Large Berry phases in layered graphene

Brillouin zones of graphene systems possess Dirac points, where band degeneracies occur. We study the variety of (and large magnitude of) phases that the electronic states can acquire when a uniform time-dependent electric field carries the electrons around one or more Dirac points in a nonconcentric fashion. An experimentally accessible determination of excess Berry phases is proposed involving the Zitterbewegung of electronic current near an orthogonality point in adiabatic motion.