Metastable $\pi$-junction between an s$_\pm$-wave and an s-wave superconductor

TL;DR

This paper investigates a Josephson junction between an s±-wave superconductor and an s-wave superconductor, revealing how sign-changing order parameters can cause time-reversal symmetry breaking and complex energy-phase behaviors.

Contribution

It introduces a Ginzburg-Landau model to explain how interfacial frustration in s±-wave and s-wave superconductor contacts leads to novel phenomena like time-reversal symmetry breaking.

Findings

Time-reversal symmetry breaking at the interface.

Unusual energy-phase relations with multiple minima.

Possible explanation for half-integer flux quantum transitions.

Abstract

We examine a contact between a superconductor whose order parameter changes sign across the Brillioun zone, and an ordinary, uniform-sign superconductor. Within a Ginzburg-Landau type model, we find that if the the barrier between the two superconductors is not too high, the frustration of the Josephson coupling between different portions of the Fermi surface across the contact can lead to surprising consequences. These include time-reversal symmetry breaking at the interface and unusual energy-phase relations with multiple local minima. We propose this mechanism as a possible explanation for the half-integer flux quantum transitions in composite niobium--iron pnictide superconducting loops, which were discovered in a recent experiment [Chen et al., Nature Phys. \textbf{6},260 (2010)].