Comment on the Kohn Phonon Drag Effect in the Thermoelectricity of Graphite

Abstract

Jay-Gerin and Maynard proposed a theory to interpret the anomaly of thermoelectric power (TEP) of graphite taking place near 35 K. They ascribed the anomaly to the phonon drag effect due to the so-called “Kohn phonons” satisfying q_⊥=2k_F⊥, where k_F⊥ denotes the maximum Fermi momentum perpendicular to the c-axis. Their theory is based on an assumption applicable only to the cylindrical Fermi surface. If we take into account the ellipsoidal Fermi surface, their theory loses its validity. Supposing that the cylindrical model is valid, their theory has several inconsistencies in itself and cannot explain the observed negative dip. From the experimental point of view, this theory conflicts with the fact that the lower the structural perfection the deeper the negative dip. Appropriate interpretation of the phonon drag TEP can be obtained by introducing the normal phonon-phonon scattering in the phonon relaxation process.