Stabilizing a massless Dirac cone within the Gaussian unitary ensemble on the surface of disordered axion insulators

In axion insulators the bulk and surface can admit distinct electronic gaps. Using a three-dimensional axion insulator for which the z-surface corresponds to a half-integer Chern insulator, we show the surface mobility gap can be closed prior to the bulk mobility gap as a function of increasing disorder strength. Closure of the surface mobility gap prior to that of the bulk stabilizes a single massless Dirac cone on the surface within the Gaussian unitary ensemble as a critical phase. The generality of these results as well as correspondence between the critical phase identified on the surface and the integer quantum Hall plateau transition are discussed.