Coherence transition in granular high temperature superconductors

We have studied experimentally the electrical conductivity and specific heat near the superconducting transition of granular samples of YBa₂Cu₃O_7−δ, YBa₂(Cu_2.98Zn_0.02)O_7−δ and GdBa₂Cu₃O_7−δ. The results show that the transition proceeds in two stages. Careful analysis of the conductivity in the regime of approach to the zero resistance state reveals the occurrence of a coherence transition, which is related to the connective nature of the granular samples. This transition occurs when the fluctuating phases of the order parameter in individual grains become long-range ordered. We obtain the exponent for fluctuation conductivity and the relevant critical temperature, T_co, which is close to the point where resistivity vanishes. The specific heat results, when analyzed as dC/dT, show a weak but reproducible cusp-like anomaly at T_co. This finding gives strong support to the interpretation of the coherence transition as a genuine critical phenomenon.