Type-1.5 superconductivity in muliband and other multicomponent systems

Usual superconductors are classified into two categories as follows: type-1 when the ratio of the magnetic field penetration length (λ) to coherence length (ξ) with Ginzburg-Landau parameter κ=λ/ξ<1/\sqrt{2} and type-2 when κ>1/\sqrt{2}. The boundary case κ=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 ξ_1<\sqrt{2}λ<ξ_2. We also briefly overview the works on single-component regime $κ\approx 1/\sqrt{2}$ and comment on recent discussion by Brandt and Das in the proceedings of the previous conference in this series.

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