DOUBLE RESISTIVE SUPERCONDUCTING TRANSITION IN SM2-XCEXCUO4-Y

We report a systematic study of the double resistive superconducting transition in carefully prepared polycrystalline samples of the electron-doped superconducting series Sm2-xCexCuO4-y for 0.12 less-than-or-equal-to x less-than-or-equal-to 0.20. From high-resolution x-ray-diffraction measurements, the solubility limit of Ce in this series is slightly greater than 0.16. An intrinsic double resistive transition present in all superconducting compounds is attributed to the granularity of these polycrystalline samples. There is a partial transition when the grains become superconducting at a temperature T(c1), and coupling between grains at a lower temperature T(c2) apparently completes the transition. Only the transition associated with coupling between grains is observed in magnetic-susceptibility measurements due to inhomogeneous grains and large penetration depths. From compositional dependences of resistive and magnetic measurements, the superconducting volume fraction increases linearly with increasing dopant concentration up to the solubility limit, while the best coupling between grains is at a dopant concentration of 0.15. Thus, there is a subtle relationship between doping and coupling in this series. Also, arguments for a true thermodynamic phase transition at T(c2) are given.