Anomalous absence of pressure effect on transition temperature in the electron-doped superconductor Nd1.85Ce0.15CuO4–δ

Abstract

THE recent discovery of the electron-doped superconductors1 added fresh fuel to controversies about the mechanism of high-temperature superconductivity in copper oxide compounds. The superconducting transition temperature (Tc) of almost all the hole-doped copper oxide compounds increases with increasing pressure2. The pressure coefficient has a large positive value compared with a typical conventional (BCS) superconductor. A systematic survey of the TcS of Y–Ba–Cu–O and La–Sr–Cu–O compounds has shown that Tc is strongly correlated with hole concentration3–5, but the Hall coefficient is only weakly dependent on pressure6,7. These results indicate that the large change in Tc with pressure is caused, not by a change in hole concentration, but by other factors related to changes in the interatomic distances. Here we report that the newly discovered electron-doped superconductor Nd,1.85Ce0.15CuO4–δ exhibits almost no pressure effect on Tc up to 2.5 GPa, in remarkable contrast to the hole-doped superconductor Nd1.3Ce0.3Sr0.5CuO4–δ. The Cu–O pyramids characteristic of the hole-doped compounds lose their apical oxygens to become square planes in the electron-doped materials. If the pressure effect on Tc does arise from changes in bond lengths, the difference in behaviour of the two compounds points to the involvement of the bond between copper and apical oxygen, which is missing in the electron-doped material.