Magnetic field-induced gapless state in multiband superconductors

TL;DR

This paper theoretically explores a new gapless superconducting state in multiband superconductors under magnetic fields, revealing a first-order phase transition and phase diagram features, with suggestions for experimental detection.

Contribution

It introduces a novel gapless state induced by magnetic fields in multiband superconductors and analyzes its phase transition characteristics.

Findings

Discovery of a new gapless superconducting state in multiband systems.

Identification of a first-order metamagnetic phase transition at zero temperature.

Phase diagram features including a critical region near the transition.

Abstract

We investigate theoretically the properties of s-wave multiband superconductors in the weak coupling (BCS) limit in the presence of pair-breaking effects of magnetic field. It is shown that a qualitatively new gapless superconducting state must appear in quasi-2D superconductors in magnetic fields parallel to the plane, corresponding to a Sarma state induced on one of the Fermi surfaces. The emergence of the new state in s-wave multiband superconductors in the absence of anisotropy or spin-orbit interaction is usually accompanied by a zero-temperature first-order metamagnetic phase transition. For anisotropic or non s-wave multiband superconductors the order of the zero-temperature metamagnetic transition depends on model parameters, and it may take the form of a smooth crossover. The details of the temperature-magnetic field phase diagram for multiband superconductors are investigated analytically at zero temperature and numerically at a finite temperature. It is shown the zero-temperatures first-order phase transition gives rise to a critical region on the B-T phase diagram. We suggest possible experiments to detect the new gapless state.