二维WTe_(2)晶格对称性的光学研究  

Optical study on crystal symmetry of two-dimensional WTe_(2)

作　　者：何宽鱼 邱天宇 奚啸翔 He Kuan-Yu;Qiu Tian-Yu;Xi Xiao-Xiang(National Laboratory of Solid State Microstructures and Department of Physics,Nanjing University,Nanjing 210093,China;Collaborative Innovation Center of Advanced Microstructures,Nanjing University,Nanjing 210093,China)

机构地区：[1]南京大学物理学院,固体微结构物理国家重点实验室,南京210093 [2]南京大学,人工微结构科学与技术协同创新中心,南京210093

出　　处：《物理学报》2022年第17期283-291,共9页Acta Physica Sinica

基　　金：国家自然科学基金(批准号:11774151);国家重点基础研究发展计划(批准号:2018YFA0307000)资助的课题.

摘　　要：二维WTe_(2)由于特殊的晶格对称性而衍生出量子自旋霍尔效应、非线性霍尔效应等奇异性质.确定其晶体结构的细节,是理解这些性质的重要出发点.本文利用温度、偏振依赖的拉曼光谱与光学二次谐波产生,详细研究了一至三层WTe_(2)的晶格对称性.实验发现,单层样品具有可观的二次谐波产生信号,表明其晶格中心对称性破缺,且偏振依赖符合C_(s)点群,与此前普遍认为的中心对称的1T’结构不同.双层样品具有更为显著的二次谐波产生信号,且信号强度的温度依赖与铁电相变一致,表明层间堆叠产生了更强的中心对称性破缺,提供了该体系中存在层间滑移铁电的证据.三层样品的二次谐波产生信号约为单层样品的五倍,但比双层样品弱一个数量级,表明层间堆叠导致其具有较弱的中心对称性破缺.仅单层与双层样品中出现了多个显著的二阶拉曼散射峰,其温度依赖反映出电子结构对层间耦合高度敏感.这些结果将有助于完善对原子级厚度WTe_(2)物理性质的理解.Two-dimensional WTe_(2) possesses a special crystal symmetry,leading to novel properties such as quantum spin Hall effect and nonlinear Hall effect.Determining the details of its crystal structure is essential for understanding these interesting properties.Here,we report an optical study on the crystal symmetry of monolayer,bilayer,and trilayer WTe_(2),using temperature and polarization dependent Raman spectroscopy and optical second harmonic generation(SHG).We find that monolayer WTe_(2) is noncentrosymmetric as indicated by its sizable SHG,in contrast to the commonly believed centrosymmetric 1T'structure.The polarization dependence of the SHG is consistent with the Cs point group.Bilayer WTe_(2) exhibits SHG signal more than one order of magnitude higher than in the monolayer and trilayer samples,with its temperature dependence reflecting the ferroelectric phase transition,evidencing strong inversion symmetry breaking induced by layer stacking and interlayer-sliding ferroelectricity.We also observe prominent second-order resonant Raman scattering peaks only in monolayer and bilayer WTe_(2),but not in thicker samples,and their temperature dependence indicates an electronic structure highly sensitive to interlayer coupling.These results will be useful for further exploring the properties of atomically thin WTe_(2).

关 键 词：二碲化钨 拉曼散射 光学二次谐波产生 铁电 

分 类 号：TQ136.13[化学工程—无机化工]