ANISOTROPIC PRESSURE DEPENDENCE OF CONDUCTION IN WELL-ORIENTED PYROLYTIC GRAPHITE .I. NON-OSCILLATORY EFFECTS AND ROLE OF CARRIER-CARRIER SCATTERING

Application of hydrostatic pressure preferentially increases those overlap integrals which depend solely on the interplanar spacing of graphite, resulting in an increase of carrier density n, whereby d ln n/dP = 37·5 × 10-6 bar-1. At absolute zero, the associated conductivity dependence d ln σ/dP = 28 × 10-6 bar-1 is independent of crystal direction, but as the temperature is raised towards 295°K the c axis coefficient falls to 25·5 × 10-6 bar-1 whereas the basal plane coefficient falls to near zero. Pre-irradiation with fast neutrons (1016 to 1018 nvt) restores all pressure coefficients measured on pristine material towards d ln n/dP. A qualitative explanation of these phenomena is obtained by the introduction of carrier-carrier scattering terms.