Seebeck coefficient of NaxCoO2: Measurements and a narrow-band model

We have performed Seebeck coefficient (S) measurement for a series of high-quality NaxCoO2 single crystals for 0.23 <= x <= 0.84 from 10 to 300 K. The room-temperature S values were found to be around 30-100 mu V/K and increase with increasing Na content. The temperature dependence of Seebeck coefficient of this system was found to be very sensitive for x=1/2, which is most likely due to the significant modifications of band structure as a result of the magnetic and charge orderings below 88 and 51 K. The overall observed features of the S(T) were analyzed in the framework of a narrow-band model. The physical parameters obtained from the fitting provide a satisfactory description of the composition and temperature dependences of Seebeck coefficient in the NaxCoO2 system.