We report on the magnetic field (0 Tless than or equal toBless than or equal to9 T) dependence of the longitudinal thermal conductivity kappa(T,B) of highly oriented pyrolytic graphite in the temperature range 5 Kless than or equal toTless than or equal to20 K for fields parallel to the c axis. We show that kappa(T,B) shows large oscillations in the high-field region (B>2 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.
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