A phenomenological theory of zero-energy Andreev resonant states

TL;DR

This paper develops a phenomenological theory explaining the formation of zero-energy states at the surfaces of unconventional superconductors, linking them to observable zero-bias conductance peaks and their splitting under broken time-reversal symmetry.

Contribution

It introduces a phenomenological framework emphasizing the role of pair potential sign change in ZES formation and explains ZBCP splitting and magnetic field effects.

Findings

ZES linked to sign change in pair potential

ZBCP observed as zero-bias conductance peak

Splitting of ZBCP explained by broken time-reversal symmetry

Abstract

A conceptual consideration is given to a zero-energy state (ZES) at the surface of unconventional superconductors. The reflection coefficients in normal-metal / superconductor (NS) junctions are calculated based on a phenomenological description of the reflection processes of a quasiparticle. The phenomenological theory reveals the importance of the sign change in the pair potential for the formation of the ZES. The ZES is observed as the zero-bias conductance peak (ZBCP) in the differential conductance of NS junctions. The split of the ZBCP due to broken time-reversal symmetry states is naturally understood in the present theory. We also discuss effects of external magnetic fields on the ZBCP.