PRESSURE-DEPENDENCE OF THE CARRIER CONCENTRATION IN THE ELECTRON-DOPED SUPERCONDUCTOR ND1.85CE0.15CUO4

Pressure dependence of the transition temperature T(c) in the electron-doped superconductor Nd1.85Ce0.15CuO4 is considered to be anomalous since T(c) hardly varies with pressure, in contrast to the case of the hole-doped superconductors where there is initially an increase in T(c) with pressure reaching a maximum, and then a gradual decrease with increasing pressure. Results of electronic structure calculations for Nd1.85Ce0.15CuO4 are presented in this paper which show that under pressure there is a small electron transfer from the CuO2 planes to the Nd sites, This decrease in the carrier (electron) concentration in the CuO2 planes is unfavorable for an increase in T(c), and predicts instead a modest decrease assuming T(c) to be proportional to the carrier concentration. We obtain a value of dT(c)/dp approximately -1.7 K/GPa. In the hole-doped superconductors, a similar charge transfer from the CuO2 planes to the rare earth sites increases the hole concentration in the CuO2 plane, and hence raises T(c). Thus our calculations show that the same mechanism which enhances the T(c) under pressure in the hole-doped superconductors, is responsible for its anomalous dependence in the electron-doped superconductors. Under pressure the rare earth element plays the major role and the role of the face or apical oxygen is only secondary.