HOPPING CONDUCTIVITY IN FULLY OXYGENATED PRBA2CU3OY, YBA2CU2COOY, ANDPRBA2CU2COOY

We report results of a comparative study of three nonmetals related to the 1-2-3 cuprate superconductors: fully oxygenated PrBa2Cu3Oy (PBCO) , YBa2Cu2CoOy (YBCCO), and PrBa2Cu2CoOy (PBCCO). We measured the resis tivity up to 300 K and the absolute thermopower (TEP) up to 400 K. The temperature dependence of the resistivity follows the Mott variable-r ange-hopping (VRH) law rho = rho0exp(T0/T)1/4 in PBCO up to approximat ely 100 K and in the other two systems at least up to room temperature . The results of the TEP measurements show that in YBCCO and in PBCCO the VRH probably takes place via energy states lying mainly below the chemical potential, while in PBCO this process utilizes states lying s ymmetrically on both sides of the chemical potential. In PBCO the TEP exhibits a cusp at low temperatures that is probably associated with t he Neel point of the Pr sublattice. In PBCCO we found at low temperatu res a sharp sign reversal of the TEP, from p type at high temperatures to n type at low temperatures, which needs further investigation.