GRANULAR BEHAVIOR IN POLYCRYSTALLINE SM2-XCEXCUO4-Y COMPOUNDS

This work reports a systematic study of polycrystalline samples of Sm2 -xCexCuO4-y (0.15 less-than-or-equal-to x less-than-or-equal-to 0.18) obtained from a sol-gel precursor and subjected to different cooling r ates after reduction. A double resistive superconducting transition is a common feature of all samples studied, suggesting that this is an i ntrinsic property of these polycrystalline compounds. At an upper temp erature T(ci), there is a fairly sharp drop in the magnitude of the el ectrical resistivity, which is followed by a well-defined plateau down to a lower temperature T(cj). At this temperature, Josephson coupling between superconducting islands is believed to complete the transitio n to the zero resistance state. From the compositional dependence of e lectrical resistivity, we infer that T(ci) decreases slightly from x = 0.15 through x = 0.18, while T(cj) shows a maximum for x = 0.16. With increasing excitation current, no significant changes in the behavior of the electrical resistivity between T(ci) and T(cj) are observed, w hile a dramatic broadening and a shift of the transition at T(cj) towa rds lower temperatures are found. Magnetic-susceptibility measurements reveal appreciable diamagnetism just below the coupling temperature T (cj) 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 approxim ately 6 and 300 angstrom, in good agreement with recent estimates obta ined from magnetoresistance measurements on polycrystalline Sm2-xCexCu O4-y (0.14 less-than-or-equal-to x less-than-or-equal-to 0.17) samples and with both neutron-diffraction studies and Mossbauer spectroscopy measurements in the isomorphic compound Nd2-xCexCuO4-y (0.0 less-than- or-equal-to x less-than-or-equal-to 0.18). All the observed macroscopi c properties, as well as the absence of a peak in the specific heat at T(ci), are discussed within the framework of a granular superconducto r model. In addition, we give qualitative arguments suggesting the imp ortance of charging effects in the macroscopic properties of these pol ycrystalline samples.