Topologically protected flat zero-energy surface bands in non-centrosymmetric superconductors

TL;DR

This paper investigates topologically protected flat zero-energy surface bands in non-centrosymmetric superconductors, linking their existence to the bulk gap's topological properties and surface orientation, with implications for experimental detection.

Contribution

It provides a general condition for the existence of zero-energy surface bands in non-centrosymmetric superconductors using quasiclassical theory.

Findings

Zero-energy surface bands are related to the sign change of gap functions.

Surface orientation significantly affects the tunneling conductance signature.

Zero-bias peaks can serve as fingerprints of topologically non-trivial NCS.

Abstract

Nodal non-centrosymmetric superconductors (NCS) have recently been shown to be topologically non-trivial. An important consequence is the existence of topologically protected flat zero-energy surface bands, which are related to the topological characteristics of the line nodes of the bulk gap via a bulk-boundary correspondence [Schnyder and Ryu, arXiv:1011.1438]. In this paper we examine these zero-energy surface bands using a quasiclassical theory. We determine their spectrum and derive a general condition for their existence in terms of the sign change of the gap functions. A key experimental signature of the zero-energy surface bands is a zero-bias peak in the tunneling conductance, which depends strongly on the surface orientation. This can be used as a fingerprint of a topologically non-trivial NCS.