Unusual specific-heat contribution in exfoliated graphite

The specific heat of Grafoil, an artificial graphite made by Union Carbide, has been measured at temperatures as low as 100 mK, thus extending substantially the range investigated up to the present. An unusual contribution to the specific heat is found which obeys a T^ law (with negative and smaller than 1). This effect is attributed to localized electronic states associated with defects in the crystal structure of the graphite sample. We compare our results to those obtained in doped semiconductors and to the theoretical predictions of Bhatt and Lee.