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Magnetism Superconductivity

Superconductivity is a peculiar state of matter that characterizes many metals. Roughly, its specific features are charge transport without energy dissipation (zero electrical resistance) and the expulsion of magnetic flux from the material (similar, but not entirely equivalent to perfect diamagnetism).

This state is present below a critical temperature Tc . The critical temperture is highest for worse (more resistive) metals, such as alloys (Nb-Ti, 9.2 K) and compounds (MgB2, 39 K, Nb3Sn, 18.3 K), with respect e.g. to Al (3 K), Hg (4.2 K). The state is totally absent in the best metals Cu, Ag, Au. The superconducting state of these metals is well described by a subtle effective attraction between electron pairs, due to lattice vibrations, in the theory of Bardeen-Cooper and Schrieffers (BCS, 1956), the first microscopic model of superconductivity.

Electrical resistance of Hg goes to zero below 4.2K as discovered in 1911 by Kamerling Onnes

The unit cell of YBa2Cu3O6+x, reaching T c =92 K for x=0.95.

The discovery of the so-called high temperature superconductivity in layered copper perovskites by Mueller and Bednorz (1986) revealed that the maximum experimental Tc (134 K, at ambient pressure in HgBa2Ca2Cu3O8) are obtained in very bad metals, close to a transition to an insulating antiferromagnet state. The transition can be tuned either by chemical or by physical means, influencing the density of charge carriers in the bad metal. The origin of this stronger superconductivity is still lacking full theoretical understanding, but most likely the underlying magnetic interaction play a role similar to that of phonons in BCS. Different models could explain the phenomenon, and no simple way has been found yet of validating experimentally one of them against the others.

Similar, but not identical mechanisms are at play in cuprates (copper perovskites) and in iron pnictides, discovered in 2007 by Hosono. The latter reach a maximum Tc of 56 K (SmFeAsO0.85F0.15) and are also composed of layers with a transition metal. Unconventional superconductors of yet another sort are found also in fullerene interacalated with alkaline metals.

The most peculiar features are encountered when these systems are driven across the transition from magnetic insulators to very strongly correlated metals, either by pressure or by fine chemical substitution. This is the region that we are currently investigating with local magnetic probes, like muons and NMR.

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Page last modified on June 02, 2011, at 07:03 PM