A Type-II superconductor is a superconductor characterized by the formation of vortex lattices in magnetic field. It has a continuous second order phase transition from the superconducting to the normal state within an increasing magnetic field. The possibility of type-II superconductivity was theoretically predicted by Alexei Alexeyevich Abrikosov, for which a Nobel Prize in Physics was awarded in 2003.[1]
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Type-II superconductors are usually made of metal alloys or complex oxide ceramics, whereas most superconducting pure metals are Type-I superconductors. All high temperature superconductors are Type-II superconductors, and (as of early 2008) comprise mostly complex copper oxide ceramics. While most pure metal or pure element superconductors are Type-I, niobium, vanadium, and technetium are pure element 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, niobium-tin).
Niobium-tin was discovered in 1954.
Other Type-II examples are the cuprate-perovskite ceramic materials which have achieved the highest temperatures to reach the superconducting state. 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. However, the lattice of graphene that was experimented with is also believed to be a Type-II superconductor.
In 2001 Magnesium diboride was discovered to be a type-II SC with useful properties. Until the discovery of the iron-arsenide family, It was the highest temperature superconductor not containing copper.
Strong superconducting electromagnets (used in MRI scanners, NMR machines, and particle accelerators) often use niobium-titanium or, for higher fields, niobium-tin.
The continuous transition to a vortex state (flux penetration) occurs at first critical magnetic field Hc1 and system loses superconductivity at second critical magnetic field Hc2 (the upper critical field).[2]
Ginzburg–Landau theory defines 2 parameters: The coherence length of a superconductor, related to the mean free path of its charge carriers, and a penetration depth.
The earlier London penetration depth is the penetration distance of a weak magnetic field.
In a Type-II superconductor, the coherence length is smaller than the London penetration depth. This is known as the vortex state, as the flux lines run through narrow regions of non superconducting material, surrounded by vortices of supercurrents protecting the rest of the superconductor. The vortices can arrange themselves in a regular structure known as the vortex lattice, also named the Abrikosov vortex, after Alexei Alexeyevich Abrikosov, who was awarded the 2003 Nobel Prize in Physics for his pioneering contributions.[3]