Pressure-induced hole doping of the Hg-based cuprate superconductors.

We investigate the electronic structure and the hole content in the copper-oxygen planes of Hg-based high T(c) cuprates for one to four CuO2 layers and hydrostatic pressures up to 15 GPa. We find that with the pressure-induced additional number of holes of the order of 0.05e the density of states at the Fermi level changes by approximately a factor of 2. At the same time, the saddle point is moved to the Fermi level accompanied by an enhanced k(z) dispersion. This finding explains the pressure behavior of T(c) and leads to the conclusion that the applicability of the van Hove scenario is restricted. By comparison with experiment, we estimate the coupling constant to be of the order of 1, ruling out the weak coupling limit.