拓扑半金属Sb薄膜的分子束外延生长、能带结构调控和原位同步辐射ARPES研究

Topological insulator with strong spin-orbit coupling, a new state of quantum matter, is currently one of the hot topics in condensed matter physics. Sb, a bulk semimetal, has a gapless spin-polarized surface state like topological insulators, which will be a promising candidate for high-temperature spin current sources. However, due to the limited fabrication methods, Sb has attracted less attention. In this work, atomically flat Sb thin films with macroscopically large area are directly prepared on Si(111) from 1 BL to 100 BL by molecular beam epitaxy and studied by in-situ scanning tunneling microscopy and synchrotron-radiation angle-resolved photoemission spectroscopy. Using variable photon energies and polarizations, the topological surface states of Sb films are successfully isolated from bulk states over the entire Brillouin zone and their different spin structures are identified. Due to the large quantum confinemet length, quantum size effect are quite sensitive in Sb films. With decreasing thickness of confinement, the surface states and bulk states of Sb films are effectively tuned, which causes the transitons from three-dimensional topological semimetal to three-dimensional topological insulator and from topologically nontrivial state to topologically trivial state. Besides, Cr and Cu atoms are doped in Sb thin films, respectively. By breaking time-reversal symmetry with proximity to a magnetic material and by breaking gauge symmetry due to proximity to a superconductor, an energy gap is expected to be induced into the topological surface state. Finally, the influence of O2 adsorption on topological surface states will be explored. This work will be a basis of unique property investigations and ultimate applications of topological surface states in Sb films.

拓扑绝缘体是一类自旋轨道耦合作用引起的新量子物质态，已成为当前凝聚态物理研究的热点之一。半金属Sb具有与拓扑绝缘体相同的自旋极化金属表面态，是高温自旋流源的理想候选人，但因制备方法的局限一直研究较少。本项目利用分子束外延技术，直接在Si(111)表面，精确可控地生长1 BL至100 BL层厚的大面积原子级平整Sb薄膜，并对其实现Cr、Cu、 O2等磁性、非磁性原子的可控掺杂和吸附。利用原位同步辐射角分辨光电子能谱，通过改变光子能量在整个布里渊区分离体能带和表面能带，通过改变光子极化获得能带自旋信息，系统研究量子尺寸效应和掺杂效应对Sb薄膜能带结构的调控。从三维到二维、从有磁到无磁，实现Sb薄膜从拓扑半金属到拓扑绝缘体、从拓扑非平庸态到拓扑平庸态的转变，探索磁有序、超导相与Sb薄膜拓扑表面态的相互作用规律，明确氧化对Sb薄膜拓扑表面态的影响，为Sb薄膜拓扑表面态独特性质的研究和应用奠定基础。

量子调控对拓扑、超导薄膜材料的拓扑表面态、体态及超导电性有重要影响，在低维拓扑、超导量子器件的制备和应用中起到关键作用。本项目中，我们对单层Sb(111)薄膜、(Bi0.2Sb0.8)2Te3薄膜以及Fe-Te-Se薄膜的量子尺寸效应分别进行了研究，主要取得的成果如下：（1）利用分子束外延技术，首次获得了三维拓扑绝缘体薄膜衬底上的单层Sb(111)薄膜，发现其能带结构在Bi2Te3和Sb2Te3两种衬底上基本相同，指出界面应力和电荷转移是导致不同衬底间能带色散和芯能级出现微小差异的主要原因；（2）利用低温同步辐射角分辨光电子能谱技术，系统地研究了2 QL至20 QL层厚的拓扑绝缘体(Bi0.2Sb0.8)2Te3薄膜，发现Dirac点与价带顶的能量间距随薄膜厚度的降低而逐渐增大，有助于降低敏感测量中薄膜体载流子浓度，再次成为单层Sb薄膜生长衬底界面的理想选择之一；（3）观测到了尺寸效应对Fe-Te-Se超导薄膜超导电性的调制规律，建立并揭示了其内在的沙漏机制。以上成果揭示了量子尺寸效应对材料拓扑表面态、体价态以及超导电性的影响，为调制拓扑材料、超导材料电子结构及性质提供了一种新的方法，同时也为它们在未来量子器件的应用中奠定基础。