Type-1.5 Superconductivity in Multiband and Other Multicomponent Systems

Abstract

Usual superconductors are classified into two categories as follows: type-1 when the ratio of the magnetic field penetration length (\lambda) to coherence length (\xi) with Ginzburg-Landau parameter \kappa=\lambda/\xi <1/\sqrt{2} and type-2 when \kappa >1/\sqrt{2}. The boundary case \kappa =1/\sqrt{2} is also considered to be a special situation, frequently termed as "Bogomolnyi limit". Here we discuss multicomponent systems which can possess three or more fundamental length scales and allow a separate superconducting state, which was recently termed "type-1.5". In that state a system has the following hierarchy of coherence and penetration lengths \xi_1<\sqrt{2}\lambda<\xi_2. We also briefly overview the works on single-component regime $\kappa \approx 1/\sqrt{2}$ and comment on recent discussion by Brandt and Das in the proceedings of the previous conference in this series.