Heat Treatment Effects on the Superconductivity and Crystal Structure of Nd_1.85Ce_0.15CuO₄ Studied Using a Single Crystal

We performed systematic post-growth heat treatments on sizable single crystals of Nd_1.85Ce_0.15CuO₄. The superconducting transition temperature, T_c, 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_c equals 18 K and 25 K under identical Ce-concentrations. Neutron scattering experiments initially observed nonmagnetic super-structure peaks at (π, π) 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 Gd_2-xCe_xCuO₄. A possible relationship between atomic displacements and superconductivity and the role of reduced heat treatments are discussed from the viewpoint of carrier localization.