[en] Suppression of superconducting property in presence of inhomogeneous spin distribution in GdBa_2Cu_3_−_xCo_xO_6_._9 has been studied. A superconducting sample without Co exhibits superconducting transition at 56.1 K. The current-voltage (I-V) characteristics exhibit nonlinear to linear transformation above a certain temperature. Two other samples (i) with Co and (ii) without Cu are found to be nonsuperconducting with very high resistive in nature at lower temperature. Localization length decreases with the increase in Co substitution. Suppression of the superconducting transition has been attributed to the change in the magnetic fluctuations induced by the randomness in spin substitution. An exponent has been extracted to understand the current-voltage behaviour. Kosterlitz-Thouless (KT) transition may be affected strongly by the shifting of magnetic fluctuations.

[en] Inter-granular Sn particles affect transport and magnetic properties both in the normal and superconducting states of polycrystalline GdBa${}_2$Cu${}_3$O${}_{7-<!--\; ???\; -->\mathrm{\delta <!--\; ??\; -->}}$ (GdBCO) superconductors drastically. Normal state resistivity at 300 K increases ~ 18.0 times as the concentration of Sn is increased. An unusual upturn in resistivity as a function of temperature curve with a dip is observed in GdBCO with 3.0 wt% of Sn in which suppression of superconducting phase transition has been observed. Current-voltage (IV) curves below T${}_c$ exhibit nonlinear features in composites of GdBCO and Sn. Magnetization (M) as a function of magnetic field (H) curves at 2.0 -20.0 K in GdBCO shifts strongly with the addition of 1.0 wt% Sn. We have extracted irreversible magnetization, lower critical field (H${}_{c1}$) and magnetic critical density (J${}_{cm}$) as a function of H at several T. The maximum pinning force density is 0.4 GN/m${}^3$ in GdBCO which is reduced in the Sn added GdBCO.