Transport and magnetic properties of polycrystalline Sm _2-xCe_xCuO_4-y

A systematic study of transport and magnetic properties of polycrystalline Sm_2-xCe_xCuO_4-y (0.15≤x≪0.18) specimens obtained from a sol-gel precursor reveals a striking double resistive superconducting transition. One of the resistive transitions takes place at a temperature T_ci and is almost insensitive to Ce concentration, applied magnetic field, and current density, while the other transition occurs at a lower temperature T_cj and is very sensitive to Ce content, applied magnetic field, and current density. Alternating-current magnetic susceptibility χ_ac measurements reveal two interesting features. There is only one contribution to the diamagnetic signal below ∼20 K, which is particularly evident in the resistive component χ] of χ_ac where a peak at T_cj can be identified. The application of an applied magnetic field shifts the χ] peak to lower temperatures and reveals a spin-glass-like behavior. All the results can be satisfactorily explained in terms of weakly linked Josephson junctions between small superconducting islands. The small islands have a superconducting transition at a higher temperature T_ci, and the long-range phase coherence is achieved through Josephson coupling at lower temperatures T_cj. All the results suggest a superconducting glass state in a disordered Josephson network.