Pressure-induced charge distribution effect on superconducting critical temperature of HgBa2Ca2Cu3O8+delta

According to the local density approximation for electronic structure calcu lations, the doping level of CuO2 layer does increase under pressure by a r elatively small amount, less than 0.02 additional holes per Cu atom at 9.2G Pa. Thus, it seems necessary to consider mechanisms other than changes in d oping level for the increase of T-c under pressure. Based on the data obtai ned from the powder neutron diffraction experiments up to a pressure of 8.5 GPa, we have calculated the valences of Cu and the pressure-induced change of the hole concentration in the inner and outer CuO2 layers in Hg-1223 sam ples by using the method of Bond Valence Sums (BVS). The results are rather good and could provide a possible explanation for the large increase of su perconducting critical temperature in this material under pressure.