Intergranular transport properties of polycrystalline Sm1.82Ce0.18CuO4-y under low applied magnetic fields

We report on intergranular transport properties of polycrystalline samples of Sm1.82Ce0.18CuO4-y exhibiting the double resistive superconducting trans ition. These samples are comprised of small islands which undergo a superco nducting transition at a higher temperature T-ci similar to 21 K and the lo ng-range phase coherence is achieved through Josephson coupling at a lower temperature T-cJ similar to 10 K. We have performed several transport measu rements including magnetic field and temperature dependence of electrical r esistance, R(T,H), and I-V characteristics in these samples. Such measureme nts were taken under low applied magnetic fields 0 less than or equal to H less than or equal to 200 Oe and at temperatures T < T-cJ < 10 K in order t o investigate the phase correlations of the superconducting order parameter throughout the material. The results of magnetoresistance R(T, H) enabled us to conclude that dissipation due to Josephson flux flow is governed by a n intergranular activation energy given by U-J(T,H) proportional to (1 - T/ T-cJ)(H-0.6). In addition, we have found an irreversible behavior of R(H) c urves, which shows a crossover field H=H* similar to 30 Oe. We also observe d that H* is excitation current independent and that I-cJ(H*) approximate t o 0). The combination of these results has indicated that the phase of the superconducting order parameter is depressed at intergranular regions or mo re appropriately at the grain boundaries in these superconducting granular materials. (C) 1999 EIsevier Science B.V. All rights reserved.