Visualizing the in-Gap States in Domain Boundaries of Ultra-Thin Topological Insulator Films

Ultra-thin topological insulators provide a platform for realizing many exotic phenomena such as the quantum spin Hall effect,and quantum anomalous Hall effect.These effects or states are characterized by quantized transport behavior of edge states.Experimentally,although these states have been realized in various systems,the temperature for the edge states to be the dominating channel in transport is extremely low,contrary to the fact that the bulk gap is usually in the order of a few tens of milli-electron volts.There must be other in-gap conduction channels that do not freeze out until a much lower temperature.Here we grow ultra-thin topological insulator Bi_(2)Te_(3) and Sb_(2)Te_(3)films by molecular beam epitaxy and investigate the structures of domain boundaries in these films.By scanning tunneling microscopy and spectroscopy we find that the domain boundaries with large rotation angles have pronounced in-gap bound states,through which one-dimensional conduction channels are suggested to form,as visualized by spatially resolved spectroscopy.Our work indicates the critical role played by domain boundaries in degrading the transport properties.

Modification of the Hybridization Gap by Twisted Stacking of Quintuple Layers in a Three-Dimensional Topological Insulator Thin Film

Twisting the stacking of layered materials leads to rich new physics. A three-dimensional topological insulator film hosts two-dimensional gapless Dirac electrons on top and bottom surfaces, which, when the film is below some critical thickness, will hybridize and open a gap in the surface state structure. The hybridization gap can be tuned by various parameters such as film thickness and inversion symmetry, according to the literature. The three-dimensional strong topological insulator Bi(Sb)Se(Te) family has layered structures composed of quintuple layers(QLs) stacked together by van der Waals interaction. Here we successfully grow twistedly stacked Sb_2Te_3 QLs and investigate the effect of twist angels on the hybridization gaps below the thickness limit. It is found that the hybridization gap can be tuned for films of three QLs, which may lead to quantum spin Hall states.Signatures of gap-closing are found in 3-QL films. The successful in situ application of this approach opens a new route to search for exotic physics in topological insulators.