Effect of Pressure on Superconducting Properties

Abstract

It is shown that the existence of superconductivity in a material, and its critical temperature, depends strongly on pressure. Several parameters are pressuredependent: (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 Rb 3 C 60 than for K 3 C 60 and decreasing with pressure in both cases, Cs 3 C 60 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.

On the Heat Capacity of Pure Elements and Phases

“…According to the authors, phase transitions involving spin, charge, lattice, and orbital degrees of freedom, which under certain circumstances for a given level of lattice thermal vibration associated with special band structure, would define a state of matter, the so-called superconducting state. The concurrence among those phase transitional mechanisms would easily explain how a superconducting state would be achieved at higher temperatures by increasing the pressure 25 , which is also a variable of state, such as the solute composition 26 . According to Souza et al 24 , entropy change is associated with a first-order transition, no matter its nature, and can be directly obtained by integrating specific heat over the temperature T .…”

On the ac magnetic susceptibility of a room temperature superconductor: anatomy of a probable scientific fraud

“…In 2009, Debessai, Matsuoka, Hamlin, Schilling and Shimizu reported in Physical Review Letters that Eu metal had become the 53rd known elemental superconductor in the periodic table [10,11]. The result has not been independently reproduced to date but is generally assumed to be true [12][13][14][15][16][17][18].…”