Pressure-induced phonon softening and electronic topological transition in HgBa₂CuO₄

Total energy local density calculations for the effects of pressure on the lattice parameters, bond lengths, electronic structure, and A_1g phonon frequency in HgBa₂CuO₄ have been carried out in order to understand the role of pressure in increasing the T_c of mercury-based superconductors. Theoretically determined zero-pressure lattice parameters and phonon frequencies are found to be in good agreement with experiment. An electronic topological transition is found to occur when the van Hove singularity (vHS) is shifted close to the vicinity of E_F by pressure which causes considerable phonon softening and anomalous behavior of the c-axis length, the Hg-O(2) bond, and the Ba z coordinate. A set of experiments that might be able to detect the presence of the vHS close to E_F is proposed.