Field-induced coexistence of $s_{++}$ and $s_{\pm}$ superconducting states in dirty multiband superconductors

TL;DR

This paper explores how external magnetic fields and currents induce a complex coexistence of $s_{++}$ and $s_{ p}$$ states in dirty multiband superconductors, challenging the notion of mutually exclusive states.

Contribution

It reveals that external fields cause a smooth crossover and coexistence of different superconducting states, with implications for vortex and boundary phenomena in multiband superconductors.

Findings

Coexistence of $s_{++}$ and $s_{ p}$ states in certain regions.

Magnetic fields induce a crossover between pure $s_{ p}$ and $s_{++}$ states.

Regions near vortices can host different states than the bulk.

Abstract

In multiband systems, such as iron-based superconductors, the superconducting states with locking and anti-locking of the interband phase differences, are usually considered as mutually exclusive. For example, a dirty two-band system with interband impurity scattering undergoes a sharp crossover between the $s_{\pm}$ state (which favors phase anti locking) and the $s_{++}$ state (which favors phase locking). We discuss here that the situation can be much more complex in the presence of an external field or superconducting currents. In an external applied magnetic field, dirty two-band superconductors do not feature a sharp $s_{\pm}\to s_{++}$ crossover but rather a washed-out crossover to a finite region in the parameter space where both $s_{\pm}$ and $s_{++}$ states can coexist for example as a lattice or a microemulsion of inclusions of different states. The current-carrying regions such as the regions near vortex cores can exhibit an $s_\pm$ state while it is the $s_{++}$ state that is favored in the bulk. This coexistence of both states can even be realized in the Meissner state at the domain's boundaries featuring Meissner currents. We demonstrate that there is a magnetic-field-driven crossover between the pure $s_{\pm}$ and the $s_{++}$ states.