ANISOTROPIC COMPRESSIBILITY AND EFFECTS OF PRESSURE ON THE ELECTRONIC-STRUCTURE AND T(C) OF HG-BASED SUPERCONDUCTORS

Total energy local density calculations of the effects of pressure on the HgBa2Can-1CunO2n+2+delta (n = 1, 2, 3) superconductors show that the compressibility is an isotropic. Estimates of the bond compressibilities show that the Cu-O (2) (apical) bond is extremely compressible, while the Hg-O(2) bond is the least compressible. A simple model is proposed to describe compressibilities of Hg-1212, Hg-1223 and Hg-1234 on the basis of results obtained for the infinite layer (Sr1-xCax)1-yCuO2 and Hg-1201 compounds. The energy location of the van Hove singularity (vHs) - important in several theories of high-T(c) superconductivity - is found to move towards E(F) with pressure, thereby showing a correspondence between the number of holes (from dopants) needed to shift E(F) down to the vHs and the pressure needed to be applied to a fixed dopant material to achieve the best T(c). Some consequences for understanding the pressure effects on the T(c) of the Hg superconductors and possible experiments are suggested.