Juan C. Medina Pantoja

Measurements of basal plane longitudinal ${\rho }_b(B)$ and Hall ${\rho }_H(B)$ resistivities were performed on highly oriented pyrolytic graphite samples in a pulsed magnetic field up to $B=50\mathrm{T}$ applied perpendicular to graphene planes, and temperatures $1.5\mathrm{K}\le T\le 4.2\mathrm{K}$. At $B>30\mathrm{T}$ and for all studied samples, we observed a sign change in ${\rho }_H(B)$ from electron- to holelike. For our best quality sample, the measurements revealed the enhancement in ${\rho }_b(B)$ for $B>34\mathrm{T}$ ($T=1.8\mathrm{K}$), presumably associated with the field-driven charge density wave or Wigner crystallization transition. In addition, well-defined plateaus in ${\rho }_H(B)$ were detected in the ultraquantum limit revealing possible signatures of the fractional quantum Hall effect in graphite.

Applied magnetic field induces metal-insulator and reentrant insulator-metal transitions in both graphite and rhombohedral bismuth. The corresponding transition boundaries plotted on the magnetic field—temperature $\left(B\text{−}T\right)$ plane nearly coincide for these semimetals and can be best described by power laws $T\sim {\left(B\text{−}{B}_c\right)}^{\kappa }$, where $B_c$ is a critical field at $T=0$ and $\kappa =0.45\pm 0.05$. We show that insulator-metal-insulator (I-M-I) transformations take place in the Landau level quantization regime and illustrate how the insulator-metal transformation in graphite samples with a moderate anisotropy transforms into a cascade of I-M-I transitions, related to the quantum Hall effect in strongly anisotropic, quasi-two-dimensional graphite samples. We discuss the possible coupling of superconducting and excitonic correlations with the observed phenomena, as well as the signatures of quantum phase transitions associated with the M-I and I-M transformations.