Moiré materials are formed when two-dimensional crystals are overlaid with a small difference in lattice constant or orientation. When the two-dimensional crystals are semimetals or semiconductors, the low energy states of moiré materials are described by periodic continuum models and have the electronic properties of artificial crystals with lattice constants on the tens of nanometer scale, allowing the number of electrons per atom to be varied widely using electrical gates. My talk will focus on the particular case of graphene bilayer moiré materials, which exhibit a rich set of strongly correlated electron states, including superconductors and insulating orbital magnets, when twisted close to a magic relative orientation angle at which the electron velocity at the Fermi level vanishes. Electronic correlations in Magic Angle Twisted Bilayer Graphene (MAtBG) are strong because the low-energy moiré superlattice bands are very narrow and because the flat bands form an octet that is the direct product of spin, valley, and sublattice internal degrees of freedom. I will discuss efforts, still very much in progress, to settle on answers to some of the following questions. Does the flat-band dispersion that remains at the magic twist angle play a key role in controlling the phase diagram? How does octet symmetry breaking depend on the moiré band filling factor? Is superconductivity in MAtBG mediated by electron-phonon interactions or by some other mechanism?
Speaker: Allan Macdonald, University of Texas at Austin
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