Josephson effect in two-band superconductors

TL;DR

This paper theoretically investigates the Josephson effect in two-band superconductors, revealing complex phase-dependent current components and conditions under which the current-phase relationship deviates from sinusoidal behavior due to time-reversal symmetry breaking.

Contribution

It introduces a detailed theoretical model for Josephson currents in two-band superconductors considering band hybridization and phase differences, highlighting novel current components.

Findings

Josephson current has three components depending on phase differences.

Band hybridization generates a new component of the Josephson current.

Time-reversal symmetry breaking can cause non-sinusoidal current-phase relations.

Abstract

We study theoretically the Josephson effect between two time-reversal two-band superconductors, where we assume the equal-time spin-singlet $s$-wave pair potential in each conduction band. %as well as the band asymmetry and the band hybridization in the normal state. The superconducting phase at the first band $\varphi_1$ and that at the second band $\varphi_2$ characterize a two-band superconducting state. We consider a Josephson junction where an insulating barrier separates two such two-band superconductors. By applying the tunnel Hamiltonian description, the Josephson current is calculated in terms of the anomalous Green's function on either side of the junction. We find that the Josephson current consists of three components which depend on three types of phase differences across the junction: the phase difference at the first band $\delta\varphi_1$, the phase difference at the second band $\delta\varphi_2$, and the difference at the center-of-mass phase $\delta(\varphi_1+\varphi_2)/2$. A Cooper pairs generated by the band hybridization carries the last current component. In some cases, the current-phase relationship deviates from the sinusoidal function as a result of time-reversal symmetry breaking down.