Unconventional properties of Ln2-xCexCuO4-y (Ln=Pr, Nd, Sm, Eu) compounds

We report on the synthesis, structural, and general physical properties of electron-doped superconductors with general formula Ln(2-x)Ce(x)CuO(4-y); Ln = Pr, Nd, Sm, Eu. This family of oxides provide a remarkable opportunity to study the relationship between structural and physical properties since, by moving along the 4f rare earth series, the evolution of several transport and magnetic properties can be nicely correlated to the steric effects associated with the lanthanide contraction. For example, there is a continuos decrease of the superconducting critical temperature T-ci with decreasing ionic radius of the lanthanide, i. e., T-ci similar to 25 K for Pr, similar to 20 K for Sm, and similar to 13 K for Eu. In addition, superconductivity has not been found in the Gd2-xCexCuO4-y system. Compounds with these stoichiometries also exhibit a great variety of macroscopic physical phenomena including metal-insulator transition, granular superconductivity, and superconductor-insulator transition. These cooper oxide materials are yet another class of magnetic superconductors in which antiferromagnetic order of the Ln ions; Ln = Nd, Sm; coexists with superconductivity. In this work we focus on two unconventional properties of these cuprates: (1) an increased of the electrical resistance below the superconducting transition which has been observed in granular Sm2-xCexCuO4-y; and (2) the appearance of granular superconductivity at temperatures for which there is no evidence for a bulk Meissner effect. The first feature is associated with an abrupt increase of the electrical resistance below the superconducting transition T-ci. We have found that the resistance excess, Delta R(T,B), is proportional to (T/T-ci)(-4) dose to T-ci and shows a crossover to a (T/T-ci)(-1/4) dependence at lower temperatures. We argued that the increase of Delta R close to T,i is due to the depletion of the concentration of normal carriers as predicted by the semi-phenomenological two-fluid theory of superconductivity. The second feature has been observed by using field-dependent; 0 less than or equal to B less than or equal to 7 T; rf surface resistance measurements. The data are described with an extended form of the Coffey-Clem theory and discussed within a granular superconducting scenario. These measurements also indicate that near T-ci -dB(c2)(c)/dT similar to 0.4 T/K, for B parallel to c axis.