Far infrared transmission experiments are performed on ultrathin epitaxial graphite samples in a magnetic field. The observed cyclotron resonance-like and electron-positron-like transitions are in excellent agreement with the expectations of a single-particle model of Dirac fermions in graphene, with an effective velocity ofc = 1.03×10 6 m/s. The electronic properties of graphite have recently become the center of considerable attention, following experiments on graphite monolayers (graphene) [1] and epitaxial graphene [2], which led to the discovery of an unusual sequence of quantum Hall effect states [3,4] and an energy-dependent mass. The considerable interest in two-dimensional graphite is fuelled by its particular band structure and ensuing dispersion relation for electrons, leading to numerous differences with respect to "conventional" two-dimensional electron systems (2DES) [2,5,6,7,8,9,10,11,12]. The band structure of graphene is considered to be composed of cones located at two inequivalent Brillouin zone corners at which the conduction and valence bands merge. In the vicinity of these points the electron energy depends linearly on its momentum: E( − → p ) = ±c| − → p |, which implies that free charge carriers in graphene are governed not by Schrödinger's equation, but rather by Dirac's equation for zero rest mass particles, with an effective velocityc, which replaces the speed of light. With the application of an external magnetic field, the Dirac energy spectrum evolves into Landau levels with energies given bywhere n scans all positive (for electrons) and negative (for holes) integers and -very importantly -zero. E 1 may be understood as a characteristic energy introduced by the magnetic field. The square root dependence on B and Landau level index n is in stark contrast to "conventional" 2D electrons, where E n = (n+ 1 2 ) eB/m, (n ≥ 0), and the Landau levels are equally spaced.The unusual sequence of quantum Hall effect states and an energy-dependent electron effective mass [3,4], found in magneto-resistance measurements, are consistent with the model of Dirac particles. Here we report a magneto-spectroscopy study of the optical properties of ultrathin epitaxial graphite layers, in which we directly probe the dependence of the energy of electrons on their momentum.The experiments were performed on graphene layers grown in vacuum by the thermal decomposition method [2,12], on single crystal (4H) SiC. These epitaxial graphene structures are routinely characterized using low energy electron diffraction, Auger electron spectroscopy, X-ray diffraction, scanning tunnelling microscopy and atomic force microscopy. The results of these measurements in combination with angular resolved photoelectron spectroscopy and transport data indicate that the graphitized part of this type of structure consists of a few (3-5) graphene layers [2,12]. We investigated two such (unpatterned) structures, with dimensions of about 4 x 4 mm 2 , which both show a similar behavior.The far infra-red transmission of the samples wa...

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Multilayer epitaxial graphene is investigated using far infrared transmission experiments in the different limits of low magnetic fields and high temperatures. The cyclotron-resonance-like absorption is observed at low temperature in magnetic fields below 50 mT, probing the nearest vicinity of the Dirac point. The carrier mobility is found to exceed 250,000 cm2/(V x s). In the limit of high temperatures, the well-defined Landau level quantization is observed up to room temperature at magnetic fields below 1 T, a phenomenon unusual in solid state systems. A negligible increase in the width of the cyclotron resonance lines with increasing temperature indicates that no important scattering mechanism is thermally activated.