Superconductivity in Pr2Ba4Cu7O15−δ

We synthesized Pr2Ba4Cu7O15−δ (Pr247) using a powder-sintering method under high-pressure oxygen and measured the electrical resistivities of samples with various oxygen deficiencies, δ, which were prepared by annealing in vacuum. The occurrence of superconductivity at about 15 K was observed after reduction treatment at 400 °C for 6 h or more in vacuum. An optimum doping at which Tc takes the maximum value was found. The Hall coefficient for the superconducting Pr247 oxide changed its sign from positive to negative at 120 K with decreasing temperature, indicating that electrons are the main carriers. We assume that the superconductivity arises in the so-called CuO double chains, unlike those in other high-Tc superconductors.