Superconductivity – Garth Wilkinson

Garth Wilkinson outlined the history of the discovery of superconductivity in metallic elements at low temperature, and the early theories of its operation, ranging from the phenomenological to the fundamental Bardeen – Cooper – Schrieffer (BCS) theory of low temperature superconductivity (LTS) which depends on the macroscopic manifestation of quantum coherence between paired electrons.

This gives rise to physical properties such as zero DC electrical resistance below a critical temperature, the exclusion of magnetic flux below a critical value from bulk materials (the Meissner effect), the quantisation of flux tubes in Type II superconductors, and coherent tunnelling of super-currents through thin insulating layers (the Josephson effect). The latter enables the most sensitive magnetometers & electromagnetic wave detectors known, and voltage calibration standards of unprecedented accuracy.

The application of BCS theory to “dirty” superconductors was described, and led on to the search for high temperature (ideally room temperature) superconductors (HTS) which has been pursued for several decades. However, still the most practically useful materials for drawing into wires for manufacture of superconducting magnets remain as Nb3Ti & Nb3Sn, discovered in the 1950s. The former is used in the Large Hadron Collider, and the latter is hoped for in the International Thermonuclear Experimental Reactor, where cooling with liquid He4 is still required.

Attempts to push towards higher critical temperatures have reduced to “alchemy” of complex alloys & crystal structures. While these can be used for “toy” demonstrations, they still don’t lead to many hoped-for practical applications (e.g. efficient power transmission, etc.) Garth showed an extract from a TED video of a talk & demonstration of HTS by Boaz Almog (http://www.ted.com/talks/boaz_almog_levitates_a_superconductor#t-144141). There is no universally agreed theory to explain HTS, there being 2 main candidates: spin density wave coupling between electrons on neighbouring lattice sites, & BCS-like interlayer coupling in layered structures. The highest critical temperature found so far is 203 K in H2S at a very high pressure of 150 GPa. Such materials are of academic interest, but little practical use at present.

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