Transport properties of a Josephson-coupled network in a superconductive ceramic of YBa2Cu4O8

Ceramic YBa2Cu4O8 samples composed of sub-micron size grains are considered as random Josephson-coupled networks of 0 and π junctions, and they show successive phase transitions. The first transition occurs inside each grain at Tc1 and the second transition occurs among the grains at Tc2 (> Tc1), where a negative divergence of nonlinear susceptibility is found. This critical phenomenon at Tc2 suggests the onset of the chiral-glass phase, as predicted by Kawamura and Li. We measured the temperature dependencies of the current-voltage characteristics of the samples and derived the linear and nonlinear resistivities. With decreases in temperature, linear resistivity decreased monotonously and remained at a finite value at temperatures less than Tc2, while nonlinear resistivity diminished continuously for temperatures moving towards Tc2. These results are consistent with the theoretical predictions.