Surface instability in nodal noncentrosymmetric superconductors

TL;DR

This paper investigates the stability of zero-energy surface states in nodal noncentrosymmetric superconductors, revealing a temperature-driven transition that breaks time-reversal symmetry and induces surface currents and spin polarization.

Contribution

It demonstrates that surface flat bands persist over a broad temperature range and identifies a spontaneous symmetry-breaking transition at lower temperatures.

Findings

Surface flat bands are stable below the bulk transition temperature.

A second transition leads to time-reversal symmetry breaking.

Surface bands shift from zero energy, becoming dispersive with induced currents.

Abstract

We study the stability of topologically protected zero-energy flat bands at the surface of nodal noncentrosymmetric superconductors, accounting for the alteration of the gap near the surface. Within a selfconsistent mean-field theory, we show that the flat bands survive in a broad temperature range below the bulk transition temperature. There is a second transition at a lower temperature, however, below which the system spontaneously breaks time-reversal symmetry. The surface bands are shifted away from zero energy and become weakly dispersive. Simultaneously, a spin polarization and an equilibrium charge current develop in the surface region.