Topological phase transition with nanoscale inhomogeneity in (Bi$_{1-x}$In$_{x}$)$_{2}$Se$_{3}$

TL;DR

This study reveals that chemical disorder caused by indium doping in Bi2Se3 leads to nanoscale inhomogeneity and phase separation, challenging the traditional understanding of topological phase transitions which involve band gap closure.

Contribution

It provides the first detailed microscopic investigation showing that chemical disorder can suppress band inversion and induce phase separation without a clear band gap closure.

Findings

No band gap closure observed across the transition.

In defects effectively suppress band inversion.

Nanoscale phase separation of topological and normal insulator regions.

Abstract

Topological insulators are a class of band insulators with non-trivial topology, a result of band inversion due to the strong spin-orbit coupling. The transition between topological and normal insulator can be realized by tuning the spin-orbit coupling strength, and has been observed experimentally. However, the impact of chemical disorders on the topological phase transition was not addressed in previous studies. Herein, we report a systematic scanning tunneling microscopy/spectroscopy and first-principles study of the topological phase transition in single crystals of In doped Bi$_2$Se$_3$. Surprisingly, no band gap closure was observed across the transition. Furthermore, our spectroscopic-imaging results reveal that In defects are extremely effective "suppressors" of the band inversion, which leads to microscopic phase separation of topological-insulator-like and normal-insulator-like nano regions across the "transition". The observed topological electronic inhomogeneity demonstrates the significant impact of chemical disorders in topological materials, shedding new light on the fundamental understanding of topological phase transition.