Semi-Meissner state

Type-1.5 superconductors are multicomponent superconductors characterized by two or more coherence lengths, at least one of which is shorter than the magnetic field penetration length ${\displaystyle \lambda }$, and at least one of which is longer. This is in contrast to singe-component superconductors, where there is only one coherence length ${\displaystyle \xi }$ and the superconductor is necessarily either type 1 (${\displaystyle \xi >\lambda }$) or type 2 (${\displaystyle \xi <\lambda }$) (often a coherence length is defined with extra ${\displaystyle 2^{1/2}}$ factor, with such a definition the corresponding inequalities are ${\displaystyle \xi >{\sqrt {2}}\lambda }$ and ${\displaystyle \xi <{\sqrt {2}}\lambda }$). When placed in magnetic field, type-1.5 superconductors should form quantum vortices: magnetic-flux-carrying excitations. They allow magnetic field to pass through superconductors due to a vortex-like circulation of superconducting particles (electronic pairs). In type-1.5 superconductors these vortices have long-range attractive, short-range repulsive interaction. As a consequence a type-1.5 superconductor in a magnetic field can form a phase separation into domains with expelled magnetic field and clusters of quantum vortices which are bound together by attractive intervortex forces. The domains of the Meissner state retain the two-component superconductivity, while in the vortex clusters one of the superconducting components is suppressed. Thus such materials should allow coexistence of various properties of type-I and type-II superconductors.

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