Massless electrons and fractionally charged particles in graphene
Andrea Young, Massachusetts Institute of Technology

Materials are all made up of the same basic ingredients, electrons and atomic nuclei. Yet through their mutual interactions, the electrons and nuclei can behave in radically different ways than they would were they isolated in vacuum. I'll discuss one particularly dramatic instance of this kind of emergent behavior in the thinnest known

material: graphene. Graphene is a single atom thick sheet of carbon atoms, arranged in a honeycomb lattice. Remarkably, the electrons in graphene, via their interaction with the carbon atoms, "lose" their mass, behaving as though they were moving at precisely the speed of light. I'll use graphene to illustrate the general notion of an effective theory, in which a complex system of interacting massive electrons in ions is described by a simple, noninteracting theory for a different kind of particle--in this case, massless electrons. These new "quasiparticles" exist in a real material that we can manipulate in the lab, allowing new kinds of experiments which would never be possible with true ultrarelativistic fermions such as might be produced in a particle accelerator. In fact, interactions of electrons in semiconductors and graphene can enable the creation and study of completely new kinds of particles without analogues in particle physics, including quasiparticles which possess only a rational fraction of the charge of a single electron.

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