Two-band superconductors: Hidden criticality deep in the superconducting state

TL;DR

This paper reveals hidden critical phenomena in two-band superconductors caused by the weaker band's independent critical temperature, leading to unusual coherence length behavior near the hidden critical point, with potential experimental detection.

Contribution

It uncovers a hidden criticality deep within the superconducting state of two-band superconductors, characterized by a non-monotonic coherence length peak and a universal scaling law.

Findings

Coherence length of the weaker band peaks near the hidden critical point.

Peak magnitude scales as gamma^(-1/3), indicating universal critical behavior.

Hidden criticality can be observed via vortex core imaging.

Abstract

We show that two-band superconductors harbor hidden criticality deep in the superconducting state, stemming from the critical temperature of the weaker band taken as an independent system. For sufficiently small interband coupling $\gamma$ the coherence length of the weaker band exhibits a remarkable deviation from the conventional monotonic increase with temperature, namely, a pronounced peak close to the hidden critical point. The magnitude of the peak scales proportionally to \gamma^(-\mu), with the Landau critical exponent \mu = 1/3, the same as found for the mean-field critical behavior with respect to the source field in ferromagnets and ferroelectrics. Here reported hidden criticality of multi-band superconductors can be experimentally observed by, e.g., imaging of the variations of the vortex core in a broader temperature range. Similar effects are expected for the superconducting multilayers.