Growth and anisotropic resistivity of ${\mathrm{PrBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ and ${\mathrm{Pr}}_2{\mathrm{Ba}}_4{\mathrm{Cu}}_7{\mathrm{O}}_{15-y}$ single crystals: A direct probe of metallic Cu-O double chains

We have successfully grown single crystals of ${\mathrm{PrBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ (Pr124) and ${\mathrm{Pr}}_2{\mathrm{Ba}}_4{\mathrm{Cu}}_7{\mathrm{O}}_{15-y}$ (Pr247) by means of a flux method under high pressure oxygen gas of 11 atm in ${\mathrm{Al}}_2{\mathrm{O}}_3$ or MgO crucibles. The most appropriate starting composition for the crystal growth was found to be Pr:Ba:Cu=1:2:3 for both Pr124 and Pr247, by examining the generated phases in samples prepared with widely varying composition and sintering temperature. The temperature dependence of resistivity of Pr124 single crystals exhibited no superconductivity, at least down to 2 K. Resistivity along the b-axis direction showed a highly conductive behavior, of which the origin is attributed to the double-chain site because of the large anisotropy between a- and b-axis resistivities. The anisotropy in resistivity between the a- and b-axis directions was found to be about 17 at room temperature, 275 at 85 K, and 190 at 5 K.