70970.3045.283.57This workth.(GGA-PBE)e 9.9567.14610.415134.8634exp.e10.2336.95510.503136.034a Calculations including the SO coupling.bAt room pressure.cAbove 3.2GPa.dAround 12-12.6GPa.eAround 14-14.4GPa.an absorption coefﬁcient with two termsα(E )=A E 2+Be−E gap −E (1)where the ﬁrst one corresponds to the free-carrier contribution and the second one corresponds to the Urbach tail of the fundamental absorption edge.Equation (1)was used to ﬁt the calculated transmittance spectra to the experimental ones.The dotted line in Fig.1was calculated with Equation (1)by using only A and E gap as ﬁtting parameters,where E gap =159meV at roompressure.FIG.1.(Color online)Experimental transmittance of a 7-μm-thick α-Bi 2Te 3sample at room pressure outside the DAC (solid line).Dotted line indicates the ﬁt of the experimental spectrum.Figure 2shows the Bi 2Te 3transmittance spectrum for several pressures up to 5.5GPa.Above that pressure the signal-to-noise ratio is too low to determine the optical bandgap energy.Figure 3shows the pressure dependence of the optical bandgap of Bi 2Te 3,as determined from the previously described procedure.The pressure coefﬁcient turns out to be −6.4±0.6meV /GPa.This pressure coefﬁcient of the optical bandgap is close to the value we obtained for the pressure dependence of the indirect bandgap from ab initio calculations (−10meV /GPa).From this result it appears that,even if the sample becomes opaque at 5.5GPa,Bi 2Te 3still has a ﬁnite bandgap of some 120meV.FIG.2.(Color online)Experimental transmittance of α-Bi 2Te 3at different pressures up to 5.5.GPa.A shift of the absorption edge to low energies is observed with increasing pressure.HIGH-PRESSURE VIBRATIONAL AND OPTICAL STUDY...PHYSICAL REVIEW B84,104112(2011)FIG.3.(Color online)Pressure dependence of the optical bandgap ofα-Bi2Te3according to reﬂectance(red squares)and to transmittance(black circles)measurements.Sample opacity above5.5GPa seems to be then a result of the free-carrier absorption tail shifting to higher energies as the carrier concentration increases.Consequently,the sample opacity is likely caused by the overlap of the free-carrier absorption tail with the fundamental-absorption tail rather than a real closure of the bandgap.We have to note that our pressure coefﬁcient of the optical bandgap is somewhat smaller in module than the pressure coefﬁcient previously reported for the indirect bandgap:−22meV/GPa19;−12meV/GPa below3GPa;and−60meV/GPa above3GPa.20We have to consider that the estimation of these pressure coefﬁcients in Refs.19and20were indirectly obtained from the pressure dependence of the electrical conductivity and those estimations suffer considerable errors since they assume that the change in resistivity is only due to the change of the indirect bandgap energy,which is not a well-founded assumption in extrinsic degenerate semiconductors.In order to conﬁrm our results on optical absorption we have performed high-pressure reﬂectance measurements in a3-μm-thick sample whose results are shown in Fig.4.The reﬂectance spectrum also exhibits a large interference fringe pattern in the transparency region,with amplitude decreasing to lowand high photon energies.The reﬂectance spectrum at6GPaFIG.4.(Color online)Experimental reﬂectance ofα-Bi2Te3at different pressures.shows that the sample exhibits a clear onset of the fundamental absorption edge at around120meV and also that the free-carrier absorption edge,even if it has shifted to higher energies, has not overlapped the fundamental absorption.Therefore our reﬂectance measurements allow us to conﬁrm the results obtained from absorption measurements.Furthermore,the bandgap pressure coefﬁcient,as determined from the shift of the photon energy at which interferences disappear,agrees with the one determined from the transmission spectra.At 7GPa,a clear change in the reﬂectance occurs,with a large increase of the reﬂectance by80%in the low-energy range.A large reﬂectance minimum(not shown here)appears at some 4000cm−1(500meV),suggesting a phase transition to a metallic phase.The metallic nature of the high-pressure phases is in good agreement with previously reported resistivity measurements.17,21,28–30If the reﬂectance minimum is taken as an estimation of the plasma frequency of the high-pressure phase above7GPa,the carrier concentration would be larger than1021cm−3(assuming the same dielectric constant as in the rhombohedral phase).If the dielectric constant inβphase is much smaller,the carrier concentration should be close to1022cm−3,which is more consistent with the observed superconducting behavior.28–30The shift of the free-carrier absorption tail follows the in-crease of the free-carrier plasma frequency.Then the pressure dependence of the plasma frequency can be estimated from the shift of the photon energy at which the free-carrier absorption tail quenches the interference fringe pattern.Reﬂectance measurements outside the cell show that the plasma frequency at ambient pressure is below50meV,consistently with the hole concentration that is of the order of7·1018cm−3,as measured by Hall effect.At4.3GPa interference fringes are observed down to some60meV(560cm−1).This upper limit to the plasma frequency would correspond to hole concentration of lower than1019cm−3,typical of a degenerate semiconductor.This increase in the hole concentration should result in a Burstein-Moss positive contribution to the optical bandgap, which explains the discrepancy between the experimental and theoretical value of the bandgap pressure coefﬁcient.The bandgap around5GPa is in fact smaller than the measured optical gap.Given the band structure of Bi2Te3,67with six equivalent minima in the valence band,the density of states is very large and the hole concentration per minimum would be only of some1.5×1018cm−3,which would lead to a Burstein-Moss shift of some50meV for a hole effective mass of0.09m0.68Then even taking into account the Burstein-Moss shift,Bi2Te3at5GPa would still be a low-gap semiconductor. In fact this estimation of the Burstein-Moss shift is based on the ambient-pressure electronic structure.At pressures above the ETT transition the density of states in the valence-band maximum is expected to be much larger as the ellipsoids merge into a thoroidal ring,as proposed by Istkevitch et al.69 Consequently,the Burstein-Moss shift above the ETT should be much lower than50meV.Finally,we must note that our analysis of the optical absorption edge in Bi2Te3have not allowed us to detect any change in the pressure dependence of the indirect bandgap around3GPa to conﬁrm the presence of an ETT as observed in other works.20,29,31,32,35,36The very small change in the pressure coefﬁcient of the indirect bandgap seems not toR.VILAPLANA et al.PHYSICAL REVIEW B 84,104112(2011)FIG.5.Experimental Raman spectra of α-Bi 2Te 3at pressures between room pressure and 7.4GPa.be affected by the ETT since there is no change in volume but in volume compressibility,and the change is very subtle to be measured in our transmission or reﬂection spectra in comparison with the drastic effects observed in transport measurements or even in the parameters of the Raman modes (as will be discussed in the next section).B.Raman scattering of α-Bi 2Te 3under pressureThe rhombohedral structure of α-Bi 2Te 3is a centrosym-metric structure that has a primitive cell with one Te atom located in a 3a Wyckoff position and the remaining Bi(2)and Te(2)atoms occupying 6c Wyckoff sites.Therefore,group theory allows 10zone-center modes,which decompose in the irreducible representations as follows 70:10=2A 1g +3A 2u +2E g +3E u .(2)The two acoustic branches come from one A 2u and a doubly degenerated E u mode,while the rest correspond to optic modes.Gerade (g)modes are Raman active while ungerade (u)modes are infrared (IR)active.Therefore,there are four Raman-active modes (2A 1g +2E g )and four IR-active modes (2A 2u +2E u ).The E g modes correspond to atomic vibrations in the plane of the layers,while the A 1g modes correspond to vibrations along the c axis perpendicular to the layers.42–44,50Figure 5shows the experimental Raman spectra of α-Bi 2Te 3at different pressures up to 7.4GPa.We have observed and followed under pressure three out of the four Raman-active modes.The E g mode calculated to be close to 40cm −1has not been observed in our experiments as it was also not seen in previous Raman-scattering measurements at room and high pressures.42,50,71–73Figure 6(a)shows the experimental-pressure dependence of the frequencies of the three ﬁrst-orderRaman modes measured in α-Bi 2Te 3,and Table II summarizes our experimental and theoretical ﬁrst-order Raman-mode frequencies and pressure coefﬁcients in the rhombohedral phase.Our experimental frequencies at room pressure are in good agreement with those already measured in Ref.42and Ref.50and with those recently measured in Refs.45and 71–73.On the other hand our theoretical frequencies at room pressure are also in good agreement with those reported in Ref.49without SO coupling (see Table II )and are slightly larger than those calculated including SO coupling (see Ref.49).In Fig.6(a)it can be observed that all the measured Raman modes exhibit a hardening with increasing pressure.The experimental values of the pressure coefﬁcients of the Raman-mode frequencies are in a general good agreement with our theoretical calculations and with the values reported in Ref.50up to 0.5GPa;however,a decrease of the pressure coefﬁcient of two modes around 4.0GPa should be noted [see dashed lines in Fig.6(b)].We have attributed the less positive pressure coefﬁcient of these two Raman modes to the pressure-induced ETT observed in Sb 2Te 3and Bi 2Te 3.20,29,31,32,35,36In fact in a previous study in Sb 2Te 3under pressure we have found a change in the pressure coefﬁcient of the frequency of all modes measured.51In order to support our hypothesis we also plot as Fig.6(b)the pressure dependence of the full width at half maximum (FWHM)of the three measured Raman modes.Curiously,it is observed that the FWHM changes its slope around 4GPa thus conﬁrming an anomaly related to the ETT.Therefore,both our results of the pressure dependence of the frequency and linewidth give support to the observation of the ETT around 4.0GPa in α-Bi 2Te 3similarly to the case of α-Sb 2Te 3.51As previously commented,anomalies in the phonon spec-trum are also expected for materials undergoing a ETT and have been observed in Sb 1.5Bi 0.5Te 3.15In the latter work the high-frequency A 1g mode was not altered near the ETT in good agreement with our measurements;however,we have noted a change both in the lower A 1g and the higher-frequency E g modes.Since A 1g modes are polarized in the direction perpendicular to the layers while the E g modes are polarized along the layers,our observation of a less positive pressure coefﬁcient at 4.0GPa of both modes in α-Bi 2Te 3suggests that the ETT in Bi 2Te 3is related to a change of the structural compressibility of both the direction perpendicular to the layers and the direction along the layers.This seems not to be in agreement with Polian et al.’s observations,which suggest that the ETT in Bi 2Te 3only affects the plane of the layers.36Consequently,more work is needed to understand the mechanism of the ETT in this material.To conclude this section regarding the rhombohedral structure of α-Bi 2Te 3,we want to make a comment on the pressure coefﬁcients of the Raman modes of this structure in comparison to those recently measured in α-Sb 2Te 3.51It is known that in chalcogenide laminar materials,the two lowest frequency E and A modes are usually related to shear vibrations between adjacent layers along the a -b plane and to vibrations of one layer against the others along the c axis,respectively.It has been commented that the E mode displays the smallest pressure coefﬁcient due to the weak bending force constant between the interlayer distances (in our case,Te-Te distances)while。

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Preparation of Polymer/Silica Composite Nanoparticles Bearing Carboxyl Groups on the Surface viaEmulsiﬁer-Free Emulsion CopolymerizationZHONG ZENG,JIAN YU,ZHAO-XIA GUOInstitute of Polymer Science and Engineering,Department of Chemical Engineering,School of Materials Science and Engineering,Tsinghua University,Beijing100084,ChinaReceived3November2004;accepted30January2005DOI:10.1002/pola.20764Published online in Wiley InterScience().ABSTRACT:Polymer/silica organic/inorganic composite nanoparticles bearing carboxylgroups on the surface were prepared via the emulsiﬁer-free emulsion copolymeriza-tion of methyl methacrylate and sodium methacrylate(NaMA).Carboxyl groups weregenerated by the addition of hydrochloric acid at the end of the copolymerization.Two methods of NaMA addition were studied:batch and two-stage procedures.Thebatch procedure allowed only a limited number of carboxyl groups to effectively bondto the composite nanoparticles.In contrast,the number of carboxyl groups could bealtered over a wide range with the two-stage procedure.Fourier transform infraredspectroscopy and chemical titration were independently used to quantify the numberof carboxyl groups,giving values close to each other and to the feed.A kinetic studyindicated that the copolymerization followed a mechanism different than that foundearlier.The average size of the composite nanoparticles was approximately40nm,asmeasured by both transmission electron microscopy(TEM)and laser scattering,andtheir polydispersity index was close to1,indicating a fairly narrow size distribution.TEM photographs of the composite nanoparticles showed a multilayered core–shell structure with one silica bead as the core and with poly(methacrylate acid)as the outmost shell.V C2005Wiley Periodicals,Inc.J Polym Sci Part A:Polym Chem43:2826–2835,2005Keywords:composite nanoparticles;core-shell polymers;emulsiﬁer-free;emulsion;copolymerization;functionalization of polymersINTRODUCTIONPolymer/inorganic composite nanoparticles are of growing interest not least because of their extensive applications asﬁllers in polymer-based nanocomposites.1–3The advantages of such composite nanoparticles over pure inor-ganic nanoparticles are obvious:not only is the composite material strengthened by the inor-ganic moiety,but the compatibility between the ﬁller and the matrix is improved by the polymer moiety.Such composite nanoparticles can be prepared mainly by the encapsulation or graft-ing of polymers onto the surface of inorganic nanoparticles.An enormous amount of work has been reported,involving various inorganic par-ticles such as silica,4–6calcium carbonate,7tita-nium oxide,8–10and alumina11(among them, silica is the most widely studied).Encapsulation can be roughly divided into two types,physical and chemical,according on whether there is any chemical bonding at the interface between the inorganic nanoparticles andCorrespondence to:J.Yu(E-mail:yujian03@mail. )Journal of Polymer Science:Part A:Polymer Chemistry,Vol.43,2826–2835(2005) V C2005Wiley Periodicals,Inc.2826the polymer.Although physical encapsulation is simple,12,13chemical encapsulation is more inter-esting because of the stronger interfacial interac-tion provided by the covalent bonding at the inter-face.14,15There are mainly two approaches to achieve chemical encapsulation.One involves the use of inorganic nanoparticles pretreated with a silane coupling agent bearing a polymerizable dou-ble bond as a comonomer during the polymeriza-tion of vinyl monomers.16,17The other uses pre-treated silica as a macroinitiator for living polymerization.6,18,19For the former,the main polymerization techniques include emulsion and dispersion polymerization.Covalent bonds are formed between nanoparticles and polymer chains during encapsulation.For the latter,polymeriza-tion starts from the surface of inorganic nanopar-ticles,and encapsulation is formed as polymeriza-tion proceeds.As for the grafting of polymers onto inorganic nanoparticles,there are three main approaches that depend on the order of polymerization and grafting:grafting-from,20grafting-through,21and grafting-onto.22With the grafting-from approach, pretreated inorganic particles bearing initiating groups such as azo groups are used as initiators, and consequently polymerization starts from the surface of inorganic particles.The grafting-through approach uses pretreated inorganic nano-particles as comonomers of vinyl monomers.Graft-ing is achieved during polymerization.It may not necessarily form encapsulation;this depends on the polymerization conditions.As for the grafting-onto approach,prepolymers containing reactive groups are attached to inorganic particles.Although composite nanoparticles often show better dispersion and compatibility when used asﬁllers for polymers,the interfacial interaction between the composite nanoparticles and the polymer matrix is based only on physical com-patibility offered by the polymer shell and can certainly be improved further by the formation of chemical bonds,just like in the case of reac-tive blending.23For this purpose,composite nanoparticles bearing functional groups such as carboxyl groups(called reactiveﬁllers)are needed and can potentially be useful in reinforc-ing polymers having functional groups that can react with the functional groups of theﬁller.As part of an ongoing project aimed at the prep-aration of reactiveﬁllers,we synthesized epoxy-functionalized polystyrene/silica composite nano-particles with a core–shell structure via emulsion polymerization.24A mixture of ionic and nonionicemulsiﬁers was used to stabilize the particles, which could partly remain in the latices as impur-ities(it is known that the emulsiﬁers used in emulsion polymerization are very hard to remove completely)and consequently limit the applica-tions of the latices to some degree.Emulsiﬁer-free emulsion polymerization has been receiving considerably more attention in recent years because it can produce clean and monodisperse latices.25–28The emulsiﬁer-free systems are often not truly free of an emulsiﬁer in the strictest sense as the name indicates.The monomer or comonomer usually contains a part that resembles the structure of an emulsiﬁer at one end of the molecular chain.Such a monomer or comonomer can play the role of an emulsiﬁer while polymerizing.29Sodium methacrylate (NaMA)is one such comonomer.It is an ionic vinyl monomer with sodium carboxylate salt at one end of the molecule and a double bond at the other end,and it has been used to conduct emulsiﬁer-free emulsion copolymerization.30 In this study,the emulsiﬁer-free emulsion copolymerization of methyl methacrylate(MMA) was carried out to prepare reactive polymer/silica composite nanoparticles with NaMA as a comono-mer.Carboxyl groups were generated after the copolymerization by neutralization with hydro-chloric acid(HCl);this made the composite nano-particles reactive.With their interesting charac-teristics(clean surface,reactive and ionizable properties,and core–shell structure),such func-tional composite nanoparticles are expected to be used not only as reactiveﬁllers but also in a wide range of applications such as protein carriers, microcapsules,water puriﬁers,and polymer cata-lysts.The method and amount of NaMA addition, the copolymerization kinetics,the carboxyl con-tent,and the morphology of the composite nano-particles are investigated here in detail. EXPERIMENTALMaterialsNanometer silica1065nm in diameter was acquired from Zhoushan Mingri Nanomaterial, Ltd.(China),and used after drying in vacuo at 1058C for12h.The pretreatment of silica with 3-methacryloxypropyltrimethoxysilane(MPTMS) was carried out with a previously published pro-cedure.24The monomers,both MMA and metha-crylate acid(MAA),were distilled under reduced pressure before use.NaMA was obtained by the POLYMER/SILICA COMPOSITE NANOPARTICLES2827neutralization of MAA with an equal molar amount of NaOH at08C.Ammonium persulfate (APS)was freshly recrystallized from water.All other reagents were used as received. CopolymerizationThe emulsiﬁer-free emulsion copolymerization was carried out under a nitrogen atmosphere in a250-mL,four-neckedﬂask with a mechanical stirrer,thermometer,and condenser.For the whole process,the reaction was in a water bath, and the stirring rate wasﬁxed at150rpm. Batch ProcedureNaMA was introduced into a reactor charged with distilled water at408C.After20min of stirring at408C,the temperature was raised to 508C,and then a mixture of SiO2and MMA, which was treated by ultrasonic irradiation for 10min just before it was used,was added.After 10min of stirring at508C,the system was raised to608C,and a solution of APS in water was added to initiate the copolymerization.The reaction proceeded at808C for2h,and then it was raised to and held at908C for30min more. Excessive HCl was added to translate the acryl-ate into carboxyl groups;this also led to demul-siﬁcation.The product was washed thoroughly with hot water and then dried.Two-Stage ProcedureTheﬁrst stage was similar to the batch proce-dure.After1h of stirring at808C,the second stage of copolymerization was started by the feeding of aqueous NaMA(at a rate of10mL/h)dropwise to the system.After the completion of aqueous NaMA addition,it was stirred at808C for30min and then was raised to and held at 908C for30min more.Excessive HCl was added to translate the acrylate into carboxyl groups; this also led to demulsiﬁcation.The recipes of all runs are listed in Table1.The yield and conversion of the monomers were deter-mined by the gravimetric method as follows: Yieldð%Þ¼Total product(g)Total monomer(g)and SiO2ðgÞÂ100 Conversionð%Þ¼Polymer former(g)Monomer used(g)Â100To estimate the strength of the interaction between either PMMA and silica or PMMA and poly(methacrylate acid)(PMAA),the sample was extracted with chloroform for12h with a Soxh-let apparatus,and then the binding efﬁciency was calculated as follows:Binding efficiency(%)¼Polymer grafted(g)Polymer formed(g)Â100CharacterizationA latex sample was used directly for the mor-phology observation and particle size and size distribution determination with a JEOL200CX transmission electron microscope(dyed by RuO4 for20min)or a Zetaparticle HS3000laser scat-tering(LS)particle size and z-potential analyzer. Na was used to denote the number of silica beads per particle,and it was be calculated with a formula reported by Bourgeat-Lami andTable1.Recipes of Emulsiﬁer-Free Emulsion CopolymerizationRun No.NaMA(mmol)MMA(mL)APS(mmol)H2O(mL)HCI(mmol) Portion1Portion2A a B bBatch series B1 2.1—15.00.2850.0— 5.0 B2 2.4—15.00.2850.0— 5.0B3 2.7—15.00.2850.0— 5.0B4 3.0—15.00.2850.0— 6.0B5 3.3—15.00.2850.0— 6.0 Two-stage series T1 2.7 5.515.00.2850.0 5.010.0 T2 2.713.715.00.2850.0 5.020.0T3 2.730.215.00.2850.010.050.0T4 2.746.715.00.2850.015.0100.0T5 2.763.215.00.2850.020.0100.0a Added before theﬁrst part of NaMA was introduced.b Used to dissolve the second part of NaMA.2828ZENG,YU,AND GUOLang.31The core–shell structure of composite nanoparticles can be considered well-deﬁned,that is,only one silica bead per particle,when the value of Na ranges from 0.95to 1.05.The number-average diameter (D n )and the weight-average diameter (D w )were calculated with the following equations:D n ¼XiN i D i .X iN i :D w ¼XiN i D 4i.XiN i D 3i :where N i (i ¼1,2,...)is the number of the par-ticles with the size of D i (i ¼1,2,...).Both N i andD i are given by LS measurement.At least 105par-ticles (i.e.,Pi N i ,measured by LS)were counted for each calculation.The polydispersity index (PDI)of the particle size was expressed as D w /D n .A value ranging from 1.00to 1.05can be regarded as a monodisperse distribution of the particle size.Determination of the Contents of Carboxyl Groups The contents of carboxyl groups in the ﬁnal car-boxyl-functional composite nanoparticles were determined by both Fourier transform infrared (FTIR)and chemical titration methods.In the for-mer,products bearing various contents of carboxyl groups after Soxhlet extraction were analyzed by FTIR with a Nicolet 560FTIR spectrometer.The vibration peak of the carbonyl group (1738cm À1)was taken as the reference peak.Thus,the area ratio of the carboxyl peak (1703cm À1)to the refer-ence peak could be calculated.The quantiﬁcations of the carboxyl contents (with respect to PMMA)were determined,with a calibration curve obtained from mixtures of MAA and MMA with different ratios.In the latter (i.e.,the titration method),a sample containing a product with a known mass was swollen in 1,4-dioxane for 24h.Then,a standardized NaOH solution with a known volume was introduced to neutralize the carboxyl group of the sample.The excess NaOH was then titrated by a standardized solution of an HCl–dioxane reagent with cresol red as an indica-tor.Experimental error due to dissolved CO 2was minimized by the performance of the titration under a nitrogen atmosphere.The difference between the number of moles of NaOH originally added and that neutralized by HCl equaled the number of moles of carboxyl groups.32Data from a minimum of three sets of these analyses were averaged.RESULTS AND DISCUSSIONBatch ProcedureIn this emulsiﬁer-free emulsion copolymeriza-tion system,the hydrophilic ionic acrylate end and the hydrophobic carbon–carbon double-bond end made NaMA amphiphilic,so the role of NaMA was twofold:emulsiﬁer and comonomer.Because silica was pretreated with MPTMS,it was hydrophobic and could be well dispersed in MMA after an ultrasonic treatment.WhenaScheme 1.Preparation of polymer/silica composite nanoparticles bearing carboxyl groups on the surface.POLYMER/SILICA COMPOSITE NANOPARTICLES 2829mixture of MPTMS-treated silica and MMA was introduced into the system containing the emul-siﬁer(NaMA),it broke and formed small drop-lets under the drive of emulsiﬁcation and shear, and MMA absorbed onto the hydrophobic sur-face of MPTMS-treated silica.As an emulsiﬁer, NaMA molecules covered the surfaces of the droplets and stabilized them.The hydrophilic ionic acrylate ends made NaMA molecules approach a water phase rather than mix with MMA homogeneously and go inside the droplets. Meanwhile,the hydrophobic double-bond ends admixed with the oil droplets and were oriented toward the center of the droplets,which could copolymerize with MMA and act as a comono-mer(Scheme1).Our previous work on the encapsulation of MPTMS-treated silica by poly-mers via emulsion polymerization has shown that the amount of the emulsiﬁer is very impor-tant with respect to the binding efﬁciency.24 When excess emulsiﬁers were used,free latices formed,and this led to decreased binding efﬁ-ciency.When the amount of the emulsiﬁers was not sufﬁcient,partial demulsiﬁcation easily hap-pened,especially in the presence of polar como-nomers,and this also led to decreased binding efﬁciency.Therefore,a series of copolymeriza-tions with different amounts of NaMA were per-formed,as shown in Table1.Figure1shows the effects of the amount of NaMA on the yield and binding efﬁciency.The trends of these two parameters are similar to those observed in our previous work mentioned previously in which sodium dodecyl sulfonate (SDS)was used as the emulsiﬁer.It is clear that the optimal amount of NaMA was around 2.7mmol,at which both the yield and binding efﬁciency were higher than90%.With either less or more NaMA,the binding efﬁciency decreased, and partial demulsiﬁcation or the formation of free latices occurred,respectively.It seems that NaMA behaves like the normal emulsiﬁer SDS.The average size(D n)and size distribution (PDI)of the composite nanoparticles obtained with different recipes are listed in Table2.With an increasing amount of NaMA,the average particle size decreased gradually.Again,NaMA showed a typical behavior of a normal emulsi-ﬁer.Only when the amount of NaMA was in the range of the optimal value did the obtained com-posite nanoparticles have a narrow size distribu-tion,and they could be considered monodisperse because the PDI was equal to1.04.With less or more NaMA,either the agglomeration of par-ticles(due to less emulsiﬁer)or the formation of free latices without a silica core widened the size distribution.Two-Stage ProcedureAs mentioned previously,in the batch procedure, only with the optimal amount of NaMA feeding could the composite materials be obtained with a high yield and binding efﬁciency.Therefore, the number of functional groups that could be effectively bound to the composite nanoparticles was limited.To obtain composite nanoparticles with a wide range of functional groups,a two-stage procedure in view of NaMA addition was investigated.In theﬁrst stage,an optimal amount of NaMA was copolymerized with MMA (as recipe B3)so that latex seeds could be formed in a high yield and binding efﬁciency.In the second stage,NaMA was added dropwise to the system to keep NaMA under starved condi-tions to avoid the formation of freelatices.Table2.Evaluation of the Encapsulation withDifferent RecipesRecipe D n(nm)PDI NaB164.6 1.13—B253.8 1.14—B338.5 1.04 1.08B437.4 1.11—B535.2 1.17—T139.6 1.04 1.14T237.9 1.030.95T340.1 1.07 1.04T441.5 1.05 1.06T540.8 1.080.94 2830ZENG,YU,AND GUOFigure2shows the yield and binding efﬁ-ciency versus the total amount of NaMA feed-ing.As expected,good encapsulation was obtained when the amount of NaMA varied over a wide range(from10to70mmol).The yields were all higher than90%,and the binding efﬁ-ciencies were superior to85%.The high efﬁ-ciency can be explained by the following points. First,the latex seeds obtained during theﬁrst stage had a very high binding efﬁciency(95%). Second,when NaMA was added to the emulsion system during the second stage,theﬁrst-stage copolymerization of MMA and MAA had justﬁn-ished,as indicated by the kinetic curve(shown later in Fig.5),the terminal free radicals of the copolymer chains were still alive,and the newly added NaMA could still copolymerize to the existing copolymer chains.Third,NaMA added during the second stage was adsorbed onto the latex particle and copolymerized almost instan-taneously because it was kept under starved conditions.33There was no formation of extra micelles,and this prevented the formation of free latices[pure poly(sodium methacrylate) (PNaMA)].Fourth,as the copolymerization of NaMA proceeded,the particles grew,the surfa-ces of the particles became larger,and more NaMA could be adsorbed and accommodated on the surface without saturation ever being reached.As shown in Table2,the average particle sizes were all around40nm with different amounts of NaMA feeding,and the particle size distributions were all narrow.In this two-stage procedure,the latex seed that formed during the ﬁrst stage had a fairly narrow particle size dis-tribution(PDI¼1.04).During the second stage, the polymerization of NaMA occurred on the surface of the seeds,and little new latices formed because NaMA was kept under starved conditions.Therefore,theﬁnal composite par-ticles had a narrow size distribution. Determination of Carboxyl GroupsThe carboxyl contents of the composite nanopar-ticles were quantiﬁed independently with two methods:a physical method(FTIR)and a chemi-cal method(titration).Figure3shows the FTIR spectra of the products with different recipes. The band at1703cmÀ1in the spectra can be attributed to the C¼¼O stretching vibration of the carboxyl group,proving that the carboxyl group was bound onto the composite particles. In comparison with the spectrum of PMMA/ silica composite nanoparticles prepared previ-ously,the extra characteristic peaks of PNaMA at1566cmÀ1can hardly be observed,and this suggests that almost all of the salt was trans-lated into carboxyl groups.The intensity of the C¼¼O band increased with an increasing amount of NaMA feeding.The titration method was calibrated with known concentrations of acrylic acid,acetic acid, and HCl.As shown in Figure4,the average error in a set of(at least three)duplicate analy-ses for one compound at one carboxyl concentra-tion was63.2%.The correlation between the numbers of moles of carboxyl groups initially in each sample and that measured was linear with a slope of1.0027and an intercept of3.2Â10À5. Figure3.FTIR spectra of composite nanoparticles with different MAA feed concentrations:(a)0,(b)10, (c)20,(d)30,and(e)40%.Therefore,we can conclude that the titration method provided an accurate measurement of the number of carboxyl groups in our test samples.The results obtained with the two methods are listed in Table 3.The values obtained by the two different methods are close to each other for almost all the samples and show only small deviations from the amounts of added NaMA.For example,in recipe T3,when the amount of added NaMA was 23.2%(mol %to MMA),that is,the theoretical amount of carboxyl groups was 23.2%(mol %to MMA),the value quanti-ﬁed by FTIR was 22.9%,and that measured by titration was 21.8%.The content of carboxyl groups increased with an increasing amount of NaMA feeding;this indicated that the content of carboxyl groups in the composite nanoparticles could be controlled by the feed.KineticsFigure 5shows the kinetic curves,with recipe T2as an example,with respect to the conver-sion and binding efﬁciency versus time.The sec-ond part of NaMA was fed dropwise into the sys-tem during the period marked with broken lines.The conversion–time curve is similar to that of the control experiment in the absence of silica (not shown),and this suggests that silica had almost no effect on the copolymerization kinetics.The conversion of the monomers (MMA and NaMA)increased rapidly during the initial 30min and then leveled off around 95%at about 50min.The copolymerization of the monomers added in the ﬁrst stage was almost completed before the start of the second-stage addition of NaMA,which was carried out at 60min.During the dropwise addition of NaMA in the second stage,the conversion was almost unchanged (remaining at ca.95%),and this sug-gested that NaMA was kept under starved con-ditions.Because NaMA played the role of a normal emulsiﬁer in the ﬁrst stage,the curve of the binding efﬁciency versus the time in this period was the opposite of that of Espiard and Guyot,5who carried out the encapsulation of silica through the emulsion polymerization of ethyl acrylate (EA).In comparison with their system,the main differences in our case were the method of silica addition and the amount of the emulsiﬁer.In their system,an aqueous silica solution was charged to the reactor before the addition of any other ingredients,and the mono-mer was fed dropwise after the initiator was introduced.However,the mixture of silica and MMA was added together in our case,andMMATable 3.Contents of Grafted PMAA Determined by Two MethodsRecipe Theoretical Amount of COOH (mol %to MMA)Actual Amount of COOH(mol %to PMMA)FTIR Titration B3 1.9 1.1 1.6T1 5.8 4.6 5.3T211.611.210.5T323.222.921.8T434.930.133.4T546.543.740.22832ZENG,YU,AND GUOwas well adsorbed onto the surface of MPTMS-treated silica before it was introduced to the emulsion system.Besides,a relatively large amount of the emulsiﬁer(NaMA in theﬁrst stage)was used in our case,whereas the concen-tration of the emulsiﬁer in their system was much lower than that of ours.We did an experi-ment with a procedure similar to that reported by Espiard and Guyot while keeping other parameters unchanged,and the binding efﬁ-ciency of the product was only11%,much lower than that obtained with our procedure(ca. 90%).Another experiment was also carried out with much less emulsiﬁer(0.2mmol of NaMA) and with other parameters unchanged;it resulted in only15%binding efﬁciency,which agreed with the trend shown by Figure1.The results of these two experiments emphasize the importance of both parameters,that is,the method of silica addition and the amount of the emulsiﬁer(NaMA in theﬁrst stage).In our case,as mentioned in the Batch Proce-dure section,hydrophobic MPTMS-treated silica dispersed well in MMA,and then the mixture was introduced into the system.Under the drive of emulsiﬁcation and shear,the mixture was separated into small droplets of MMA contain-ing MPTMS-treated rger pure MMA droplets hardly existed because if they had existed,they would have broken and formed smaller pure MMA droplets,which would have polymerized and formed free polymer latices ﬁnally.Because the binding efﬁciency was nearly100%(shown in Fig.5),there was almost no free polymer latex.Therefore,after the free radicals of the initiator came into the droplets, copolymerization may have taken place in the droplets containing the MPTMS-treated silica particles,which were surrounded by MMA,as shown in Scheme 1.Polymers were formed around the surface of silica and were covalently bound to silica when silica reacted as a comono-mer.The binding efﬁciency during the initial 15min was very low,whereas the conversion of the monomers increased rapidly during this period.This suggests that the polymer that formed during the initial15min hardly bonded to silica;that is,the double bonds of the MPTMS-treated silica practically did not react. It is probable that the initiator radicals entered the silica/MMA/NaMA droplets from the outside and initiated preferentially the monomers (MMA and NaMA)that were near the surfaces of the droplets(Scheme1).As the copolymeriza-tion proceeded,the growing radicals had more chance to reach the center of the droplets and react with the double bonds of silica,forming grafting,and the binding efﬁciency increased accordingly.Moreover,the binding efﬁciency achieved its maximum at about40min and then leveled off,whereas the monomer conversion was still increasing at that time.It can be con-cluded that the reaction of the double bonds on the surface of silica was almost completed after 40min of copolymerization.The monomers con-tinued polymerizing at the polymer shell and propagated onto the polymer chains(rather than form new polymer chains)after the complete reaction of silica;this was similar to the shell growth mechanism presented by Yan et al.34 Therefore,the reaction of silica in our case mainly happened from15to40min,and so the binding efﬁciency increased rapidly after15min of reaction and reached its maximum at about 40min of reaction.In the case of Espiard and Guyot,5MPTMS-treated silica was added in the beginning,and the initiator was added before the dropwise addition of the monomer.Hence,the double bonds of MPTMS-treated silica had a greater chance to be initiated during the early stage of polymerization,and grafting could start from the surface of silica.As the polymerization pro-ceeded,more emulsiﬁer was needed tostabilize Scheme2.Polymerization mechanism of Espiard and Guyot’s system[PEA¼poly(ethyl acrylate)].POLYMER/SILICA COMPOSITE NANOPARTICLES2833the growing latices.However,the amount of the emulsiﬁer was extremely small,and the mono-mer added in the later stage tended to form pure polymer latex,following the homogeneous nucleation mechanism of emulsiﬁer-free emul-sion polymerization;this resulted in decreased grafting efﬁciency,as shown in Scheme 2.MorphologyTransmission electron microscopy (TEM)photo-graphs of primary silica beads and carboxyl-functionalized composite nanoparticles taken from the emulsion after copolymerization was ﬁn-ished and dyed by RuO 4are shown in Figure 6.The multilayered core–shell structure can be ob-served in the image of the ﬁnal particles [Fig.6(b)],which is different from that of the primary silica beads [Fig.6(a)].In each composite nanopar-ticle,the dark core is made of only one silica bead;this is proved by both its size and the cal-culation of Na (as shown in Table 2)and sug-gests the formation of a well-deﬁned core–shell structure.The surrounding light layer is PMMA,which cannot be dyed by RuO 4.The out-most dark shell is PMAA containing carboxyl groups,which is dyed by RuO 4.This kind of par-ticle morphology is in agreement with our expectations.The average particle size deter-mined by TEM was about 40nm with a narrow size distribution,which agreed with that meas-ured by LS (as shown in Table 2).CONCLUSIONSThe emulsiﬁer-free emulsion copolymerization of MMA and NaMA has been successfully used to prepare functional polymer/silica composite nano-particles.It has been shown that the method of NaMA feeding is very important with respect to the yield and binding efﬁciency,and only the two-stage procedure allows effective control of the carboxyl content,which was measured by two different methods (FTIR and titration).The kinetic study suggests that the copolymerization follows a mechanism different than that found earlier.Polymers formed during the initial stage practically do not graft onto silica,and the mono-mers continue polymerizing,following the shell growth mechanism,after silica has completely reacted;therefore,the binding efﬁciency increases rapidly after 15min and reaches its maximum before the copolymerization ﬁnishes.As demon-strated by TEM,the composite nanoparticles have a multilayered core–shell structure with silica as the core and with PMAA as the outmost shell.The particles sizes are around 40nm with a narrow size distribution.REFERENCES AND NOTES1.Chen,J.F.;Wang,G.Q.;Zeng,X.F.;Zhao,H.Y.;Cao,D.P .;Yun,J.;Tan,C.K.J Appl Polym Sci 2004,94,796–802.Figure 6.TEM photographs of (a)primary silica beads and (b)carboxyl-functionalized composite nanoparticles.2834ZENG,YU,AND GUO。

小角中子散射Neutron scattering

The BIX-3 diffractometer, equipped with an NIP, is dedicated to protein crystallography using a monochromatized neutron beam source (installed at the JAERI).
Light scattering
electromagnetic waves
X-Ray scattering
electromagnetic waves – short wavelength
Neutron scattering
particles
(Electron scattering)
charged particles
Usually: Combination of them
Neutron Scattering Length
of biological relevant elements [10-12 cm]
[F. Sears (1986), H. Glättli und M. Goldmann (1987)]
Spin Contrast Variation
Time of flight Laue diffraction pattern •Diffraction patterns are 3D: (x,y,λ)
•Reduced reflection overlap.
•Reduced background •Enhanced signal-tonoise
Comparison of L4 position
50S subunit E.coli
Neutron Scattering
ca. 100 Å 50S subunit T.thermophilus

TEM 投射电镜 - Lecture 8 -电子衍射 Electron Scattering

through specimen thickness
Consider instead specimen of N atoms / unit thickness. Total cross section for scattering from specimen:
QT
= NσT
=
Noσ Tρ A
N = # atoms / unit volume r = density A = atomic weight
Wave perspective:
– Coherent - maintains phase
– Incoherent - does not maintain phase
Scattering terminology
Forward scattering - thin samples
– Elastic forward scattering is usually low angle (1-10°), coherent
Electron from outer shell to fill the hole in the inner shell
Energy required is
characteristic of the
atom
Inelastic scattering
Characteristic X-rays - nomenclature
Two types:
– Radiolysis: Inelastic scattering ⇒ ionization which breaks bonds
– “Knock-on” damage: direct displacement of atoms
Electron scattering

金的等离子共振等综述

a b s t r a c t
This review describes the fundamental aspects of laser–gold nanoparticle (Au NP) interaction that leads to nanoscale energy deposition to the surroundings through light ampliﬁcation and heat generation. Besides the importance of the primary process in physics and chemistry, application of the light–NP interaction has attracted signiﬁcant interest from various areas ranging from analytical chemistry to material chemistry and biomedicine. Here we consider both mechanistic and application aspects. Our attention is focused on pulsed-laser-induced fast processes that revealed the heating–cooling dynamics of electrons, lattice (particle), and particle’s environment. On the application side, we focus on material fabrication and processing that beat diffraction-limited resolution. Together, we will shed a light on the essence of research activities carried out in the past 10 years. In addition to an abundance of latest information obtained from currently available literature, this review includes ﬁgures obtained by our own calculations to provide readers with a better understanding of the basics of the optical properties and energy and heat-transfer processes of Au NPs, which are not familiar to photochemists. © 2012 Elsevier B.V. All rights reserved.

小波去噪在拉曼单模光纤传感系统中的应用

ｔｉａｅｒｐｓｓａｃｍｕａｉｅａｅａｅａｇｒｈａｄａｗｖｌｔｔｎｆｒｓｇａｒｃｓｉｇｍｅｈｄｔｒｃｓｈｈｓｐｐｒｐｏｏｅｕｌｔｖｒｇｌｏｉｍｎａｅｅｒｓｍｉｎｌｐｏｅｓｎｔｏｏｐｏｅｓｔｅｖｔａｏＲｍａｃｔｒｎｉｎ．Ｄｂｖｌｔｆｎｔｎｉｕｅｔｖｅｏｏｉｏａｅｓｙｍａｌｂｓｍｕａｉｇｈｅａｎｓａｔｉｇｓｇａｅ１５ｗａｅｅｕｃｉｓｓｄｗｉｆｅｄｃｍｐｓｔｎｌｙｒ．Ｂｔｉｌｔ，ｔｅｒ－ｏｈｉｉａｎｓｉｈｗｔａｈｉｎｌｐｏｅｓｎｔｏａｌｎｔｓｇａｎｉｅｅｆｃｉｅｙｅａｎｄｓｅｉｎｌｖｒｌ，ｕｔｓｏｈｔｔｅｓｇａｒｃｓｉｇｍｅｈｄｃｎｅｉａｅｉｎｌｏｓｆｔｖｌ，ｒｔｉｅｉｄｓｇａｅｙｗｅｌｓｍｉｅｒｓｏｔｎｈｙｔｍ￣ｍｅｓｒｍｅｔｉｎｍｐｏｅｔｅｓｓｅｐｒｒａｃ．ｈｒｔｅｓｓｅｅａｕｅｎｍｅａｄｉｒｖｈｙｔｍｅｆｍｎｅｔｏＫＥＹＷＯＲＤＳ：ａｎ；ＳｎｌＲｍａｉｇｅ—ｍｏｅｆｅ；Ｃｍｕａｉｅａｅａｅｄｂｒｕｌｔｖｒｇ；Ｗａｅｅｅ—ｎｉｎｉｖｖｌｔｄｏｓｇｉ
中图分类号：Ｎ４Ｔ２７文献标识码：Ｂ

黑砷磷室温太赫兹探测器

黑砷磷室温太赫兹探测器董卓陈捷朱一帆杨洁王中长张凯 Room-temperature terahertz photodetectors based on black arsenic-phosphorus DONG Zhuo, CHEN Jie, ZHU Yi-fan, YANG Jie, WANG Zhong-chang, ZHANG Kai
收稿日期：2020-09-30；修订日期：2020-10-13 基金项目：国家自然科学基金资助项目（No. 61927813，No. 61875223，No. 61922082）；国家重点研发计划资助项目
（No. 2016YFE015700） Supported by National Natural Science Foundation of China (No. 61927813， No. 61875223， No. 61922082); National Key R & D Program中图分类号：TN382
文献标志码：A
doi：10.37188/CO.2020-0175
理机制和优异材料特性的高灵敏度、便携式太赫兹探测器的研制刻不容缓。黑砷磷是一种新型二维材料，其带隙和输运
特性随化学组分可调，在光电探测领域被广泛关注。目前基于黑砷磷的研究集中在红外探测方面，而对于太赫兹探测的
应用未见报道。本文介绍了一种基于黑砷磷的天线耦合太赫兹探测器。实验结果表明，在探测过程中存在两种不同的
探测机制，并且两者之间存在竞争关系。通过改变黑砷磷的化学组分可以定制不同的探测机制，使其达到最优响应性
能。在平衡材料带隙和载流子迁移率的情况下，探测器实现了室温下对 0.37 THz 电磁波的灵敏探测，其电压响应度和噪
声等效功率分别为 28.23 V/W 和 0.53 nW/Hz1/2。

电离层对星载P波段合成孔径雷达成像的影响

电离层对星载P波段合成孔径雷达成像的影响姚佰栋;时晶晶【摘要】This paper analyzed the relationship between the signal’s carrier frequency, bandwidth, the ionospheric total electronics content(TEC) and scintillation intensity with P band synthetic aperture radar(SAR)’s image resolution. The point target imaging simulation results shown that SAR’s range resolution was closely related with the signal’s carrier frequency, bandwidth and the ionospheric TEC when the wave propagated in the ionosphere, and the ionosphere also caused the SAR image offset in range direction. In addition the azimuth resolution is mainly inlfuenced by the ionospheric scintillation effect, the azimuth resolution decreased rapidly with the enhancement of scintillation’s intensity. While in strong scintillation conditions, the azimuth resolution declined seriously, even could not image.%文章分析了信号载频、带宽、电离层总电子量（Total Electronics Content,TEC）以及闪烁强度等因素对于P波段合成孔径雷达（Synthetic Aperture Radar，SAR）成像分辨率的影响，并进行了点目标成像仿真研究。

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a r X i v :n u c l -e x /0506010v 1 7 J u n 2005WM-05-109Parity-Violating Electron Scattering from 4He and the Strange Electric Form Factorof the NucleonK.A.Aniol,1D.S.Armstrong,2T.Averett,2H.Benaoum,3P.Y.Bertin,4E.Burtin,5J.Cahoon,6G.D.Cates,7C.C.Chang,8Y.-C.Chao,9J.-P.Chen,9Seonho Choi,10E.Chudakov,9B.Craver,7F.Cusanno,11P.Decowski,12D.Deepa,13C.Ferdi,4R.J.Feuerbach,9J.M.Finn,2S.Frullani,11K.Fuoti,6F.Garibaldi,11R.Gilman,14,9A.Glamazdin,15V.Gorbenko,15J.M.Grames,9J.Hansknecht,9D.W.Higinbotham,9R.Holmes,3T.Holmstrom,2T.B.Humensky,16H.Ibrahim,13C.W.de Jager,9X.Jiang,14L.J.Kaufman,6A.Kelleher,2A.Kolarkar,17S.Kowalski,18K.S.Kumar,mbert,Violette,6J.LeRose,9D.Lhuillier,5N.Liyanage,7D.J.Margaziotis,1M.Mazouz,19K.McCormick,14D.G.Meekins,9Z.-E.Meziani,10R.Michaels,9B.Moﬃt,2P.Monaghan,18C.Munoz-Camacho,5S.Nanda,9V.Nelyubin,7,20D.Neyret,5K.D.Paschke,6M.Poelker,9R.Pomatsalyuk,15Y.Qiang,18B.Reitz,9J.Roche,9A.Saha,9J.Singh,7R.Snyder,7P.A.Souder,3R.Subedi,21R.Suleiman,18V.Sulkosky,2W.A.Tobias,7G.M.Urciuoli,11A.Vacheret,5E.Voutier,19K.Wang,7R.Wilson,22B.Wojtsekhowski,9and X.Zheng 23(The HAPPEX Collaboration)1California State University,Los Angeles,Los Angeles,California 90032,USA2College of William and Mary,Williamsburg,Virginia 23187,USA3Syracuse University,Syracuse,New York 13244,USA 4Universit´e Blaise Pascal/CNRS-IN2P3,F-63177Aubi`e re,France 5CEA Saclay,DAPNIA/SPhN,F-91191Gif-sur-Yvette,France6University of Massachusetts Amherst,Amherst,Massachusetts 01003,USA7University of Virginia,Charlottesville,Virginia 22904,USA 8University of Maryland,College Park,Maryland 20742,USA9Thomas Jeﬀerson National Accelerator Facility,Newport News,Virginia 23606,USA10Temple University,Philadelphia,Pennsylvania 19122,USA11INFN,Sezione Sanit`a ,00161Roma,Italy12Smith College,Northampton,Massachusetts 01063,USA 13Old Dominion University,Norfolk,Virginia 23508,USA14Rutgers,The State University of New Jersey,Piscataway,New Jersey 08855,USA15Kharkov Institute of Physics and Technology,Kharkov 310108,Ukraine16University of Chicago,Chicago,Illinois 60637,USA 17University of Kentucky,Lexington,Kentucky 40506,USA18Massachusetts Institute of Technology,Cambridge,Massachusetts 02139,USA 19Laboratoire de Physique Subatomique et de Cosmologie,38026Grenoble,France20St.Petersburg Nuclear Physics Institute of Russian Academy of Science,Gatchina,188350,Russia21Kent State University,Kent,Ohio 44242,USA22Harvard University,Cambridge,Massachusetts 02138,USA 23Argonne National Laboratory,Argonne,Illinois,60439,USA(Dated:June 7,2005)We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from 4He at an average scattering angle θlab =5.7◦and a four-momentum transfer Q 2=0.091GeV 2.From these data,for the ﬁrst time,the strange electric form factor of the nucleonG s E can be isolated.The measured asymmetry of A PV =(6.72±0.84(stat)±0.21(syst))×10−6yields a value of G sE =−0.038±0.042(stat)±0.010(syst),consistent with zero.PACS numbers:13.60.Fz;11.30.Er;13.40.Gp;14.20.Dh,25.20.Bf,24.85.+pThe complex structure of the nucleon goes well beyond its simplest description as a collection of three valence quarks.The sea of gluons and qqsea:there being no valence strange quarks,any strange quark contributions must be eﬀects of the sea.Thus a quark ﬂavor decomposition of the various properties ofthe nucleon becomes of signiﬁcant interest.In particular,a prominent open question is the strange quark contribu-tions to the distributions of charge and magnetization.The use of weak neutral current interactions as key to providing a quark ﬂavor separation of nucleon currents has been discussed for nearly 20years [1].The Z 0bo-son interaction with the nucleon is described using form factors which are sensitive to a diﬀerent linear combi-nation of the light quark distributions than arise in the more familiar electromagnetic form factors.Thus,when2 combined with electromagnetic form factor data for thenucleon and the assumption of charge symmetry,neu-tral current measurements allow the disentangling of thecontributions of the u,d and s quarks[2,3,4].Recently,experimental techniques have developed tothe point of enabling measurements of suﬃcient preci-sion to access strange-quark eﬀects.The strange quarkcontributions to the charge and magnetization of the nu-cleon are encoded in the strange electric and magneticform factors,G s E and G s M,analogs of the usual Sachsform factors G E and G M.The neutral current interaction can be accessed us-ing parity-violating electron scattering,in which longi-tudinally polarized electrons are scattered from unpo-larized targets.The cross section asymmetry A PV=(σR−σL)/(σR+σL)is formed,whereσR(L)is the crosssection for right(left)handed electrons.This asymme-try,while typically tiny,of order a few parts per mil-lion(ppm),is caused by the interference of the weak andelectromagnetic amplitudes,and so it isolates the neutralcurrent form factors.Recently,results of parity-violating electron scatteringmeasurements on the proton at forward angles[5,6,7],and on the proton and deuteron at backward angles[8]have been reported.Each of these individual experimentsis sensitive to diﬀerent linear combinations of G s E,G s Mand the axial form factor G ZpA .No individual experiment shows compelling evidence for non-zero strange quark eﬀects.However,many avail-able model calculations predicting signiﬁcant strange form factors are allowed by the data.It is desirable to carry out complementary measurements that could help disentangle the contributions from the various form fac-tors.In this paper,we report on experiment E00114,the ﬁrst measurement of A PV for a4He target,which is sen-sitive to just one of the form factors:G s E,i.e.the strange quark charge distribution in the nucleon[2,9].Elastic electron scattering from4He is an isoscalar 0+→0+transition and therefore allows no contribu-tions from magnetic or axial-vector currents.The parity-violating asymmetry at tree-level is given by[3]A He PV=G F Q22 4sin2θW+G s E3 correlated asymmetries in the electron beam were main-tained below0.075ppm in intensity,0.005ppm in energy, 7nm in position and4nrad in angle.The data sample consists of roughly3million pairs of windows,corresponding to a60hour period.For each window,a distributed data acquisition system(DAQ) collects data from the polarized source electronics and the integrated response of the beam monitors and the rmation on the helicity of the beam,de-layed by8windows,is included in the data stream.To protect against false asymmetry from electronic pick-up, no signal carries the helicity information away from the source region without this8window delay.The only cuts applied to the data are to remove peri-ods of either low beam current or rapidly changing cur-rent,or when a spectrometer magnet is oﬀ.No helicity-dependent cuts are applied.The helicity-dependent asymmetry in the integrated detector response,normalized to the average beam in-tensity during each window,is computed for each pair of windows and then corrected forﬂuctuations in the beam trajectory to form the raw asymmetry A raw.To ﬁrst order,ﬁve correlated parameters describe the aver-age trajectory of the beam during a window:energy,and horizontal and vertical positions and angles.Two independent methods are used to calibrate the detector sensitivity to each beam parameter,and thus remove the beam-induced random and systematic eﬀects from the raw detector-response asymmetry.Theﬁrst uses a calibration subset of helicity windows,where each beam parameter is modulated periodically around its av-erage value by an amount large compared to nominal beamﬂuctuations.The other method applies linear re-gression to the window pairs used in the physics analysis. These techniques yield results which diﬀered by a neg-ligible amount compared to theﬁnal statistical error.Fi-nal results are obtained using the modulation technique. The cumulative correction for A raw due to beam trajec-tory and energy asymmetry is−0.026ppm.The small beam intensity asymmetry of0.075ppm would induce a false asymmetry proportional to any alin-earity in the detectors and beam current ing dedicated runs,the alinearity of the detector system is determined to be less than1%and the relative alinearity between the beam monitors and the detectors is found to be less than0.2%.A half-wave(λ/2)plate is periodically inserted into the source laser optical path.This passively reverses the sign of the electron beam polarization,and hence the sign of A raw,while leaving many possible systematic eﬀects un-changed.Roughly equal statistics were accumulated with and without theλ/2plate,thereby suppressing many systematic eﬀects.Figure1shows A raw for all the data, grouped by periods of constantλ/2-plate state.The physics asymmetry A He PV is formed from A raw by correcting for beam polarization,backgrounds,andﬁniteData Set Number)-6(1AsymmetryFIG.1:Raw detector asymmetry A raw for both spectrome-ters,broken down by data set.The step pattern represents the insertion/removal of aλ/2plate at the beam source,which shouldﬂip the sign of the measured asymmetry.acceptance:A He PV=K1− i f i,(2) where P b is the beam polarization,f i are background fractions and A i the associated background asymmetries, and K accounts for the range of kinematic acceptance. The beam polarization is measured in the experimental hall using a Compton polarimeter[16]which provides a continuous,non-invasive measurement simultaneous with data collection.Averaged over the run,the polarization is determined to be P b=(86.9±1.7)%.This result is consistent within error with results from dedicated runs to measure polarization using a Møller polarimeter[13]. Tracking chambers,part of the standard HRS detector package[13],are used to track individual events at the focal plane during dedicated,low-current runs in order to determine the average kinematics and to study back-grounds to the integrating measurement.The total background is found to comprise<3%of the detector signal,of which inelastic scattering from4He is the largest component.The inelastic fraction is deter-mined to be(1.6±0.8)%of the total detectedﬂux by extrapolating the rise of the inelastic signal above the elastic radiative tail into the low-momentum edge of the detector—see Figure2.This contribution is dominated by quasi-elastic(QE)scattering.The rescattering of electrons from various spectrome-ter apertures is another source of background.This is studied by varying the central spectrometer momentum in dedicated runs,and is determined to be(0.6±0.6)%, dominated by QE scattering.Contributions from ex-posed iron pole-tips in the dipole are negligible. Background due to the aluminum windows of the cryo-4) (MeV)0(p-pE v e n t sFIG.2:Measured momentum diﬀerence from the central mo-mentum (p 0)of one spectrometer,at the focal plane (ﬁlledhistogram).Quasielastic scattering from 4He dominates the data at low momenta.The vertical lines indicate the edges of the Cerenkov detector acceptance.genic target is measured using an aluminum target with thickness matching the radiation length of the full target cell.The background is (0.7±0.1)%of the total detected rate;studies at low current to track individual counting events show that this contribution is dominated by QE scattering.The corrections to the measured asymmetry for the QE scattering backgrounds discussed above are deter-mined using the predicted [17]asymmetry of −1.6ppm for both aluminum and 4He,with a conservative 50%er-ror assumed.The background corrections to A raw are small and are listed in Table I.The average Q 2is determined to be Q 2 =(0.091±0.001)GeV 2by dedicated low-current runs.Determina-tion of Q 2at this level requires precision measurement of the absolute scattering angle.Due to nuclear recoil,the scattering angle can be determined from the momentum diﬀerence between electrons elastically scattered from hy-drogen and from a heavy nucleus.A water-cell target provided a target containing hydrogen and the heavier oxygen nuclei.The scattering angle into pinholes of a sieve collimator at the entrance of each spectrometer is measured using this method to a precision of 0.3%.Results from the two spectrometers agree within the statistical errors and are averaged together.After all corrections,the asymmetry is found to be A He PV =6.72±0.84±0.21ppm,where the ﬁrst error is statistical and the second systematic.Individual contributions to the systematic error are detailed in Table I.The theoretical value for the asymmetry from Eq.1,under the assumption that G s E =0,including the (small)vector electroweak radiative corrections [3],isA He PV |G s E =0=7.483ppm.The eﬀect of purely electro-magnetic radiative corrections is negligible due to the spin independence of soft photon emission and the smallCorrection (ppm)Target windows 0.058±0.012QE 4He 0.129±0.070Rescattering 0.049±0.050Beam Asyms.−0.026±0.102Alinearity 0.000±0.077Normalization Factors Polarization P b 0.869±0.017Acceptance K 1.000±0.001Q 2Scale 1.000±0.010TABLE I:Corrections to A raw and systematic uncertainties.momentum acceptance of the detectors.For the elec-tromagnetic form factor G γT =0Ewe have used a recent phenomenological ﬁt to the world data at low Q 2[18],with a total uncertainty of 2.6%.Comparing A He PV |G s E =0to our measured A He PV we extract the value of the strange electric form factor G s E =−0.038±0.042±0.010,which is consistent with zero.The ﬁrst uncertainty is statisti-cal and the second is systematic,including those due to radiative corrections and electromagnetic form factors.There have been numerous attempts to calculate strange form factors using a host of models and theo-retical approaches (see [4]and references therein).Avail-able calculations do not even agree on the sign of G s E ,and predicted values range from −0.08to +0.08.The present result disfavors models with large positive val-ues,e.g.[20,21].We have also made a measurement of A PV from the proton at a very similar Q 2,which is reported in an ac-companying paper [19].The combination of the two mea-surements,as well as with previous measurements at thesame Q 2[7,8]allows access to both G s E and G sM sepa-rately [19].A new run of this experiment is scheduled for Summer 2005,which is expected to improve the sta-tistical precision by a factor of 3,along with a modest reduction in the systematic error.In summary,we have made the ﬁrst measurement of the parity-violating asymmetry in elastic electron scat-tering from 4He,which is uniquely sensitive to the strange electric form factor G s E .The result obtained is consistent with zero,and constrains models of the strangeness in the nucleon.We thank the superb staﬀof Jeﬀerson Lab for their contributions to the success of this work.This work was supported by DOE contract DE-AC05-84ER40150,Modiﬁcation No.M175under which the Southeastern Universities Research Association (SURA)operates the Thomas Jeﬀerson National Accelerator Facility,and by the Department of Energy,the National Science Foun-dation,the Istituto Nazionale di Fisica Nucleare (Italy),the Commissariat `a l’´Energie Atomique (France),and theCentre National de Recherche Scientiﬁque (France).5[1]D.B.Kaplan and A.Manohar,Nucl.Phys.B310,527(1988);R.D.Mckeown,Phys.Lett.B219,140(1989).[2]D.H.Beck,Phys.Rev.D39,3248(1989).[3]M.J.Musolf et al.,Phys.Rept.239,1(1994).[4]K.S.Kumar and P.A.Souder,Prog.Part.Nucl.Phys.45,S333(2000);D.H.Beck and R.D.McKeown,Ann.Rev.Nucl.Part.Sci.51,189(2001); D.H.Beck andB.R.Holstein,Int.J.Mod.Phys.E10,1(2001).[5]K.A.Aniol et al.,Phys.Rev.Lett.82,1096(1999);K.A.Aniol et al.,Phys.Lett.B509,211(2001).[6]F.E.Maas et al.,Phys.Rev.Lett.93,022002(2004)[7]F.E.Maas et al.,Phys.Rev.Lett.94,152001(2005)[8]D.T.Spayde et al.,Phys.Lett.B583,79(2004);T.M.Ito et al.,Phys.Rev.Lett.92,102003(2004). [9]M.J.Musolf,R.Schiavilla and T.W.Donnelly,Phys.Rev.C50,2173(1994)[10]S.Ramavataram,E.Hadjimichael and T.W.Donnelly,Phys.Rev.C50,1175(1994).[11]M.J.Musolf and T.W.Donnelly,Phys.Lett.B318,263(1993)[12]K.A.Aniol et al.,Phys.Rev.C69,065501(2004)[13]J.Alcorn et al.,Nucl.Instrum.Meth.A522,294(2004).[14]P.Brindza et al.,IEEE Trans.on Applied Superconduc-tivity,11,no1,1294(2001).[15]T.Maruyama et al.physics/0412099.[16]S.Escoﬃer et al.,physics/0504195.[17]M.J.Musolf and T.W.Donnelly,Nucl.Phys.A546,509(1992)[Erratum-ibid.A550,564(1992)].[18]J.Friedrich and Th.Walcher,Eur.Phys.J.A17,607(2003);we adopt theirﬁt version1.[19]K.A.Aniol et al.,nucl-ex/0506011.[20]H.Weigel et al.,Phys.Lett.B353,20(1995)[21]A.Silva,H.C.Kim and K.Goeke,Phys.Rev.D65,014016(2002)[Erratum-ibid.D66,039902(2002)].。