The Nernst-Ettingshausen coefficient in conductors with a narrow conduction band: Analysis and application of its results to HTSC materials

A theoretical analysis is made of the Nernst-Ettingshausen coefficient Q fo r the case of a narrow conduction band present in the band spectrum of a ma terial. It is shown that the presence of such a band results in a qualitati ve change in the mechanism responsible for the Nernst-Ettingshausen effect as compared to the classical case of a broad conduction band and that the b ehavior of this coefficient reveals a number of specific features that are different from the case of the classical theory of transport coefficients i n semiconductors and metals. It is demonstrated that the pattern of the Q(T ) relation in the case of a narrow band is drastically affected by the asym metry of the dispersion curve, whereas the other features of the band spect rum and of the properties of the carrier system, including the character of the energy dependence of the relaxation time, are less significant and, in a first approximation, can be disregarded. The calculated Q(T) curves are in qualitative agreement with the experimental relationships obtained for d oped HTSCs of the YBa2Cu3Oy system. The possibility of using this approach for a complex analysis of the experimental temperature dependences of the f our transport coefficients in the normal phase of HTSC materials is demonst rated. (C) 2001 MAIK "Nauka/Interperiodica".