THERMAL TRANSPORT AND PERCOLATIVE TRANSITION IN THE AG-BPSCCO COMPOSITE SYSTEM

The thermal conductivity of the Ag-BPSCCO composite system is studied in the temperature range from 15 K to 200 K. The percolation threshold value of Ag (f(c)) is estimated to be 0.25, which is rather close to the three-dimensional percolation threshold (0.16). The observed values of critical exponents near the percolation threshold, t'(f>f(c)) and s'(f<f(c)), are 1.08 and 0.63, respectively. The observed value of t' is not consistent with the universal value (t=2) in the metal-insulator mixture. This discrepancy is discussed within the framework of the scaling theory. In the first order approximation, the critical behavior below f(c) in the Ag-BPSCCO composite is determined by that in the superconductor-normal conductor composite. The thermal conductivity above f(c) is explained by the critical property of the good conductor-bad conductor composite with the finite conductivity ratio.