Studies on thermoelectric power and percolation in BSCCO system

Abstract

Thermoelectric power (TEP) studies have been carried out on Pb-doped Bi_1.8Pb_0.2Sr₂Ca₂Cu₃O₁₀ and undoped Bi₂Sr₂Ca₂CuCu₃O₁₀ specimens between 77 and 300 K. The TEP of the Pb-doped specimen decreases linearly with increasing temperature between the temperature of the TEP peak (123 K) and 300 K. The TEP drops to zero at 103 K, the temperature at which the resistivity becomes zero. The undoped specimens show a two step resistive transition at 110 and 80 K with still a finite resistivity at 80 K. The TEP, however, drops to zero at 80 K. This confirms our earlier inference of a percolation concentration of the superconducting fraction in a normal matrix for zero TEP much smaller than the usual percolation for zero resistivity and supports recent theoretical calculations done by two of us (Rajput and Kumar). The general features such as the TEP peak, the temperature variation above the peak and the change of sign around 300 K can be well accounted for by assuming conduction through holes and electrons as carriers and a strong phonon-carrier interaction.