Super salt

Bechgaard, an organic salt, has extraordinary qualities of superconductivity

Published: Thursday 31 July 1997

 Superconductivity could be us though superconductivity was discovered by the Dutch physicist Heike Kamerlingh Onnes about 85 years ago, it remains an enigma in many ways. The discovery of high temperature superconductivity and the numerous potential applications, from more powerful particle accelerators to high speed trains have been responsible for the enormous amount of research in the area.

I J Lee and his team at the department of physics, State University of New York, Buffalo, have reported that superconductivity in a particular organic salt is extraordinarily resilient to the application of large magnetic fields. This would add to the present understanding of why certain materials are superconductors and others are not ( Physical Review Letters , Vol 78, May 5, 1997).

Superconductivity was explained in 1957 by the physicists John Bardeen, Leon N Cooper, and John Robert Schrieffer of the United States. The bcs theory of superconductivity, so known for the names of the above-mentioned scientists, is based on the pairing of electrons (known as Cooper pairs) due to the attraction between them.

Certain materials are superconductors due to the crystal structure of the material and the forces between the electrons. Some organic materials were also found to be superconductors in the early 1980s. These materials, it was believed, had a fundamentally different force which leads to the formation of Cooper pairs.

Lee and his team have discovered that Bechgaard salt (tetramethyltetraselena-fulvalene hexafluorophosphate) an organic salt, displays superconductivity that can survive magnetic fields up to six tesla (the unit to measure magnetic flux density), compared to other materials where it is destroyed in a field of two tesla. The group believes that the reason for this phenomenon is the peculiar structure of the material. In one of the directions, there is very high metallic conductivity while in the transverse directions, the weak forces between the molecules lead to much lower conductivity.

One of the ways in which a superconductor can be forced into the normal state is by applying a magnetic field. The weakest magnetic field that will cause this transition is called the critical field. Type ii superconductors, of which the organic salt is an example, show a peculiar trait when subjected to a magnetic field. A sample of this type (in the form of a long thin cylinder) was exposed to a decreasing magnetic field oriented with the sample. The increase in magnetisation, instead of occurring suddenly at the critical field, set in gradually. At fields higher than the critical field, the sample becomes normal, instead of a superconductor.

This was observed by Lee and his team while studied superconductivity in the organic salt in very low temperatures. The applications of such organic superconductors are not very many (because unlike high temperature superconductors, their transition temperatures are low). These are, however, crucial for the testing of models for Type- ii superconductors. A lot of work needs to be done to understand the phenomenon which is producing such unusual results. With technological advances in the near future, it will be possible to achieve much higher fields and lower temperatures.

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