Anomalously high conductivity in bromine-intercalated graphite*

We have found that when graphite is intercalated with bromine, the \textit{ab}-plane ($c$-axis) conductivity sharply increases (decreases). Characterization of the Br intercalated samples by exposure time, weight uptake, sputter Auger spectroscopy and X-ray diffraction show a Br concentration that is uniformly distributed within a graphite host having an expanded interplanar spacing $d_{c}$. The \textit{ab}-plane conductivity is enhanced by several orders of magnitude in the temperature range from 300~K down to 1.7~K and shows no sign of saturation with increasing Br concentration. Hall measurements confirm a pronounced increase in the density of negative carriers consistent with an increased optical reflectivity (below 3000~cm$^{-1})$. The inferred plasma frequencies and extrapolated dc conductivities are consistent with the transport measurements. The diamagnetic susceptibility decreases with increasing Br concentration and follows a temperature dependence from which a Fermi energy that increases with increasing Br concentration is extracted. By increasing $d_{c}$, the \textit{ab}-plane conductivity of Br intercalated graphite begins to resemble the additive contributions of parallel connected doped graphene sheets and thus has implications for carbon based electronics.