Granular behavior in polycrystalline Sm2-xCexCuO4-y compounds

This work reports a systematic study of polycrystalline samples of Sm2-xCexCuO4-y (0.15≤x≤0.18) obtained from a sol-gel precursor and subjected to different cooling rates after reduction. A double resistive superconducting transition is a common feature of all samples studied, suggesting that this is an intrinsic property of these polycrystalline compounds. At an upper temperature Tci, there is a fairly sharp drop in the magnitude of the electrical resistivity, which is followed by a well-defined plateau down to a lower temperature Tcj. At this temperature, Josephson coupling between superconducting islands is believed to complete the transition to the zero resistance state. From the compositional dependence of electrical resistivity, we infer that Tci decreases slightly from x=0.15 through x=0.18, while Tcj shows a maximum for x=0.16. With increasing excitation current, no significant changes in the behavior of the electrical resistivity between Tci and Tcj are observed, while a dramatic broadening and a shift of the transition at Tcj towards lower temperatures are found. Magnetic-susceptibility measurements reveal appreciable diamagnetism just below the coupling temperature Tcj suggesting that superconducting properties are really confined to small regions, with size comparable to the London penetration depth. The average size of these regions are estimated to be between ∼6 and 300 , in good agreement with recent estimates obtained from magnetoresistance measurements on polycrystalline Sm2-xCexCuO4-y (0.14≤x≤0.17) samples and with both neutron-diffraction studies and Mössbauer spectroscopy measurements in the isomorphic compound Nd2-xCexCuO4-y (0.0≤x≤0.18). All the observed macroscopic properties, as well as the absence of a peak in the specific heat at Tci, are discussed within the framework of a granular superconductor model. In addition, we give qualitative arguments suggesting the importance of charging effects in the macroscopic properties of these polycrystalline samples.