Magnetic-field-induced electronic phase transitions in semimetals in high magnetic fields

We report an experimental study of the magnetic-field-induced electronic phase transitions in semimetals (graphite and Pi) in pulsed high magnetic fields up to several hundred teslas. Far-infrared and millimetre-wave spectroscopies were employed to investigate the electric conductivity and the optical transitions between the Landau levels. In graphite a phase transition was observed in the mm-wave transmission through a specially designed strip-line system with a sample on one of the walls. Evidence of a density-wave phase transition was found in the infrared transmission at a transition field of about 30-40 T. Cyclotron resonance in very high field revealed that the n = 0 spin-up level is depopulated above a field of B = 54 T, while the n = 0 spin-down level persists under the Fermi level at least up to 200 T. In Pi, anomalous structures were observed in the strip-line mm-wave transmission spectra as well as the infrared transmission spectra, indicating a semimetal-to-semiconductor transition at around 85 T.