ABSENCE OF PRESSURE-INDUCED CHARGE-TRANSFER IN SUPERCONDUCTING YBA2CU3O7

The pressure dependence of T(c) in the 90 K superconductor YBa2Cu3O7 is anomalously low (dT(c)/dP approximately 0.5-1 K/GPa), in contrast to the oxygen deficient YBa2Cu3O7-delta compound which shows a large pressure dependence (dT(c)/dP approximately 4-7 K/GPa) for delta approximately 0.2-0.4. It has been suggested that T(c) in the 90 K superconductor has already reached its optimum value, and thus no further increase in T(c) can be expected. Results of electronic structure calculations are presented in this paper which show that, contrary to the conclusions drawn from bond-valence-sum arguments, there is an extremely minor change in the hole concentration in the CuO2 planes as a function of applied pressure in the 90 K superconductor. We therefore conclude that the lack of charge transfer from the CuO2 planes is the essential reason behind the anomalously low pressure dependence of T(c) in this 90 K superconductor.