Ginzburg–Landau theory introduced the
superconducting coherence length ξ in addition to
London magnetic field penetration depth λ. According to Ginzburg–Landau theory, in a type-II superconductor \lambda/\xi >1/\sqrt{2}. Ginzburg and Landau showed that this leads to negative energy of the interface between superconducting and normal phases. The existence of the negative interface energy was also known since the mid-1930s from the early works by the London brothers. A negative
interface energy suggests that the system should be unstable against maximizing the number of such interfaces. This instability was not observed until the experiments of Shubnikov in 1936 where two critical fields were found. In 1952 an observation of type-II superconductivity was also reported by Zavaritskii.
Fritz London demonstrated that a magnetic flux can penetrate a superconductor via a topological defect that has integer phase winding and carries quantized magnetic flux. Onsager and Feynman demonstrated that quantum vortices should form in superfluids. A 1957 paper by
A. A. Abrikosov generalizes these ideas. In the limit of very short coherence length the vortex solution is identical to London's fluxoid, If a superconductor is cooled in a field, the field can be trapped, which can allow the superconductor to be suspended over a magnet, with the potential for a frictionless joint or bearing. The worth of flux pinning is seen through many implementations such as lifts, frictionless joints, and transportation. The thinner the superconducting layer, the stronger the pinning that occurs when exposed to magnetic fields. ==Materials==