Substitution of Nd at Ba site in Gd123

Structural and superconducting behaviors of Gd(Ba_2−x Nd_x)Cu₃O_7+δ with 0.0 ≤ x ≤ 0.8 have been studied by X-ray diffraction and electrical and magnetic measurements. The XRD patterns show the samples with x ≤ 0.5 are isostructure 123 phase, and an orthorhombic–tetragonal phase transition occurs at x = 0.2. The transition temperature decreases linearly with increasing Nd content. This behavior is also observed in the curves of the room-temperature resistivity versus Nd doping. The metal–insulator and superconductor–insulator transitions occur at x _c^MIT = 0.3 and x _c^SIT = 0.35, respectively. The conduction mechanisms of the normal-state resistivity are the Coulomb gap (CG) for x < x _c^MIT, two-dimensional variable-range hopping (2D-VRH) at x _c^MIT ≤ x ≤ x _c^SIT, and 3D-VRH for x > x _c^SIT. The pinning energy derived from the thermally activated flux creep model, and coupling energy derived from the Ambegaokar–Halperin theory decrease with increasing applied magnetic field as a power-law relation, and decrease as Nd content increases. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)