Magnetothermal conductivity of highly oriented pyrolytic graphite in the quantum limit

We report on the magnetic field $(0\mathrm{}\mathrm{T}<~B<~9\mathrm{T})$ dependence of the longitudinal thermal conductivity $\kappa \mathrm{}(T,B)\mathrm{}$ of highly oriented pyrolytic graphite in the temperature range $5\mathrm{K}<~\mathrm{T}<~20\mathrm{K}$ for fields parallel to the c axis. We show that $\kappa \mathrm{}(T,B)\mathrm{}$ shows large oscillations in the high-field region $(B>\mathrm{}2\mathrm{}\mathrm{T})$ where clear signs of the quantum-hall effect are observed in the Hall resistance. With the measured longitudinal electrical resistivity we show that the Wiedemann-Franz law is violated in the high-field regime.