Observation of the surface hybridization gap in the electrical transport properties of the ultrathin topological insulator (Bi$_{1-x}$Sb$_{x}$)$_2$Te$_3$

TL;DR

This study investigates the hybridization gap in ultrathin (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ topological insulator films, revealing an insulating phase at low temperatures in 6 nm films, advancing understanding of quantum spin Hall states.

Contribution

First experimental observation of the hybridization gap in ultrathin (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ films through transport measurements, highlighting the conditions for quantum spin Hall phase realization.

Findings

Insulating phase observed in 6 nm films at low temperatures.

Hybridization gap evidenced by transport measurements.

Magnetic field effects on the gap remain partially unresolved.

Abstract

We study the three-dimensional topological insulator (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ in its ultrathin limit i.e. when the thickness is of the same order as the surface state penetration depth. It is expected that in this limit a hybridization gap opens at the Dirac point, which gives rise to a quantum spin Hall (QSH) or insulating phase, depending on the material thickness. We fabricate (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ Hall bars with a thicknesses of 6 and 9 nm and measure an insulating phase around the Dirac point for low bias and at sub-Kelvin temperatures only in samples fabricated from the 6 nm films, which indicates the presence of a hybridization gap. The effect of a perpendicular magnetic field on the hybridization gap is studied but remains partially unresolved. The results form an important step towards experimentally realizing the quantum spin Hall state via hybridization in ultrathin films of (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$, yet, they also expose a knowledge gap regarding transport measurements in these systems.