Nernst effect and dimensionality in the quantum limit

The Nernst effect has recently emerged as a very sensitive, yet poorly understood, probe of electron organization in solids(1-4). Graphene, a single layer of carbon atoms set in a honeycomb lattice, embeds a two-dimensional gas of massless electrons(5) and hosts a particular version of the quantum Hall effect(6,7). Recent experimental investigations of its thermoelectric response(8-10) are in agreement with the theory conceived for a two-dimensional electron system in the quantum Hall regime(11,12). Here, we report on a study of graphite(13), a macroscopic stack of graphene layers, which establishes a fundamental link between the dimensionality of an electronic system and its Nernst response. In striking contrast with the single-layer case, the Nernst signal sharply peaks whenever a Landau level meets the Fermi level. Thus, the degrees of freedom provided by finite interlayer coupling lead to an enhanced thermoelectric response in the vicinity of the quantum limit. As Landau quantization slices a three-dimensional Fermi surface, each intersection of a Landau level with the Fermi level modifies the Fermi-surface topology. According to our results, the most prominent signature of such a topological phase transition emerges in the transverse thermoelectric response.