Josephson effect in the cuprates: microscopic implications

TL;DR

This paper investigates the Josephson effect in cuprate superconductors, proposing that quantum fluctuations of pairing symmetry explain deviations from BCS theory predictions in current-phase relations.

Contribution

It introduces a model where quantum fluctuations of pairing symmetry account for anomalous Josephson junction behaviors in cuprates, differing from standard BCS expectations.

Findings

Experimental values of $I_1R_N$ are smaller than BCS predictions.

The ratio $I_2/I_1$ is larger than BCS predictions.

Quantum fluctuations of pairing symmetry explain these anomalies.

Abstract

In the tunnel limit, the current-phase relation of Josephson junctions can be expanded as $I(\phi)=I_1\sin\phi+I_2\sin 2\phi$. Standard BCS theory predicts that $I_1R_N\sim\Delta/e$ and $I_2/I_1\sim D$, where $R_N$ is the resistance of the junction in the normal state, $\Delta$ is the superconducting gap, and $D\ll 1$ is the junction transparency. In the cuprates, the experimental value of $I_1R_N$ ($I_2/I_1$) is much smaller (larger) than the BCS prediction. We argue that both peculiarities of the cuprates can be explained by postulating quantum fluctuations of the pairing symmetry.