Granular effects in superconducting YBa2Cu4O8 ceramic

Two characteristic transition temperatures have been observed in superconducting YBa2Cu4O8 ceramic by means of an ac inductive method. Our susceptibility data from a powdered sample, and a bulk sample at different magnetic field strengths, show that the two temperatures correspond to superconducting transition at the intergrain region Tc(inter) and at the intragrain region Tc(intra); the latter gives a weak diamagnetic signals. We found that Tc(inter) was gradually lowered when the sample was subjected to increasing quenching temperatures, while Tc(intra) was unaffected by such treatment, the latter being consistent with the oxygen stability of YBa2Cu4O8. The small susceptibility signals occuring at Tc(intra) will be discussed. (orig.)