Smearing of the quantum anomalous Hall effect due to statistical fluctuations of magnetic dopants

TL;DR

This paper investigates how statistical fluctuations of magnetic dopants cause smearing of the quantum anomalous Hall effect, revealing a semi-hard gap and its impact on low-temperature conductivity.

Contribution

It introduces a theoretical model showing how dopant fluctuations lead to a semi-hard gap and affects electronic states near the transition point in QAH systems.

Findings

Exponential tails of in-gap states near the gap edges.

Existence of a semi-hard gap despite large smearing.

Arrhenius temperature dependence of conductivity at low temperatures.

Abstract

Quantum anomalous Hall effect (QAH) is induced by substitution of a certain portion, x, of Bi atoms in a BiTe-based insulating parent compound by magnetic ions (Cr or V). We find the density of in-gap states, N(E), emerging as a result of statistic fluctuations of the composition, x, in the vicinity of the transition point, where the average gap, E_g, passes through zero. Local gap follows the fluctuations of x. Using the instanton approach, we show that, near the gap edges, the tails are exponential, ln N(E) \propto -(E_g-|E|), and the tail states are due to small gap reduction. Our main finding is that, even when the smearing magnitude exceeds the gap-width, there exists are semi-hard gap around zero energy, where ln N(E) \propto -E_g/|E| (ln E_g/|E|). The states responsible for N(E) originate from local gap reversals within narrow rings. The consequence of semi-hard gap is the Arrhenius, rather than variable-range hopping, temperature dependence of the diagonal conductivity at low temperatures.