Condensed state superconducting materials

The Fermi liquid state in metals has two common instabilities. Superconductivity (SC) is due to pair formation of electrons and (spin-, chaige-) density waves (CDW, SDW) are formed by pairing electrons and holes. Theoretically it has been snspected quite early that pair-wave functions other than s-wave as in conventional superconductors and density waves may exist. However it has taken surprisingly long to identify such unconventional condensed pair states in real materials. 

We shall distinguish two fundamental attributes of superconductivity - the state of superconductivity and the effect of superconductivity - that lead to two complementary descriptions of superconductors. On one side the state of superconductivity is characterized by the state of a conducting material, which, after the Jahn-Teller condensation, becomes an insulator with several equivalent ground states. The state of superconductivity determines all statical properties of superconductors energy gap, its temperature dependence, specific heat, density of states near the Fermi surface etc. On the other side the effect of superconductivity determines all dynamical properties of superconductors supercurrent, Meissner effect, quantization of magnetic flux, etc. We shall devote in this section just to the problem of effect of superconductivity. 

NbTi and Nb3Sn low-temperature superconductors. The exact origins of the low critical current density obsrved in these new materials is possibly the most challenging problem in condensed matter physics today. Its resolution is expected to lead to a broader and deeper understanding of solid state physics and to be helpful in defining the possibilities and limits of new superconducting technologies as well. 

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