Topologically protected flat zero-energy surface bands in noncentrosymmetric superconductors

Nodal noncentrosymmetric superconductors (NCS) have recently been shown to be topologically nontrivial. 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 [A. P. Schnyder and S. Ryu, arXiv:1011.1438]. In this Rapid Communication, 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 nontrivial NCS.