Magnetothermal Transport of Oriented Graphite at Low Temperatures

We have studied the magnetic field dependence of the thermal conductivity $\kappa(T,B)$ of highly oriented pyrolytic graphite samples at temperatures 0.2K$ \le T < 10$K and fields 0T$ \le B \le 9$T. The samples show clear deviations from the Wiedemann-Franz law with a kink behavior at fields $B \sim 0.1 $T near the metal-insulator transition observed in electrical resistivity measurements. We further show that the oscillations in the thermal conductivity at the quantum limit $B > 1 $T, which are correlated with the Landau quantization observed in Hall measurements, increase in amplitude with temperature following a $\sim T^3$ law at $T > 0.2 $K and show a maximum at $T \sim 6 $K, suggesting that they are phonons mediated.