A COMPARATIVE-STUDY OF THE PRESSURE-INDUCED CHARGE REDISTRIBUTION IN YBA2CU3O7 AND YBA2CU4O8

While the pressure dependence of T(c) in the single-chain superconductor YBa2Cu3O7 is anomalously low, the double-chain compound YBa2Cu4O8 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 YBa2Cu3O7 and YBa2Cu4O8 families of superconductors, one can consider three separate structural components: CuO2 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 CuO2 planes. The difference in behavior under pressure in YBaCu3O7 and YBa2Cu4O8 arises due to the chains. While in YBa2Cu4O8 they further accept the electronic charge, and hence further increase the hole density in the CuO2 planes, their behavior is quite the opposite in YBa2Cu3O7 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 YBa2Cu3O7. 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 YBa2Cu4O8 we obtain an increase in the hole density of approximately 0.019/CuO2. This gives a T(c) approximately 100 K, in good agreement with experiment.