T-EXPONENT IN THE PRBA2CU3O7-AG PERCOLATION SYSTEM

We have studied the electrical transport in the PrBa2Cu3O7-Ag percolation system. The resistivity ratio of the former to the latter, rho-Pr 1:2:3/rho-Ag, had been systematically "tuned" by performing the resistance measurements at various selected temperatures between 30 and 300 K. The critical volume fraction phi-c and a best value of the t exponent in the sample resistivity rho varies as (phi - phi-c)-t above the percolation threshold were determined, where phi is the Ag volume fraction. We find that phi-c = 0.135 +/- 0.005 and t = 1.91 +/- 0.02. In addition, based on the results obtained at various measuring temperatures or rho-Pr 1:2:3/rho-Ag, we show that the value of t extracted by presuming the above simple power-law behavior can be substantially smaller than the predicted value (almost-equal-to 2.0) if rho-Pr 1:2:3/rho-Ag is not sufficiently large. The scaling behavior of (rho-Ag/rho) (phi - phi-c)-t as a function of (rho-Ag/rho-Pr 1:3:3) (phi - phi-c)-(t + s) has then been investigated, where s is the critical exponent governing the divergence of conductivity in a superconductor-normal-metal composite.