Systematic study of (La1-xGdx)1.85Sr0.15CuO4 (0≤x≤1): Structure, superconductivity, resistivity, and magnetic properties

A room-temperature structural phase diagram has been determined in (La1-xGdx)1.85Sr0.15CuO4 system (0≤x≤1). There exist three stable phases (T, T* and T'), in which the local Cu-O unit is an octahedron, a pyramid, and a square, respectively. The Jahn-Teller distortion is reduced in the order of T, T*, and T'. For each phase, there is a solubility region. No magnetic ordering is found in the T and T* phase, both of which exhibit paramagnetism with a constant Gd magnetic moment consistent with that of Gd3+. In Gd2CuO4 and Gd1.85Sr0.15CuO4, the initial susceptibility indicates a Neel state in the Cu-O2 plane at TN=285 K and another magnetic transition at low temperature. TN is not sensitive to the Sr doping at all, indicating that extra holes cannot be doped onto the Cu-O2 plane. While the T* and T' phases are insulating, exhibiting a variable-range hopping behavior, the Gd-doped (La1-xGdx)1.85Sr0.15CuO4 (x≤0.1) is superconducting with Tc reducing with increasing Gd concentration. The suppression of Tc is not due to a variation of the electron-boson coupling strength which remains unchanged in the system, but correlates closely with the low-temperature resistivity anomaly. Such an anomaly can be best described by a logarithmic temperature dependence