Pressure induced self-doping and dependence of critical temperature in stoichiometry YBa2Cu3O6.95 predicted by first-principle and BVS calculations

This paper deals with the pressure effect on self-doping and critical temperature in optimum oxygen stoichiometry YBa2Cu3O6.95 of high temperature superconductor (HTS) based on a numerical study combined the first-principle with bond valence sum (BVS) calculations. The microscopic electronic properties and equilibrium ionic position configurations in the superconductor under external pressure are firstly calculated using the first-principle method. The results show that the apex oxygen in the cuprate superconductor shifts towards CuO2 plane due to pressure effect, and the minimum buckling angle of CuO2 plane is correlated with the maximum critical temperature. A BVS formalism is then utilized for evaluating the valences of all ions in the superconductor on the basis of the electronic and ionic properties and the hole concentration in both CuO2 plane and Cu-O chain are deduced. It demonstrates that the pressure-induced charge redistribution leads to a self-doping process of the hole-transfer into CuO2 plane from both Cu-O chain and Y site in the cuprate superconductor, which is the dominant mechanism of pressure effect on the superconductive properties. In order to quantitatively predict critical temperature profile of YBa2Cu3O6.95 under pressure, a modified formula describing pressure-induced charge transfer taking into account pressure dependence of the optimum hole concentration is developed. The predicted results exhibit good agreements with the experimental data in the literature, and the model parameters on the critical characteristics of the superconductor are discussed in details.