Pressure and Grain-Size Dependences of the Chiral-Glass Transition in Superconductive Ceramics of YBa2Cu3O7−y

Ceramic high-Tc superconductors may be viewed as a weakly coupled random Josephson network containing π-junctions, and a chiral-glass transition is predicted to be occurred. Grain-size and pressure dependences of the chiral-glass transition have been investigated on YBa2Cu3O7−y ceramics. With decreasing temperature, a superconducting order occurs at first inside each grain at Tc1 and furthermore among the grains at Tc2 (<Tc1). The increase in Tc2 with increase of synthesis temperature is considered as the change of strength of inter-grain Josephson coupling accompanying the growth of grain size. The Tc2 also increases with increase of pressure, however, the sharpness of the transition at Tc2 does not change. In spite of difference in Tc2's, the increasing rates of Tc2 by pressure are nearly the same.