High and Magnetic-field-dependent Surface Carriers Mobility in 3D Topological Insulators without Bulk States

TL;DR

This study reports record-high surface carrier mobility in 3D topological insulators, highlighting the absence of bulk carriers as key to high mobility and analyzing magnetic field effects on surface state transport.

Contribution

It introduces a novel application of the two-liquid model to reveal unprecedented surface carrier mobility and explores magnetic field effects on surface transport in topological insulators.

Findings

Surface mobility reaches ~20000 cm²/Vs in topological insulators.

Bulk-to-surface conduction transition occurs below 100 K.

Magnetoresistance and Hall effects indicate multiple surface carrier species.

Abstract

By applying the conventional two-liquid model to the magnetoresistivity tensor, we reveal a record-high carrier mobility for surface states in tetradymite topological insulators ($\sim$ 20000 cm$^2$/Vs) in both bulk crystals and thin flakes of Sn-Bi$_{1.1}$Sb$_{0.9}$Te$_2$S. Bulk crystals of this 3D topological insulator exhibit a transition from bulk to surface-dominated conductivity below 100 K, whereas in thin flakes, bulk conductivity is suppressed at even higher temperatures. Our data therefore suggest that a key ingredient for elevated mobility is the absence of bulk carriers at the Fermi level. A fingerprint of the high-mobility carriers, i.e a steep low-field magnetoresistance along with a strong Hall effect nonlinearity below 1 T, signifies the presence of at least two surface-related carrier species, even when bulk states are frozen out. To explain the magnetoresistance and the Hall effect in a wider range of magnetic fields ($>1$ T), one must assume that the carrier mobility drops with the field. The influence of Zeeman splitting on mobility and the contribution of anomalous Hall conductivity provide a much better description of the magnetoresistance and the nonlinearity of the Hall coefficient. Our data call for a revision of the surface state mobility in 3D topological insulators.