SEEBECK COEFFICIENT AND ELECTRICAL-RESISTIVITY MEASUREMENTS FOR SEMICONDUCTING ANTIFERROMAGNETIC YBA2CU3OX, UP TO 1000 K - EVIDENCE FOR PHONON DRAG AND STRONG ELECTRON-PHONON COUPLING

The Seebeck coefficient, S, and electrical resistance, R, have been me asured as functions of temperature at constant oxygen composition, z, for ceramic YBa2Cu3Oz samples with 6.197 less than or equal to z less than or equal to 6.400 and at temperatures up to approximate to 1000 K , by means of a simple experimental device which allows us to change t he temperature without changing the composition z. S(T)(z) exhibits a large peak at T approximate to 200 K which decreases as z increases. T he main object of the discussion is to understand the origin of this p eak. At first, it is shown that this S peak is independent of the Neel transformation which occurs in the above composition range. The coupl ing between the Neel process and the transport properties is revealed, as for similar systems, by a lambda point on the (partial derivative In R/partial derivative T)(z) versus T plot at T-N and by a positive h ump for In R(T)(z). Several possibilities are then discussed, notably antimagnon and phonon drag. A new experimental fact appears as a key a rgument in this discussion. The system exhibits a Fermi glass behavior with an Anderson transition occurring on changing z. At the transitio n which is observed between z = 6.350 and z = 6.400, the activated dif fusion component of S disappears. However, for z = 6.400, on the metal lic side of the transition, a T-1 term is still observed in S which is the signature of phonon drag. This phonon-drag effect appears to be u nusually strong: the maximum temperature of the S peak, up to 250 K, i s high as regards the Debye temperature, approximate to 350-400 K, and the phonon-drag contribution to S amounts to about 30% at 600 K for z = 6.400. This indicates a particularly strong electron-phonon couplin g. As a consequence, the coupling parameter lambda is expected to be h igh in these systems.