Magnetic Field Effect on the Superconducting Transition in (RE)xBa1-xCuOy

The superconducting transition of (RE) x Ba 1- x CuO y (RE: Nd, Sm, Eu, Gd, Ho, Yb, Y; 0.25≦ x ≦0.40) has been studied in magnetic fields up to 6 T. Two-step transitions have been clearly observed in the magnetic fields for most of the samples. The samples are considered to be composed of the orthorhombic phase with a higher T c around or below 90 K, and of the inhomogeneous quasi-tetragonal phase with a lower T c around or below 60 K surrounding the former phase, though the composite ratio depends on the RE-concentration x . The RE-concentration dependence of the transition curve is closely concerned with the ion radius of RE.