Type-II superconductor

[5] In their argument, a type-I superconductor had positive free energy of the superconductor-normal metal boundary.

Ginzburg and Landau pointed out the possibility of type-II superconductors that should form inhomogeneous state in strong magnetic fields.

However, at that time, all known superconductors were type-I, and they commented that there was no experimental motivation to consider precise structure of type-II superconducting state.

The theory for the behavior of the type-II superconducting state in magnetic field was greatly improved by Alexei Alexeyevich Abrikosov,[6] who was elaborating on the ideas by Lars Onsager and Richard Feynman of quantum vortices in superfluids.

[7] Ginzburg–Landau theory introduced the superconducting coherence length ξ in addition to London magnetic field penetration depth λ.

Ginzburg and Landau showed that this leads to negative energy of the interface between superconducting and normal phases.

The thinner the superconducting layer, the stronger the pinning that occurs when exposed to magnetic fields.

Other type-II examples are the cuprate-perovskite ceramic materials which have achieved the highest superconducting critical temperatures.

These include La1.85Ba0.15CuO4, BSCCO, and YBCO (Yttrium-Barium-Copper-Oxide), which is famous as the first material to achieve superconductivity above the boiling point of liquid nitrogen (77 K).

Due to strong vortex pinning, the cuprates are close to ideally hard superconductors.

Superconductive behavior under varying magnetic field and temperature. The graph shows magnetic flux B as a function of absolute temperature T . Critical magnetic flux densities B C1 and B C2 and the critical temperature T C are labeled. In the lower region of this graph, both type-I and type-II superconductors display the Meissner effect (a). A mixed state (b), in which some field lines are captured in magnetic field vortices, occurs only in Type-II superconductors within a limited region of the graph. Beyond this region, the superconductive property breaks down, and the material behaves as a normal conductor (c).
Quantum vortices in a 200-nm-thick YBCO film imaged by scanning SQUID microscopy [ 1 ]
Position memory due to vortex pinning in a high temperature superconductor