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.
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.
These quantum vortices repel each other and thus tend to form uniform vortex lattices or liquids.
[1] 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.
To produce them would require the application of a magnetic field stronger than what a superconducting condensate can sustain.
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[2] that the type-I/type-II dichotomy could be broken in a multi-component superconductors, which possess multiple coherence lengths.
It was also pointed out that systems which have phase transitions between different superconducting states such as between
should rather generically fall into type-1.5 state near that transition due to divergence of one of the coherence lengths.
The vortex excitations in that case have cores in both components which are co-centered because of electromagnetic coupling mediated by the field
[2] Additional condition of thermodynamic stability is satisfied for a range of parameters.
[2][3][4] It was shown that there is a range of parameters where these vortices are energetically favorable enough to be excitable by an external field, attractive interaction notwithstanding.
This results in the formation of a special superconducting phase in low magnetic fields dubbed "Semi-Meissner" state.
Such vortex clusters should coexist with the areas of vortex-less two-component Meissner domains.
In a two-band superconductor the electrons in different bands are not independently conserved thus the definition of two superconducting components is different.
[4] In 2009, experimental results have been reported[7][8][9] claiming that magnesium diboride may fall into this new class of superconductivity.
[10] More recent theoretical works show that the type-1.5 may be more general phenomenon because it does not require a material with two truly superconducting bands, but can also happen as a result of even very small interband proximity effect[6] and is robust in the presence of various inter-band couplings such as interband Josephson coupling.
[12] Type-I and type-II superconductors feature dramatically different charge flow patterns.
Type-I superconductors have two state-defining properties: The lack of electric resistance and the fact that they do not allow an external magnetic field to pass through them.
An externally applied magnetic field of sufficiently low strength is cancelled in the interior of a type-I superconductor by the field produced by the surface current.
In a type-II material, magnetic fields can penetrate into the interior, carried inside by vortices that form an Abrikosov vortex lattice.
In such materials, the external magnetic field can produce clusters of tightly packed vortex droplets because in such materials vortices should attract each other at large distances and repel at short length scales.
Thus a vortex cluster will represent two competing types of superflow.
One component will form vortices bunched together while the second component will produce supercurrent flowing on the surface of vortex clusters in a way similar to how electrons flow on the exterior of type-I superconductors.
These vortex clusters are separated by "voids," with no vortices, no currents and no magnetic field.
Movies from numerical simulations of the Semi-Meissner state where Meissner domains coexist with clusters where vortex droplets form in one superconducting components and macroscopic normal domains in the other.