Electron localization mechanism in the normal state of high-Tc superconductors

The ceramic compounds Gd1-xPrxBa2Cu3O7-y (GdPr-123) with 0.0 less than or equal to x less than or equal to 1.0, were synthesized by standard solid state reaction technique. XRD analysis shows a predominantly single phase perovskite structure with the orthorhombic Pmmm symmetry. The samples have been examined for superconductivity by measuring electrical resistivity within the temperature range 10-300 K. These measurements show a suppression of superconductivity with increasing x. It is observed that the critical Pr concentration (x(cr)) required to suppress superconductivity is about 0.45, the samples with x < 0.45 become superconducting and are metallic in their normal state, the samples with x greater than or equal to 0.45 do not become superconducting and show a semiconducting behavior above 10 K. To interpret the normal state properties of the samples, the quantum percolation theory based on localized states is applied. A cross-over between variable-range hopping (VRH) and Coulomb gap (CG) mechanisms is observed as a result of decreasing the Pr content. (C) 1998 Published by Elsevier Science Ltd.