拓扑绝缘体表面电子-声子相互作用机理的实验研究

Topological insulators (TIs) are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their surface. One of the most interesting properties of TIs is their spin helicity, whereby the spin is locked normal to the wave vector of the surface electronic state. The topological surface states should be very stable in TIs, since these spin-textured surface states are robust against spin-independent backscattering. Consequently, electron-phonon coupling (EPC) should be the dominant scattering mechanism for surface electronic states at finite temperatures. Hence, the study of EPC in TIs is of exceptional importance in assessing any potential applications. However, a consensus about the EPC strength on the surface of TIs has not been achieved by current studies, since conflicting values have been reported. And the EPC mechanism is still not clear. In this proposal we will employ Angle Resolved Photoemission Spectroscopy and state-of-the-art High Resolution Electron Energy Loss Spectroscopy to study EPC mechanism on the surface of typical three dimensional TIs, such as the systems of Bi2Se3 and Bi2Te3. We will also develop the related theory to figure out the details of the EPC physical picture such as the transition path of the electrons and the evolved specific phonon modes.

拓扑绝缘体是一种新的量子物质态，其体内的能带结构类似于普通绝缘体一样具有能隙而其表面则具有导电的金属态。拓扑绝缘体最令人关注的一种性质是其表面电子态很难被非磁性的杂质所破坏，从而使这种表面电子态非常稳定。这就使电子-声子的相互作用成为了拓扑绝缘体表面电子态最重要的散射机制。因此，对拓扑绝缘体表面电声耦合的研究对其任何潜在的应用都是非常重要的。然而，当前的研究对拓扑绝缘体表面电声耦合的强度存在很大的分歧，特别是对其散射机制，还没有明确的结论。本项目将结合角分辨光电子能谱仪和新型高分辨低能电子能量损失谱仪，对拓扑绝缘体表面的电声耦合的机理进行系统全面的研究，研究体系包括典型的三维拓扑绝缘体材料Bi2Se3和Bi2Te3等。我们还将发展相关的方法来解析出拓扑绝缘体表面电声耦合详细的物理图像，包括电子的跃迁路径以及参与相互作用的具体声子模式。

拓扑绝缘体的表面元激发是凝聚态物理中非常重要的课题，对拓扑材料的相关应用具有重要意义。在本项目的支持下，我们利用自行设计搭建的ARPES-HREELS联合系统对拓扑绝缘体的表面电子集体激发及声子色散进行了研究，包括用ARPES和HREELS分别测量了单晶解理表面的电子能带和元激发色散，发展了依据Eliashberg理论解析电声耦合图像的理论方法。此外，我们还将此方法扩展到了对FeSe/SrTiO3界面超导体系的研究中。项目取得的主要成果包括：（1）在拓扑绝缘体表面首次发现了受拓扑保护的集体激发模式；（2）依据电子-声子耦合的研究对电荷密度波根据其起源进行了分类；（3）完成了对我们新搭建的2D-HREELS谱仪的首次综合应用；（4）首次从声子的实验层面阐明了FeSe/SrTiO3界面超导体系中衬底声子对超导增强的具体影响。目前，在项目的支持下已在知名学术期刊上发表论文3篇，在相关领域产生了比较重要的影响。