Low carrier concentration crystals of the topological insulator Bi$_{2-x}$Sb$_{x}$Te$_{3-y}$Se$_{y}$: a magnetotransport study

TL;DR

This study synthesizes and characterizes Bi-Sb-Te-Se topological insulator crystals with extremely low bulk carrier density, demonstrating dominant surface conduction and 2D weak antilocalization effects.

Contribution

It refines the bulk-insulating properties of BSTS by achieving record low carrier density and quantifies the surface contribution to transport in thin samples.

Findings

High resistivity and low bulk carrier density in synthesized crystals.

Surface conduction dominates transport in thin samples, reaching 97%.

Observation of 2D weak antilocalization consistent with topological surface states.

Abstract

In 3D topological insulators achieving a genuine bulk-insulating state is an important research topic. Recently, the material system (Bi,Sb)$_{2}$(Te,Se)$_{3}$ (BSTS) has been proposed as a topological insulator with high resistivity and a low carrier concentration (Ren \textit{et al.} \cite{Ren2011}). Here we present a study to further refine the bulk-insulating properties of BSTS. We have synthesized Bi$_{2-x}$Sb${_x}$Te$_{3-y}$Se$_{y}$ single crystals with compositions around $x = 0.5$ and $y = 1.3$. Resistance and Hall effect measurements show high resistivity and record low bulk carrier density for the composition Bi$_{1.46}$Sb$_{0.54}$Te$_{1.7}$Se$_{1.3}$. The analysis of the resistance measured for crystals with different thicknesses within a parallel resistor model shows that the surface contribution to the electrical transport amounts to 97% when the sample thickness is reduced to $1 \mu$m. The magnetoconductance of exfoliated BSTS nanoflakes shows 2D weak antilocalization with $\alpha \simeq -1$ as expected for transport dominated by topological surface states.