Type-1.5 superconductivity in multiband systems: the effects of interband couplings

TL;DR

This paper explores how multiband superconductors with strong interband couplings can exhibit the unique type-1.5 superconductivity, characterized by complex vortex interactions due to multiple fundamental length scales.

Contribution

It analyzes the effects of various interband couplings on the emergence of type-1.5 superconductivity and clarifies the interpretation of fundamental length scales in such systems.

Findings

Type-1.5 superconductivity can occur with strong interband couplings.

Fundamental length scales relate to gauge field mass and mixed normal mode masses.

Vortex interactions are influenced by these multiple length scales.

Abstract

In contrast to single-component superconductors, which are described at the level of Ginzburg-Landau theory by a single parameter \kappa and are divided in type-I \kappa<1/\sqrt{2} and type-II \kappa>1/\sqrt{2} classes, two-component systems in general possess three fundamental length scales and have been shown to possess a separate "type-1.5" superconducting state. In that state, as a consequence of the extra fundamental length scale, vortices attract one another at long range but repel at shorter ranges, and therefore should form clusters in low magnetic fields. In this work we investigate the appearance of type-1.5 superconductivity and the interpretation of the fundamental length scales in the case of two bands with substantial interband couplings such as intrinsic Josephson coupling, mixed gradient coupling and density-density interactions. We show that in the presence of substantial intercomponent interactions of the above types the system supports type-1.5 superconductivity with fundamental length scales being associated with the mass of the gauge field and two masses of normal modes represented by mixed combinations of the density fields.