Magnetization and transport properties in the superconducting   Pr$_{2}$Ba$_{4}$Cu$_{7}$O$_{15-\delta}$ with metallic double-chain
  We have reported the effect of pressure on the magnetization, and transport properties in the nominal composition Pr$_{2}$Ba$_{4}$Cu$_{7}$O$_{15-\delta}$ synthesized by a sol-gel technique. A reduction treatment of the as-sintered sample in vacuum causes higher superconductivity achieving $T_{c,on}=\sim 30$ K for $\delta =0.94$. Application of hydrostatic pressure on the oxygen depleted sample enhances its onset temperature up to 36 K at 1.2 GPa, indicating the nearly optimum doping level of the charge carrier in comparison to the pressure dependence of lower $T_{c}$ samples with $\delta =0.45$. Seebeck coefficient of the superconducting sample shows a metallic conduction, followed by a clear drop below $T_{c,on}$ and is in its temperature dependence below 100 K quite different from that of the non-superconducting one. This finding strongly suggests a dramatic change of the electronic state along the CuO double chain due to the reduction treatment for the appearance of superconductivity . 
Shibuki Toshima, Michiaki Matsukawa, Taiji Chiba, Satoru Kobayashi,   Sigeki Nimori, Makoto Hagiwara
cond-mat.supr-con cond-mat.mtrl-sci