Field–Temperature Phase Diagram of Intergrain Ordering in Superconducting Ceramic YBCO

The ceramic YBa2Cu4O8 superconductor composed of submicron grains is considered a random Josephson-coupled network containing the so-called π junctions and shows successive phase transitions. With decreasing temperature, first the intragrain superconductive transition occurs inside each grain at Tc1 and then the chiral-glass transition occurs among the grains at Tc2 (< Tc1). The third transition at Tc3 (< Tc2) is the intergrain superconducting transition. We measured the nonlinear susceptibility and resistivity of the ceramic YBa2Cu4O8 superconductor to determine the field dependences of the transition temperatures Tc2 and Tc3. The phase diagram of the intergrain ordering is discussed in light of the result predicted by Kawamura.

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