Type-1.5 superconductivity in multiband systems: Effects of interband couplings

Abstract

In contrast to single-component superconductors, which are described at the level of Ginzburg-Landau theory by a single parameter $\ensuremath{\kappa}$ and are divided in type-I $\ensuremath{\kappa}<1/\sqrt{2}$ and type-II $\ensuremath{\kappa}>1/\sqrt{2}$ 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 active 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.