NMR probe of suppressed bulk conductivity in the topological insulator Bi0.5Sb1.5Te3.

Jun Kue Park,Jae Sang Lee,Do Hoon Kang,Sung Kyun Park

doi:10.1039/d1ra07194g

Abstract

The relaxation behavior in the topological insulator (TI) Bi0.5Sb1.5Te3 has been investigated using 125Te nuclear magnetic resonance spectroscopy. We systematically investigate the spin–lattice relaxation rate (1/T1) in bulk electronic states with varying particle sizes. By analyzing the 1/T1 relaxation behavior, we find that with decreasing particle sizes the electronic states in the bulk exhibit more topological insulating behavior, indicative of an increasing energy gap supported by higher thermal activation energy. Besides, the decreasing density of states at the Fermi level was observed in the massive Dirac electrons with decreasing particle size by analyzing the spin–lattice relaxation according to a theoretical model in this spin–orbit coupled system.

Highlights

Topological insulators (TIs) of layered materials with binary or ternary chalcogenides have been widely investigated due to the interest in potential applications such as spintronics and quantum computing.[1,2,3] An ideal topological insulator (TI) exhibits gapless Dirac-like edge or surface states, whereas a bulk interior is featured by an insulating energy gap.[1,2,4,5] The suppressed conductivity of the bulk interior in TIs is important as much as the edge or surface states given the applications such as eld-effect transistors, because the insulating bulk state may act in a role of switch off, but the topological conductive state acts as switch on
By analyzing the 1/T1 relaxation behavior, we find that with decreasing particle sizes the electronic states in the bulk exhibit more topological insulating behavior, indicative of an increasing energy gap supported by higher thermal activation energy
The decreasing density of states at the Fermi level was observed in the massive Dirac electrons with decreasing particle size by analyzing the spin–lattice relaxation according to a theoretical model in this spin–orbit coupled system

Summary

Introduction

Topological insulators (TIs) of layered materials with binary or ternary chalcogenides have been widely investigated due to the interest in potential applications such as spintronics and quantum computing.[1,2,3] An ideal TI exhibits gapless Dirac-like edge or surface states, whereas a bulk interior is featured by an insulating energy gap.[1,2,4,5] The suppressed conductivity of the bulk interior in TIs is important as much as the edge or surface states given the applications such as eld-effect transistors, because the insulating bulk state may act in a role of switch off, but the topological conductive state acts as switch on. In a nanoscale TI, Koumoulis et al.[6] found the metallic surface state obeying the Korringa relation, with a well-separated NMR shoulder peak arising from a greater surface-to-volume ratio. In this metallic state, the spin–lattice relaxation rate (1/T1) is determined by the interaction between the nucleus and electrons of the surface topological state. We systematically study the Dirac electron system in the bulk of a TI with varying particle sizes, according to the recent theoretical model.[10] We observed a varying carrier density and obtained activation energies by analyzing the 1/T1 data for the TI Bi0.5Sb1.5Te3

Experimental

NMR line shapes and relaxation patterns

Relaxation behavior of particle-sized topological insulators

Conclusions