Unusual temperature dependence of the resistivity of exfoliated graphites

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, ρ_⊥, 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, ρ_∥, which exceeds by 2-3 orders of magnitude the in-plane resistivity of HOPG. The temperature dependence of both ρ_⊥ and ρ_∥ 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, {}. Theoretical implications are discussed.