PRESSURE-INDUCED PHONON SOFTENING AND ELECTRONIC TOPOLOGICAL TRANSITION IN HGBA2CUO4

Results of total energy local density calculations for the effect of p ressure on the lattice parameters, bond lengths, electronic structure and A(1g) phonon frequency in HgBa2CuO4 have been carried out. The eff ects of pressure on the frequency of the apical-oxygen (O-A) A(1g) pho non in the Hg-O-A-Cu bonds are studied in order to understand the role of pressure in increasing T-c of mercury-based superconductors. Theor etically determined zero pressure lattice parameters and phonon freque ncies are in good agreement with experiments. An electronic topologica l transition that occurs when the van Hove singularity is shifted clos e to the vicinity of E(F) by pressure causes considerable phonon softe ning and anomalous behavior of c-axis length, the Hg-O-A bond and Ba z -coordinate.