Separate surface and bulk topological Anderson localization transitions in disordered axion insulators

TL;DR

This paper demonstrates that in disordered axion insulators, the bulk and surface topological transitions can occur independently, leading to unique metallic states with distinct spectral and response properties.

Contribution

It reveals the decoupling of bulk and surface topological transitions in disordered axion insulators, showing the emergence of a 2D unquantized anomalous Hall metal.

Findings

Bulk and surface mobility gaps evolve differently under disorder.

Decoupled topology yields a 2D anomalous Hall metal with GUE spectral properties.

Results suggest generality to other insulators and superconductors.

Abstract

In topological phases of matter for which the bulk and boundary support distinct electronic gaps, there exists the possibility of decoupled mobility gaps in the presence of disorder. This is in analogy with the well-studied problem of realizing separate or concomitant bulk-boundary criticality in conventional Landau theory. Using a three-dimensional axion insulator having clean, gapped surfaces with $e^2/2h$ quantized Hall conductance, we show the bulk and surface mobility gap evolve differently in the presence of disorder. The decoupling of the bulk and surface topology yields a regime that realizes a two-dimensional, unquantized anomalous Hall metal in the Gaussian unitary ensemble (GUE), which shares some spectral and response properties akin to the surface states of a conventional three-dimensional (3D) topological insulator. The generality of these results as well as extensions to other insulators and superconductors is discussed.