Effect of Pressure on Superconducting Properties

It is shown that the existence of superconductivity in a material, and its critical temperature, depends strongly on pressure. Several parameters are pressure-dependent: (1) structure, particularly bond distances, (2) Hubbard U, (3) coupling between sites, and (4) orbital occupation number. Eliashberg theory often leads to incorrect predictions, for example in A(3)C(60) with A = K, Rb, and Cs. While T (C) is correctly predicted to be higher for Rb3C60 than for K3C60 and decreasing with pressure in both cases, Cs3C60 is not superconducting at ambient pressure. The same is the case for pure metals such as Cs and Ca (superconducting at high pressure). A theory for electron pairs, similar to the Marcus model for single electrons, appears to agree with the experiment in most cases