Electrical transport mechanism for semi-conducting YBa2Cu3Oz:: A complementary study

This article is devoted to the study of a particularity in the variations of the electrical resistance, R, of YBa2Cu3Ox as a function of temperature, T, at constant composition, z, as measured in a previous study. For T > 300 K and for z near 6.3, on increasing T, InR, which is at first an increasing T-function, goes through a maximum and becomes at last a decreasing T-function. These experimental results may be satisfactorily interpreted by using the electrical transport model we have previously worked out for semiconducting (SC) YBa2Cu3Oz: these SC compounds behave as a Fermi glass with an Anderson transition occurring on changing z. The InR maximum appears to be due to the competition between two processes: the excitation of the carriers from the Fermi level to the extended states at the mobility edge and their diffusion similar to the one occurring in the metallic regime. Some consequences of this model are developed, particularly concerning a phonon drag contribution, S-g, to the Seebeck coefficient, S. The ratio S-g/S appears as exceptionally large and indicates a strong phonon-carrier coupling. (C) 1997 Elsevier Science Ltd. All rights reserved.