Topological Phase Transitions in a Hybridized Three-Dimensional Topological Insulator

TL;DR

This paper investigates topological phase transitions in 3D topological insulators as their thickness varies, revealing how hybridization induces a crossover from trivial to non-trivial states and enabling field-controlled switching between different topological phases.

Contribution

It provides experimental evidence of topological phase transitions driven by surface hybridization in 3D TIs and demonstrates control via magnetic and electric fields.

Findings

Observation of a finite conductance indicating QSH state

Mapping of the 3D to 2D topological crossover

Field-induced switching between insulating and QSH states

Abstract

As the thickness of a three-dimensional (3D) topological insulator (TI) becomes comparable to the penetration depth of the surface states, quantum tunneling between surfaces turns their gapless Dirac electronic structure into a gapped surface state. Analytical formulation suggests that the hybridization gap scales exponentially with decrease in number of layers while the system oscillates between topologically trivial and non-trivial insulators. This work explores the transport properties of a 3D TI in the inter-surface hybridization regime. By experimentally probing the hybridization gap as a function of BiSbTeSe2 thickness using three different methods, we map the crossover from the 3D to 2D state. In the 2D topological state, we observe a finite longitudinal conductance at ~2e2/h when the Fermi level is aligned within the surface gap, indicating a quantum spin Hall (QSH) state. Additionally, we study the response of trivial and non-trivial hybridization gapped states modulated by external out-of-plane magnetic and electric fields. Our revelations of surface gap-closing and/or reopening features are strongly indicative of topological phase transitions (TPTs) in the hybridization gap regime, realizing magnetic/electric field switching between band insulating and QSH states with immense potential for practical applications.