Infrared spectroscopy of 2D electron gas in high magnetic field: a case study of graphite

We present the first systematic investigation of the optical constants of HPOG graphite in magnetic fields up to 17T. The ab plane magneto-reflectance in the frequency range 15-3000 cm$^{-1}$ was measured with the field in c-axis. The optical conductivity was obtained from Kramers-Kronig analysis augmented with ellipsometry data. These experiments have allowed us to monitor the field-induced transfer of the electronic spectral weight from the Drude mode to cyclotron resonance (CR) modes. In applied fields, the conductivity in the limit of $\omega \to $0 is depleted by several orders of magnitude in accord with notoriously large positive magneto-resistance of graphite. A close examination of the lineshape of CR modes is indicative of the coexistence of carriers with 3D and 2D character. The latter mode reveals a $\sqrt H $ dependence of the cyclotron frequency long anticipated for Dirac quasiparticles with linear dispersion. In this fashion, our magneto-optics experiments have allowed us to explore novel aspects of charge dynamics in this prototypal quasi-2D material.