Type-II superconductor

The B-T diagram of type-II superconductor.

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 H_c1. The vortex density increases with increasing field strength. At a higher critical field H_c2, superconductivity is destroyed. Type-II superconductors do not exhibit a complete Meissner effect. ^[1]

History

In 1935, Rjabinin, Lev Shubnikov^[2][3] experimentally discovered the Type-II superconductors. In 1950, the theory of the two types of superconductors was further developed by Lev Landau and Vitaly Ginzburg in their paper on Ginzburg-Landau theory.^[4] 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. The theory for the behavior of the Type-II superconducting state in magnetic field was greatly improved 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.^[5]

Vortex state

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.^[5]

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.

Flux pinning

In the vortex state, a phenomenon known as flux pinning, where a superconductor is pinned in space above a magnet, becomes possible. This is not possible with type-I superconductors, since they cannot be penetrated by magnetic fields.^[6] Since the superconductor is pinned above the magnet away from any surfaces, there is the potential for a frictionless joint. 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

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 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 (77 K). Due to strong vortex pinning, the cuprates are close to ideally hard superconductors.

Important uses

Strong superconducting electromagnets (used in MRI scanners, NMR machines, and particle accelerators) often use niobium-titanium or, for higher fields, niobium-tin.

See also

• Type-I superconductor
• Ideally hard superconductor
• Conventional superconductor
• Covalent superconductors
• List of superconductors
• Superconductor classification
• Technological applications of superconductivity
• Timeline of low-temperature technology
• Type-1.5 superconductor
• Unconventional superconductor

References

1. Tinkham, M. (1996). Introduction to Superconductivity, Second Edition. New York, NY: McGraw-Hill. ISBN 0486435032.
2. Rjabinin, J. N. and Schubnikow, L.W. (1935) "Magnetic properties and critical currents of superconducting alloys", Physikalische Zeitschrift der Sowjetunion, vol .7, no.1, pp. 122–125.
3. Rjabinin, J. N.; Shubnikow, L. W. (1935). "Magnetic Properties and Critical Currents of Supra-conducting Alloys". Nature 135 (3415): 581. Bibcode:1935Natur.135..581R. doi:10.1038/135581a0.
4. Ginzburg, V.L. and Landau, L.D. (1950) Zh. Eksp. Teor. Fiz. 20, 1064
5. A. A. Abrikosov, "Type II superconductors and the vortex lattice", Nobel Lecture, December 8, 2003
6. Rosen, J., Ph.D., & Quinn, L. (n.d.). Superconductivity. In K. Cullen, Ph.D. (Ed.), Encyclopedia of physical science. Retrieved from Science Online database.