We report on far infrared magnetotransmission measurements on a thin graphite sample prepared by exfoliation of highly oriented pyrolytic graphite. In a magnetic field, absorption lines exhibiting a blueshift proportional to $\sqrt{B}$ are observed. This is a fingerprint for massless Dirac holes at the $H$ point in bulk graphite. The Fermi velocity is found to be $\tilde{c}=(1.02\pm 0.02)\times {10}^6\mathrm{m}/\mathrm{s}$ and the pseudogap $|\Delta |$ at the $H$ point is estimated to be below 10 meV. Although the holes behave to a first approximation as a strictly 2D gas of Dirac fermions, the full 3D band structure has to be taken into account to explain all the observed spectral features.

• Received 17 September 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.136403