The conduction mechanism in Gd_1−x−zCe_xCa_zBa₂Cu₃O_7−δ

We have investigated the substitution of Ce and divalent Ca with a radius similar to Gd in Gd_1−x−zCe_xCa_zBa₂Cu₃O_7−δ. We have prepared Gd_1−x−zCe_xCa_zBa₂Cu₃O_7−δ compounds with different concentrations of Ce and Ca by the standard solid-state reaction technique. X-ray diffraction (XRD) experiments are performed and the results are refined by the Rietveld method. XRD analysis shows a predominantly single-phase perovskite structure with orthorhombic Pmmm symmetry. In Gd_1−x−zCe_xCa_zBa₂Cu₃O_7−δ compounds, induced suppression of T_c due to Ce doping is compensated by hole doping with increasing Ca substitution for Gd. With the increase of Ca content, T_c increases up to an optimum value; thereafter it begins to decrease, and the resistivity curve has a bell shape. The transition temperature width also decreases for all x with the increase of z. In order to interpret the normal state electrical properties of the samples, the quantum percolation theory based on localized states is applied. Fitting the data with two-dimensional variable range hopping and three-dimensional variable range hopping models, and a Coulomb gap regime, shows that our results are consistent with the Coulomb gap regime. The results of the electrical and structural properties of Gd_1−x−zCe_xCa_zBa₂Cu₃O_7−δ-doped compounds suggest that Ca doping compensates the suppression of the superconducting state caused by the Ce content.