Thermoelectric Power of Amorphous Metals and High-Temperature Superconductors

Abstract

Measurements of the thermoelectric power of two different types of systems have been made; cold condensed amorphous metallic films and high temperature ceramic superconductors. i) Amorphous films The thermopower and resistivity of a-Tl100-x Tex (x < 30) and a-In100-xSbx (x = 16-42) have been measured in the temperature range 5-60 K. An impending metal-insulator transition is evident in the resistivity of both systems as the concentration of the non-metallic component is increased, and the temperature dependence is consistent with localization and electron-electron interaction effects. The data also gives support to recent predictions regarding behaviour of the thermopower near the metal-insulator transition. At low temperatures there is an increase in the slope of the thermopower characteristic of the enhancement due to the electron-phonon interaction. Theoretically the thermopower enhancement is calculated in terms of the Eliashberg function and this is the first time it has been seen in amorphous systems for which the Eliashberg function has also been measured. There is good agreement between the theory and the experimental results. The annealing behaviour of the films has also been monitored and an amorphous-crystalline transition has been observed below room temperature. ii) High Temperature Superconductors The thermopower of several Y Ba2 Cu3 O7 and Gd Ba2 Cu, O7 samples has been measured up to room temperature. As expected the thermopower is zero below the superconducting transition temperature, ~ 90 K, but there is a surprising precursor effect just above the transition, namely the thermopower rises sharply before falling to zero in less than 5 K. The width of the precursor correlates with the width of the transition in the resistivity suggesting that the precursor is related to superconducting fluctuations. An explanation is given for the precursor in terms of the effect of these fluctuations on both the phonon-drag thermopower which is enhanced, and the diffusion thermpower which is attenuated.