Pressure dependence of the hole concentration in the T ∗-phase superconductor Nd 2−x−yCe xSr yCuO 4

Abstract

Neutron powder diffraction data have shown recently that the application of a hydrostatic pressure in the hole-doped T ∗-phase Nd 2−x−yCe xSr yCuO 4 superconductors results in a rather uniform compression of all bond lengths, and no pronounced shortening of the planar Cu-apical oxygen (Cu-O(2)) bond length is observed within experimental error. This compound, however, exhibits a large positive pressure dependence of T c with the pressure coefficient d T c/d P∼3–4K/GPa, contrary to what would be expected on the basis of the Cu-O(2) bond length variation. Results of electronic structure calculations are presented in this paper which show that the Cu-O(2) bond length does not play an important role in determining the charge redistribution under pressure, and that the Nd-O bond lengths exercise a more important role. We find that under pressure there is a small electron transfer from the CuO 2 planes to the Nd sites which act as electron reservoirs. This results in an increase in the role concentration in the CuO 2 planes, and hence in an increase in T c. Assuming a linear relationship between T c and the hole concentration, we recover the pressure dependence of T c observed experimentally.