Bounds and anomalies of inhomogeneous anomalous Hall effects

TL;DR

This paper uses homogenization theory to establish bounds on the anomalous Hall conductivity in inhomogeneous materials and explains how apparent experimental anomalies can arise from inhomogeneities.

Contribution

It provides the first rigorous bounds on measured anomalous Hall effects in inhomogeneous conductors and explains experimental anomalies through inhomogeneity effects.

Findings

Homogenized AHC cannot exceed local AHC bounds.

Inhomogeneities can cause apparent anomalies in Hall measurements.

Examples show inhomogeneity-induced anomalies mimic topological Hall effects.

Abstract

It is well recognized that interpreting transport experiment results can be challenging when the samples being measured are spatially nonuniform. However, quantitative understanding on the differences between measured and actual transport coefficients, especially the Hall effects, in inhomogeneous systems is lacking. In this work we use homogenization theory to find exact bounds of the measured or homogenized anomalous Hall conductivity (AHC) in inhomogeneous conductors under minimal assumptions. In particular, we prove that the homogenized AHC cannot exceed the bounds of the local AHC. However, in common experimental setups, anomalies that appear to violate the above bounds can occur, with a popular example being the "humps" or "dips" of the Hall hysteresis curves usually ascribed to the topological Hall effect (THE). We give two examples showing how such apparent anomalies could be caused by different types of inhomogeneities and discuss their relevance in experiments.