Observation of Landau levels of Dirac fermions in graphite

The unique electronic behaviour of monolayer and bilayer graphene(1,2) 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(3-11) and the observation of an anomalous quantum Hall effect in both types of graphene(4,5,8). 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.