THE RELATION BETWEEN HIGH-TEMPERATURE THERMOPOWER, CONDUCTIVITY AND OXYGEN-CONTENT IN YBA2CU3O6+X

Experimental data, more detailed and complete than previously availabl e, for thermopower and conductivity in YBa2Cu3O6+x was obtained in the temperature range 600 to 850 degrees C and oxygen content varying in the limits 0.1 less than or equal to x less than or equal to 0.8 under equilibrium pressure of oxygen. The concentration dependencies of con ductivity and thermopower were derived using an equilibrium P(O-2)-T-x diagram. Isothermal plots of thermopower versus the logarithm of cond uctivity show unusual features in the extrinsic hole regime, i.e. a sm all inward curvature, an anomalously small slope and a weak temperatur e dependence were observed. The decrease in the oxygen content at high temperature results in the expected transition to intrinsic behavior and the contribution of electrons to the conductivity. The results obt ained have been interpreted in the framework of the band structure com prised of two narrow bands on the background of two broad bands. The m ain influence of the broad bands is on the position of the chemical po tential, whereas the location of the narrow bands relative to the chem ical potential of the electrons impacts the electron transport. (C) 19 98 Published by Elsevier Science S.A.