The term type-1.5 superconductor refers to a multicomponent superconductor characterized by two or more coherence lengths related to the magnetic field penetration length as follows: .
Physically it means that it has two superconducting components, which densities vary on two different characteristic length scales. One of which is larger and another is smaller than the characteristic length scale of the variation of magnetic field.
As a consequence, it has behavior different from that of type-I, where and type-II superconductors, where .
Type-1.5 superconductors should possess quantum vortices: magnetic flux carrying excitations which allow magnetic field to pass through superconductor due to a vortex-like circulation of superconducting particles. In contrast to type-II superconductors these vortices have long-range attractive, short-range repulsive interaction. As a consequence type-1.5 superconductor in magnetic field undergoes a macroscopic phase separation into domains of Meissner state (domains with expelled magnetic field) and clusters of quantum vortices which are bound together by attractive intervortex forces. The domains of 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.
Animation from numerical calculations of vortex cluster formation are available at "Numerical simulations of vortex clusters formation in type-1.5 superconductors."
Read more about Type-1.5 Superconductor: Detailed Explanation, Type-1.5 Superconductor in Mixtures of Independently Conserved Condensates, Type-1.5 Superconductivity in Multiband Systems, Microscopic Models, Current Experimental Research, Non-technical Explanation, Animations of Type-1.5 Superconducting Behavior, See Also