Critical Currents in Conventional Josephson Junctions With Grain Boundaries

TL;DR

This paper investigates how grain boundaries and associated disorder significantly reduce the critical current in Josephson junctions, impacting their performance in superconducting electronics.

Contribution

It introduces a physically realistic model of distance-dependent hopping amplitudes and demonstrates the substantial effect of grain boundaries on critical currents.

Findings

Grain boundaries cause large drops in critical current.

Disorder near tunnel barriers leads to critical current variations.

Results are relevant for superconducting electronics applications.

Abstract

It has been hypothesized that the variation of the critical currents in Nb/Al-AlO$_x$/Nb junctions is due to, among other effects, the presence of grain boundaries in the system. Motivated by this, we examine the effect of grain boundaries on the critical current of a Josephson junction. We assume that the hopping amplitudes are dependent on the interatomic distance, and derive a physically realistic model of distance-dependent hopping amplitudes. We find that the presence of a grain boundary and associated disorder is responsible for a very large drop in the critical current relative to a clean system. We also find that when a tunnel barrier is present, grain boundaries cause substantial variations in the critical currents due to the disordered hoppings near the tunnel barrier. We discuss the applicability of these results to Josephson junctions presently intended for use in superconducting electronics applications.