In superconductivity, a type-II superconductor is characterized by the formation of magnetic vortices in an applied magnetic field. This occurs above a certain critical field strength Hc1. The vortex density increases with increasing field strength. At a higher critical field Hc2, superconductivity is completely destroyed.
The idea of two types of superconductors was proposed by Lev Landau and Vitaly Ginzburg in their paper on Ginzburg-Landau theory. In their argument, a type-I superconductor had positive free energy of the superconductor-normal metal boundary. At that time, all known superconductors were type-I, and initially type-II behavior was considered unphysical. Type-II superconducting behavior was first observed in experiments by Lev Shubnikov, who investigated superconducting alloys in a magnetic field, and later by Zavaritskii. The theory for the behavior of the type-II superconducting state in magnetic field was developed by Alexei Alexeyevich Abrikosov, who was elaborating on the ideas by Lars Onsager and Richard Feynman of quantum vortices in superfluids and Fritz London's idea of magnetic flux quantization in superconductors. The Nobel Prize in Physics was awarded for the theory of type-II superconductivity in 2003.
Ginzburg–Landau theory defines two parameters: The superconducting coherence length and the London magnetic field penetration depth. In a type-II superconductor, the coherence length is smaller than the penetration depth. This leads to negative energy of the interface between superconducting and normal phases. The existence of the negative interface energy was 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, which was not observed in first experiments on superconductors, before the experiments of Shubnikov in 1936 where two critical fields were found. As was later discussed by A.A. Abrikosov these interfaces manifest as lines of magnetic flux passing through the material, turning a region of the superconductor normal. This normal region is separated from the rest of the superconductor by a circulating supercurrent. In analogy with fluid dynamics, the swirling supercurrent creates what is known as a vortex, or an Abrikosov vortex, after Alexei Alexeyevich Abrikosov. He found that the vortices arrange themselves into a regular array known as a vortex lattice.
In the extreme type-II limit, the problem of type-II superconductor in magnetic field is exactly equivalent to that of vortex state in rotating superfluid helium, which was discussed earlier by Richard Feynman in 1955.
Type-II superconductors are usually made of metal alloys or complex oxide ceramics. All high temperature superconductors are type-II superconductors. While most elemental superconductors are type-I, niobium, vanadium, and technetium are elemental type-II superconductors. Boron-doped diamond and silicon are also type-II superconductors. Metal alloy superconductors also exhibit type-II behavior (e.g. niobium-titanium and niobium-tin).
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.