Anomalous Phonon Drag Effect in Graphite

Well crystalline kish graphite exhibits a positive peak of the thermoelectric power S below 20 K besides a dip around 35 K. This peak is more enhanced by plotting β x x instead of S , where S = F x /∇ x T =β x x /σ x x · F x is an electric field and σ x x denotes the conductivity tensor. The dip is related to the normal phonon drag effect, while the peak is ascribed to the two-stage drag effect proposed by Kozlov-Nagaev. By solving the coupled Boltzmann equations of carriers and phonons, expressions of the two-stage drag effect with and without magnetic field are derived, where the Fermi surface is approximated by the ellipsoid model and the electron-phonon interaction is limited to the coupling with the in-plane vibration.