Quasiparticle Bound States and Low-Temperature Peaks of the Conductance of NIS Junctions in d-Wave Superconductors

TL;DR

This paper investigates quasiparticle bound states at the boundaries of d-wave superconductors and their impact on NIS junction conductance, revealing low-temperature peaks and effects of surface roughness.

Contribution

It provides analytical and numerical analysis of bound state energies and their influence on conductance, including effects of interface roughness, in anisotropic superconductors.

Findings

Bound states with nonzero energies exist near the surface normal.

Low-temperature conductance peaks relate to bound state energy extrema.

Surface roughness affects the visibility of conductance features.

Abstract

Quasiparticle states bound to the boundary of anisotropically paired superconductors, their contributions to the density of states and to the conductance of NIS junctions are studied both analytically and numerically. For smooth surfaces and real order parameter we find some general results for the bound state energies. In particular, we show that under fairly general conditions quasiparticle states with nonzero energies exist for momentum directions within a narrow region around the surface normal. The energy dispersion of the bound states always has an extremum for the direction along the normal. Along with the zero-bias anomaly due to midgap states, we find, for quasi two-dimensional materials, additional low-temperature peaks in the conductance of NIS junctions for voltages determined by the extrema of the bound state energies. The influence of interface roughness on the conductance is investigated within the framework of Ovchinnikov's model. We show that nonzero-bias peaks at low temperatures may give information on the order parameter in the bulk, even though it is suppressed at the surface.