A moire pattern is formed when two copies of a periodic pattern are overlaidwith a relative twist. We address the electronic structure of a twistedtwo-layer graphene system, showing that in its continuum Dirac model the moirepattern periodicity leads to moire Bloch bands. The two layers become morestrongly coupled and the Dirac velocity crosses zero several times as the twistangle is reduced. For a discrete set of magic angles the velocity vanishes, thelowest moire band flattens, and the Dirac-point density-of-states and thecounterflow conductivity are strongly enhanced.
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