Type-II superconductor

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A Type-II superconductor is a superconductor characterised by its gradual transition from the superconducting to the normal state within an increasing magnetic field. Typically they superconduct at higher temperatures and magnetic fields than Type-I superconductors. This allows them to conduct higher currents.

Materials

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, Technetium, Diamond and Silicon are pure element Type-II superconductors. Some metal alloy superconductors also exhibit Type-II behavior (e.g. niobium-titanium, niobium-tin).

Other Type-II examples are the cuprate-perovskite ceramic materials which have achieved the highest temperatures to reach the superconducting state. These include La_1.85Ba_0.15CuO₄, BSCCO, and YBCO (Yttrium-Barium-Copper-Oxide), which is famous as the first material to achieve superconductivity above the boiling point of liquid nitrogen.

Critical temperatures and critical fields

In comparison to the (theoretically) sharp transition of a Type-I superconductor above the lower temperature T_c1, magnetic flux from external fields is no longer completely expelled, and the superconductor exists in a mixed state. Above the higher temperature T_c2, the superconductivity is completely destroyed, and the material exists in a normal state. Both of these temperatures are dependent on the strength of the applied field. It is more usual to consider a fixed temperature, in which case transition (flux penetration) occurs between critical field strengths H_c1 and H_c2 (the upper critical field).^[1]

Mixed state

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, meaning that magnetic flux lines can pierce the material at high enough external fields. 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.^[2]

See also

References

1. ^ http://www-unix.mcs.anl.gov/superconductivity/phase.html H-T diagrams of Type-I and Type-II SC
2. ^ Abrikosov, A. A. (July 2004). "Nobel Lecture: Type-II superconductors and the vortex lattice". Reviews of Modern Physics 76: 975–979. doi:10.1103/RevModPhys.76.975.