Electrical Resistivity Studies On Graphite At High Pressure And Low Temperature (c-axis, Dac, Conduction, Hopg, Hydrostatic).

Abstract

High pressure is shown to give a valuable insight into the intrinsic c-axis resistivity of Highly Oriented Pyrolitic Graphite (HOPG). For the purpose of improving our understanding of the fundamental behavior of this technologically important material, additional forms of graphitic material such as Grafoil, Single Crystal Graphite (SCG) and polycrystalline natural graphite were explored for a comparative analysis. A novel technique utilizing a gasketed diamond anvil cell is described which permits four-probe resistivity measurements at pressures of up to 40 kbar and temperatures extending down to 2 K while maintaining the integrity of samples as fragile as graphite. The four-lead arrangement is designed to avoid contact and lead-wire resistances which might otherwise obscure the comparatively small resistance changes of interest typical of highly conductive materials. Our data on HOPG can be fitted well to a model describing conduction along the c-axis as composed of two components acting in parallel: an ordinary metallic one and a tunnelling conduction between crystallites. The total conductivity has been found to be a superposition of both conductivities, and their respective weights depend on the quality of the graphite material. This model accounts for the discrepancy in temperature dependence between HOPG and SCG and, in particular, the origin of the peak observed in the c-axis resistivity within polycrystalline forms of graphite. Samples of single crystal graphite of higher quality than previously reported have been examined and an asymptotic limit of T('2) has been found to govern the temperature dependence of the c-axis resistivity at low temperatures. Specimens of less perfect quality are found to have a temperature dependence of the form T('x), with x decreasing for lower quality samples.