Helical Majorana surface states of strongly disordered topological superconductors with time-reversal symmetry

TL;DR

This study investigates how strong surface disorder affects helical Majorana surface states in topological superconductors with time-reversal symmetry, revealing their robustness and critical delocalization phenomena.

Contribution

The paper provides extensive numerical analysis of the stability and properties of Majorana surface states under strong disorder in topological superconductors.

Findings

Majorana surface states are protected against weak disorder.

Strong disorder partially localizes surface states, especially near the gap.

Zero-energy extended states persist across all disorder levels.

Abstract

Noncentrosymmetric superconductors with strong spin-orbit coupling and the B phase of ${}^3$He are possible realizations of topological superconductors with time-reversal symmetry. The nontrivial topology of these time- reversal invariant superconductors manifests itself at the material surface in the form of helical Majorana modes. In this paper, using extensive numerical simulations, we investigate the stability and properties of these Majorana states under strong surface disorder, which influences both bulk and surface states. To characterize the effects of strong disorder, we compute the level spacing statistics and the local density of states of both two- and three-dimensional topological superconductors. The Majorana surface states, which are located in the outermost layers of the superconductor, are protected against weak disorder, due to their topological characteristic. Sufficiently strong disorder, on the other hand, partially localizes the surface layers, with a more pronounced effect on states with energies close to the gap than on those with energies close to zero. In particular, we observe that for all disorder strengths and configurations there always exist two extended states at zero-energy that can carry thermal current. At the crossover from weak to strong disorder the surface state wave functions and the local density of states show signs of critical delocalization. We find that at this crossover the edge density of states of two-dimensional topological superconductors exhibits a zero-energy divergence, reminiscent of the Dyson singularity of quasi-one-dimensional dirty superconductors.