Glimpse of the Neutrino in Graphene
Quantum electrodynamics and condensed-matter physics are experiencing a tantalizing convergence under the rubric of "Dirac materials"-crystals hosting charge carriers more aptly described by the Dirac equation than the Schrödinger equation. Dirac electrons comprise two-component wavefunctions and quantum symmetries intertwining spins and pseudospins with momentum; from this structure stems true electron chirality and a direct mapping to relativistic particles in a vacuum. Graphene is emerging as a prototype two-dimensional Dirac material and has been proposed as a condensed-matter nanolaboratory for relativistic particle experiments, but with exceptional accessibility since the effective speed of light is scaled down by a factor of about 300. This talk will survey our low-temperature experiments in which the unique symmetries of Dirac electrons are graphically revealed by scanning tunneling microscopy applied as a coherent nanoscale probe. By directly imaging wavefunctions, tracking quantum mechanical phase, and manipulating single atoms, we see quasiparticles surprisingly closer in phenomenology to massless relativistic neutrinos than to typical massive, non-relativistic electrons. For example, Berry's phase leads to a cancellation of backscattering and to a condensed-matter observation of the relativistic Mott scattering cross section for chiral particles. These ideas and measurements are now being extended to nanostructures and to three-dimensional topological materials.