A comparative study of the pressure-induced charge redistribution in YBa 2Cu 3O 7 and YBa 2Cu 4O 8

Abstract

While the pressure dependence of T c in the single-chain superconductor YBa 2Cu 3O 7 is anomalously low, the double-chain compound YBa 2Cu 4O 8 exhibits one of the highest positive pressure dependences. Results of electronic structure calculations are presented in this paper in order to understand this difference in behavior, and to shed some light on the charge redistribution under pressure. Our calculations show that the charge redistributions under pressure are quite subtle, and that all sites participate in this charge redistribution. It is difficult to correctly understand the pressure dependence of T c without taking into account this charge redistribution. Roughly speaking, in the YBa 2Cu 3O 7 and YBa 2Cu 4O 8 families of superconductors, one can consider three separate structural components: CuO 2 planes, CuO chains, and the ionic elements Y, Ba. There is always an increase of the electronic charge at the ionic sites under pressure, and this tends to increase the hole density in the CuO 2 planes. The difference in behavior under pressure in YBa 2Cu 3O 7 and YBa 2Cu 4O 8 arises due to the chains. While in YBa 2Cu 4O 8 they further accept the electronic charge, and hence further increase the hole density in the CuO 2 planes, their behavior is quite the opposite in YBa 2Cu 3O 7 where they transfer back the electronic charge and almost compensate for the accumulation of the electronic charge at the ionic sites. The final result is that there is an extremely small increase in the hole density under pressure in YBa 2Cu 3O 7. This difference in behavior of the chains is of structural origin, namely single chain versus double chains, which affects the densities of states in the vicinity of the Fermi level. At a pressure of 4.65 GPa in YBa 2Cu 4O 8 we obtain an increase in the hole density of ∼0.019 CuO 2 . This gives a T c ∼ 100 K, in good agreement with experiment.