In a perpendicular magnetic field, the quantized independent-electron states (Landau levels) of graphene are degenerate in the valley degree of freedom and nearly-so in spin. The momentum-space valley degeneracy traces to the equivalence of the A and B carbon sublattices, a symmetry also reflected in graphene s unique zero-energy Landau level (LL0). Here we use maps of the local density of states acquired via cryogenic scanning tunneling spectroscopy to investigate the influence of local potentials on the spatial distribution of the LL0 wavefunctions. For the quasi-freestanding top layer of multilayer epitaxial graphene, we map extended drift states at the LL0 peak energy and show that at nearby energies these states localize within weak potential fluctuations. Unexpectedly, at high magnetic fields we find hexagonally-arrayed regions where the LL0 peak energy varies on the atomic scale. The implied A/B symmetrybreaking is a real-space manifestation of locally lifting the LL0 valley degeneracy.
Citation: Nature Physics
Pub Type: Journals
graphene, STM, STS, silicon carbide