Abstract
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.
Highlights
High pressure technology plays an important role to change the physical properties of solids, and applications of high pressure have greatly enhanced the understanding of the electronic, phonon, and doping effects on the most newly emerged materials
We propose a numerical approach combined the first-principle method with bond valence sum (BVS) calculations to capture the pressure effect on self-doping and critical temperature in high temperature superconductor of optimum oxygen stoichiometry
The Cu[2]-O(4) bond length is sensitive to the charge distribution in YBCO-123,5,29 which could be interpreted by charges interaction that the negative charged apex oxygen atoms shifting towards the CuO2 planes makes it more attractive for holes to transfer to the CuO2 planes from Cu-O chains
Summary
One is the pressure-induced charge transfer from the deficient Cu-O chains to superconducting CuO2 planes, which increases the hole concentration of CuO2 planes This property has been confirmed by the measurement of Hall constant with hydrostatic pressure.. This property has been confirmed by the measurement of Hall constant with hydrostatic pressure.4,7,8 Another is the intrinsic pressure effect on Tc, which is usually independent of pressure-induced hole-transfer into CuO2 planes, and the maximum critical temperature Tc max is increased clearly in the optimally doped samples.. In consideration of pressure effect on changing hole concentration, Almasan et al. introduced a simple pressure-induced charge-transfer model (PICM) to clarify the dTc/dP behavior based on inverted parabola relation for different oxygen doping YBCO-123 superconductors. Some predictions and discussions related to the model are addressed in details
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