Non-contact mutual inductance based measurement of an inhomogeneous topological insulating state in Bi2Se3 single crystals with defects

TL;DR

This study uses a non-contact mutual inductance technique to investigate how Se vacancy defects affect the surface and bulk electrical conductivities in Bi2Se3 topological insulator crystals, revealing a temperature-dependent transition between conduction regimes.

Contribution

It introduces a non-contact method to distinguish surface and bulk conduction in topological insulators and models the inhomogeneous state caused by defects.

Findings

Surface conduction shows linear frequency dependence; bulk conduction shows quadratic dependence.

Transition from surface to bulk conduction occurs near 70 K.

Defect states create a low-conductivity bulk inhomogeneous medium.

Abstract

Pure Topological Insulating materials preserve a unique electronic state comprising of bulk insulating gap and conducting surface states. Here we use bulk Bi2Se3 single crystals possessing Se vacancy defects as a prototype topological insulator (TI) material for exploring the effect of non-magnetic disorder on the conducting properties of TIs. We employ a sensitive, non-contact, mutual inductance based technique for measuring the surface and bulk contribution to electrical conductivity in the TI. We discern the different contributions, by observing that predominant surface electrical conduction produces linear frequency dependence of the pickup signal while bulk conductivity gives rise to quadratic frequency dependence. We also see an algebraic temperature dependent surface conductivity while an activated bulk conductivity. Using the above we uncover an interplay between surface and bulk contribution to electrical conductivity in the TI as a function of temperatures. In the Bi2Se3 crystals the transformation from surface to bulk dominated electrical transport is found to occur close to 70 K. This temperature range matches well with our results from activated bulk electrical transport results which shows an activation energy scale, delta which is in the millieV range. The gap delta is much less than the bulk band gap in Bi2Se3, and which we argue is associated with defect states in the TI material. To understand our results, we propose a model of a TI comprising of an inhomogeneous low electrically conducting medium (bulk) which is sandwiched between thin two high electrically conducting sheets (surface). The inhomogeneous TI state we argue is generated by Selenium vacancies defects in Bi2Se3, which is responsible for producing an interplay between bulk and surface conductivity.