Temperature dependence of the critical current in high-Tcsuperconductors

We present a study of the temperature dependence of the critical current in bulk samples of the high-$T_c$ superconductors Y-Ba-Cu-O and Dy-Ba-Cu-O. We have found that the critical current varies linearly with ($T_c-T$). This result suggests a percolating network of Josephson-coupled superconducting grains. However, the critical current versus temperature curves are found to have a nonlinear tail close to the critical temperature. We have determined that the temperature at which the $I_c$ vs $T$ curves depart from the linear behavior in our samples agrees with what is expected according to theoretical models of granular superconductors.