Dual Topology of Dirac Electron Transport and Photogalvanic Effect in Low‐Dimensional Topological Insulator Superlattices

Abstract

are protected by time-reversal symmetry (TRS) and characterized by a nonzero Z 2 invariant. [1] The spin and momentum of topological edge states (2D TIs) or surface states (3D TIs) are orthogonally locked and robust against nonmagnetic backscattering. [2] Beyond the Z 2 -order protected band topology, the crystalline symmetry preserves the band topology in cases such as discrete rotation and mirror symmetry which defines the topological crystalline insulators (TCIs). [3] A combination of these two topological properties in one material, i.e., dual topology, defines a dual TI and offers fertile possibilities of gapping one topological surface state via breaking the respective symmetry while preserving another. [4] Therefore, the exploration of dual TIs can elucidate more degrees of freedom to manipulate the band structure and electric transport properties for designing novel topological electronic, spintronic, thermoelectric, and optical devices. [5] Dual TIs can exist in different topological forms. [6] One form refers to dual protection of surface states by multiple bulk symmetries, such as Bi 2 Te 3 [6b] predicted to be both a TI and a TCI whose topological surface state is protected by TRS and crystal symmetry simultaneously. Another interesting form is that the dual TIs can Dual topological insulators, simultaneously protected by time-reversal symmetry and crystalline symmetry, open great opportunities to explore different symmetry-protected metallic surface states. However, the conventional dual topological states located on different facets hinder integration into planar opto-electronic/spintronic devices. Here, dual topological superlattices (TSLs) Bi 2 Se 3 -(Bi 2 /Bi 2 Se 3 ) N with limited stacking layer number N are constructed. Angle-resolved photoelectron emission spectra of the TSLs identify the co existence and adjustment of dual topological surface states on Bi 2 Se 3 facet. The existence and tunability of spin-polarized dual-topological bands with N on Bi 2 Se 3 facet result in an unconventionally weak antilocalization effect (WAL) with variable WAL coefficient α (maximum close to 3/2) from quantum transport experiments. Most importantly, it is identified that the spin-polarized surface electrons from dual topological bands exhibit circularly and linearly polarized photogalvanic effect (CPGE and LPGE). It is anticipated that the stacked dual-topology and stacking layer number controlled bands evolution provide a platform for realizing intrinsic CPGE and LPGE. The results show that the surface electronic structure of the dual TSLs is highly tunable and well-regulated for quantum transport and photoexcitation, which shed light on engineering for opto-electronic/spintronic applications.