Observation of Landau levels of Dirac fermions in graphite

The unique electronic behaviour of monolayer and bilayer graphene is a result of the unusual quantum-relativistic characteristics of the so-called ‘Dirac fermions’ (DFs) that carry charge in these materials. Although DFs in monolayer graphene move as if they were massless, and in bilayer graphene they do so with non-zero mass, all DFs show chirality, which gives rise to an unusual Landau level (LL) energy spectrum and the observation of an anomalous quantum Hall effect in both types of graphene. Here we report low-temperature scanning tunnelling spectra of graphite subjected to a magnetic field of up to 12 T, which provide the first direct observations of the LLs that produce such behaviour. Unexpectedly, we find evidence for the coexistence of both massless and massive DFs in graphite, and confirm the quantum-relativistic nature of these quasiparticles through the appearance of a zero-energy LL. The dynamics of electrons inside a material is usually described within the framework of non-relativistic quantum mechanics. For parabolic energy bands the energy–momentum dispersion E= E±± h̄ k2/2m∗ resembles that of non-relativistic free particles with an effective mass m arising from interactions with the lattice. Here the energy is measured relative to the bottom of the conduction band, E+ = EC, for electron-like (+) particles, or to the top of the valence band, E− = EV, for holelike (−) particles. In the presence of a magnetic field, B, the energy for motion perpendicular to the field is quantized in a series of equally spaced LLs: