Unusual temperature dependence of the resistivity of exfoliated graphites

Abstract

The electrical resistivity of three exfoliated graphites (Grafoil, Foam, and Union Carbide UCAR-ZYX) has been measured in the temperature range 1.5-300 K. While the resistivity measured across the rolling plane, ${\rho }_{\perp }$, is comparable to the $c$-axis resistivity of highly oriented pyrolytic graphite (HOPG), the exfoliation has a large effect on the resistivity along a rolling direction, ${\rho }_{\parallel }$, which exceeds by 2-3 orders of magnitude the in-plane resistivity of HOPG. The temperature dependence of both ${\rho }_{\perp }$ and ${\rho }_{\parallel }$ is similar but highly unusual. It shows a metalliclike behavior below about 25 K and an activated character at higher temperatures. The data can be fitted well by considering two conduction mechanisms acting in parallel: an ordinary metallic one and a hoppinglike one with (temperature) exponent, ¼. Theoretical implications are discussed.