Signatures of long-range-correlated disorder in the magnetotransport of ultrathin topological insulators

TL;DR

This study reveals that long-range-correlated disorder in ultrathin topological insulators prevents the expected insulating states, leading instead to metallic behavior with characteristic magnetotransport signatures, supported by experiments and theoretical analysis.

Contribution

It demonstrates how long-range disorder destroys insulating states in ultrathin TIs, causing a transition to metallic behavior, supported by experimental data and theoretical modeling.

Findings

Disorder induces a metallic state in ultrathin TIs expected to be insulators.

Observed weak antilocalization cusp and linear magnetoresistance.

Theoretical conditions for insulator-to-metal transition based on disorder.

Abstract

In an ultrathin topological insulator (TI) film, a hybridization gap opens in the TI surface states, and the system is expected to become either a trivial insulator or a quantum spin Hall insulator when the chemical potential is within the hybridization gap. Here we show, however, that these insulating states are destroyed by the presence of a large and long-range-correlated disorder potential, which converts the expected insulator into a metal. We perform transport measurements in ultrathin, dual-gated topological insulator films as a function of temperature, gate voltage, and magnetic field, and we observe a metallic-like, non-quantized conductivity, which exhibits a weak antilocalization-like cusp at the low magnetic field and gives way to a nonsaturating linear magnetoresistance at large field. We explain these results by considering the disordered network of electron- and hole-type puddles induced by charged impurities. We argue theoretically that such disorder can produce an insulator-to-metal transition as a function of increasing disorder strength, and we derive a condition on the band gap and the impurity concentration necessary to observe the insulating state. We also explain the linear magnetoresistance in terms of strong spatial fluctuations of the local conductivity, using both numerical simulations and a theoretical scaling argument.