"Chain scenario" for Josephson tunneling with pi-shift in YBa2Cu3O7

TL;DR

This paper interprets Josephson tunneling experiments in YBa2Cu3O7 using a bilayer antiferromagnetic spin fluctuation model, revealing a pi phase shift due to the sign difference in order parameters across bands.

Contribution

It introduces a new interpretation of Josephson experiments in YBa2Cu3O7 based on a bilayer spin fluctuation model with extended s-wave symmetry and opposite signs in different bands.

Findings

Josephson currents exhibit a pi phase shift along and perpendicular to chains.

The model explains experimental observations with extended s-wave symmetry.

Cu-O chain band shares the same sign as antibonding plane band.

Abstract

We point out that all current Josephson-junction experiments probing directly the symmetry of the superconducting state in YBa2Cu3O7, can be interpreted in terms of the bilayer antiferromagnetic spin fluctuation model, which renders the superconducting state with the order parameters of extended $s$ symmetry, but with the opposite signs in the bonding and antibonding Cu-O plane bands. The essential part of our interpretation includes the Cu-O chain band which would have the order parameter of the same sign as antibonding plane band. We show that in this case net Josephson currents along and perpendicular to the chains have the phase shift equal to pi.