Resonant transport through midgap states in voltage-biased Josephson junctions of d-wave superconductors

TL;DR

This paper theoretically investigates the ac Josephson effect in voltage-biased d-wave superconductor junctions, revealing how midgap states cause resonant peaks in current-voltage characteristics and explaining experimental zero-bias conductance peaks.

Contribution

It provides an analytical model showing the role of midgap states as resonances in d-wave superconductor junctions, linking theory with experimental observations.

Findings

Resonant current peaks at finite voltage due to midgap states

Analytical formula illustrating the resonance mechanism

Explanation of zero-bias conductance peaks in experiments

Abstract

We study theoretically the ac Josephson effect in voltage biased planar junctions of d-wave superconductors. For some orientations of the superconductors a current peak is found at finite voltage in the current-voltage characteristics. We pick out the relevant physical processes and write down an analytical formula for the current which clearly shows how the midgap state acts as a resonance and produces the peak. We present a possible explanation for the zero-bias conductance peak, recently found in experiments on grain boundary junctions of high-temperature superconductors, in terms of resonant transmission through midgap state of quasiparticles undergoing multiple Andreev reflections. We note that within our framework the zero-bias conductance peak appears in rather transparent Josephson junctions of d-wave superconductors.