Universal conductivity and the electrodynamics of graphite at high pressures

We address the in-plane pressure-dependent electrodynamics of graphite through synchrotron-based infrared
spectroscopy and ab initio density functional theory calculations. The Drude term remarkably increases upon
pressure application, as a consequence of an enhancement of both electron and hole charge densities. This is due
to the growth of the band dispersion along the kz direction between the K and H points of the Brillouin zone.
On the other hand, the mid-infrared optical conductivity between 800 and 5000 cm-1 is almost flat, and very
weakly pressure dependent, at least up to 7 GPa. This demonstrates a surprising robustness of the graphene-like
universal quantum conductance of graphite, even when the interlayer distance is significantly reduced.