Nonlinear quantum critical transport and the schwinger mechanism for a superfluid-mott-insulator transition of bosons

Scaling arguments imply that quantum-critical points exhibit universal nonlinear responses to external probes. We I investigate the origins of such nonlinearities in transport, which is especially problematic since the system is necessarily driven far from equilibrium. We argue that for a wide class of systems the new ingredient that enters is the Schwinger mechanism - the production of carriers from the vacuum by the applied field-which is then balanced against a scattering rate that is itself set by the field. We show by explicit computation how this works for the case of the symmetric superfluid-Mott insulator transition of bosons.