Absence of pressure-induced charge transfer in superconducting YBa 2Cu 3O 7

Abstract

The pressure dependence of T c in the 90 K superconductor YBa 2Cu 3O 7 is anomalously low (d T c/d P∼0.5−1 K/GPa), in contrast to the oxygen deficient YBa 2Cu 3O 7−λ compound which shows a large pressure dependence (d T c/d P∼4−7 K/GPa) for λ∼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 CuO 2 planes as a function of applied pressure in the 90 K superconductor. We therefore conclude that the lack of charge transfer from the CuO 2 planes is the essential reason behind the anomalously low pressure dependence of T c in this 90 K superconductor.