Type-II superconductor

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A Type-II superconductor is one of the two main types of superconductor. It is characterised by its gradual transition from the superconducting to the normal state. Type-II superconductors tend to be made of metal alloys or complex oxide ceramics, whereas Type-I superconductors tend to be made of pure metals.

1 Materials
2 Critical temperatures
3 Mixed state
4 References
5 See also

[edit] Materials

All high temperature superconductors fall into the Type II category of superconductors, and are made of complex oxide ceramics. Some examples include the cuprate-perovskite ceramic materials which have achieved the highest temperatures to reach the superconducting state. This includes La_1.85Ba_0.15CuO₄, and YBCO (Yttrium-Barium-Copper-Oxide), which is famous as the first material to achieve superconductivity above the boiling point of liquid nitrogen, as well as the highest temperature superconductor to date: mercury thallium barium calcium copper oxide (Hg₁₂Tl₃Ba₃₀Ca₃₀Cu₄₅O₁₂₅).

While most pure metal or pure element superconductors are Type I, Niobium, Vanadium and Technetium are pure element Type II superconductors. Some metal alloy superconductors also exhibit Type II behavior.

[edit] Critical temperatures

In comparison to the (theoretically) sharp transition of a Type-I superconductor 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. Alternatively it is possible to consider a fixed temperature, in which case transition occurs between critical field strengths H_c1 and H_c2.

[edit] Mixed state

The coherence length of a superconductor is related to the mean free path of its charge carriers. Its 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) lattice, after Alexei Alexeyevich Abrikosov, who was awarded the 2003 Nobel Prize in Physics for his pioneering contributions.^[1]

[edit] References

^ which he summarized in "Nobel Lecture: Type-II superconductors and the vortex lattice", published in Reviews of Modern Physics, volume 76, July 2004, pages 975-979