Temperature and field dependencies of galvanomagnetic effects in graphite

In zero magnetic field the electrical resistivity, ρ(0, T), of highly-oriented-pyrolytic (polycrystalline) graphite decreases monotonically with decreasing temperature, becoming nearly constant below about 4 Kelvin. However, in an applied field, B_z, the transverse resistivity component, ρ_xx(B_z, T), goes through a maximum as a function of temperature at about 25°K. The size of the maximum increases as the field increases. A natural single crystal of graphite also exibits a maximum, but it is modified by Shubnikov-de Haas oscillations of ρ_xx. We show that the maximum of ρ_xx at fixed field is directly related to the unexpected field dependence of ρ_xx at low temperatures reported in earlier work, where it was observed that the transverse conductivity component, σ_xx(∼- 1/ρ_xx), decreases more slowly with field than expected on the basis of a simple two-band model. Calculations of the field and temperature dependence of σ_xx in graphite are needed, for fields just below and in the quantum limit.