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 λ {\displaystyle \lambda } , and at least one of which is longer. This is in contrast to single-component superconductors, where there is only one coherence length ξ {\displaystyle \xi } and the superconductor is necessarily either type 1 ( ξ > λ {\displaystyle \xi >\lambda } ) or type 2 ( ξ < λ {\displaystyle \xi <\lambda } ) (often a coherence length is defined with extra 2 1 / 2 {\displaystyle 2^{1/2}} factor, with such a definition the corresponding inequalities are ξ > 2 λ {\displaystyle \xi >{\sqrt {2}}\lambda } and ξ < 2 λ {\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.
Description Type-I superconductors completely expel external magnetic fields if the strength of the applied field is sufficiently low. Also the supercurrent can flow only on the surface of such a superconductor but not in its interior. This state is called the Meissner state. However at elevated magnetic field, when the magnetic field energy becomes comparable with the superconducting condensation energy, the superconductivity is destroyed by the formation of macroscopically large inclusions of non-superconducting phase. Type-II superconductors, besides the Meissner state, possess another state: a sufficiently strong applied magnetic field can produce currents in the interior of superconductor due to formation of quantum vortices. The vortices also carry magnetic flux through the interior of the superconductor. These quantum vortices repel each other and thus tend to form uniform vortex lattices or liquids. Formally, vortex solutions exist also in models of type-I superconductivity, but the interaction between vortices is purely attractive, so a system of many vortices is unstable against a collapse onto a state of a single giant normal domain with supercurrent flowing on its surface. More importantly, the vortices in type-I superconductor are energetically unfavorable. To produce them would require the application of a magnetic field stronger than what a superconducting condensate can sustain. Thus a type-I superconductor goes to non-superconducting states rather than forming vortices. In the usual Ginzburg–Landau theory, only the quantum vortices with purely repulsive interaction are energetically cheap enough to be induced by applied magnetic field. It was proposed that the type-I/type-II dichotomy could be broken in a multi-component superconductors, which possess multiple coherence lengths. Examples of multi-component superconductivity are multi-band superconductors magnesium diboride and oxypnictides and exotic superconductors with nontrivial Cooper-pairing. There, one can distinguish two or more superconducting components associated, for example with electrons belong to different bands band structure. A different example of two component systems is the projected superconducting states of liquid metallic hydrogen or deuterium where mixtures of superconducting electrons and superconducting protons or deuterons were theoretically predicted. It was also pointed out that systems which have phase transitions between different superconducting states such as between s {\displaystyle s} and s + i s {\displaystyle s+is} or between U ( 1 ) {\displaystyle U(1)} and U ( 1 ) × U ( 1 ) {\displaystyle U(1)\times U(1)} should rather generically fall into type-1.5 state near that transition due to divergence of one of the coherence lengths.
In mixtures of independently conserved condensates For multicomponent superconductors with so called U(1)xU(1) symmetry the Ginzburg-Landau model is a sum of two single-component Ginzburg-Landau model which are coupled by a vector potential
A {\displaystyle A} :
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