Delocalization Transition of a Disordered Axion Insulator

The axion insulator is a higher-order topological protected by inversion symmetry. We show that, under quenched disorder respecting symmetry on average, the topology of stays robust, and an intermediate metallic phase in which states are delocalized unavoidable at transition from to trivial insulator. derive this conclusion general arguments, classical percolation theory, numerical study 3D quantum network model simulating disordered through layer construction. find localization length critical exponent near delocalization be $\ensuremath{\nu}=1.42\ifmmode\pm\else\textpm\fi{}0.12$. further that stable even weak breaking average symmetry, up strength. also quantitatively map our effective Hamiltonian we its low-energy $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ expansion.