Effect of impurities on the electronic phase transition in graphite in the magnetic quantum limit

The electronic phase transition in graphite under strong magnetic fields is studied by magneto-transport measurement for a wide variety of single-crystal samples for a field range up to 280 kG and temperatures down to 0.41 K. The magnetic field dependence of the transition temperature is fitted to a functional form $T_c\mathrm{}\left(B\right)=T^{*}\exp \left(\frac{-B^{*}}{B}\right)$, where the parameters $T^{*}$ and $B^{*}$ are approximately 100 K and 1000 kG, respectively. This functional form is consistent with Yoshioka and Fukuyama's model of a magnetic-field-induced charge-density-wave instability. We have found two types of samples, each of which exhibit distinctive magnetotransport behavior at the onset of the phase transition. For one sample type, we have observed a suppression of $T_c$ and a less sharp conductivity change at the transition point, relative to the other type. This difference in behavior is correlated with the difference in the ionized impurity concentration between the two types of samples as deduced from the high-field Hall measurements. The differences between the types can be understood by taking account of the "pair-breaking" effect of the impurities.