Heat treatment effects on the superconductivity and crystal structure of Nd1.85Ce0.15CuO4 studied using a single crystal

We performed systematic post-growth heat treatments on sizable single crystals of Nd1.85Ce0.15CuO4. The superconducting transition temperature, T,, and its distribution in the sample are both sensitive to details of the heat treatment and sample size. With refinement of the heat treatment conditions we have succeeded in preparing two distinct superconducting samples in which T, equals 18 K and 25 K under identical Ce-concentrations. Neutron scattering experiments initially observed nonmagnetic superstructure peaks at (pi,pi) only in the reduced superconducting phase. Atomic displacement in this structure is different from that induced by in-plane rotation of CuO4 squares as previously reported for Gd2-x,CexCuO4. A possible relationship between atomic displacements and superconductivity and the role of reduced heat treatments are discussed from the viewpoint of carrier localization.