Functional renormalization group study of the interacting resonant level model in and out of equilibrium

We investigate equilibrium and steady-state nonequilibrium transport properties of a spinless resonant level locally coupled to two conduction bands of width similar to Gamma via a Coulomb interaction U and a hybridization t('). In order to study the effects of finite bias voltages beyond linear response, a generalization of the functional renormalization group to Keldysh frequency space is employed. Being mostly unexplored in the context of quantum impurity systems out of equilibrium, we benchmark this method against recently published time-dependent density matrix renormalization group data. We thoroughly investigate the scaling limit Gamma ->infinity characterized by the appearance of power laws. Most importantly, at the particle-hole symmetric point the steady-state current decays like J similar to V(J)(-alpha) as a function of the bias voltage Vt('), with an exponent alpha(J)(U) that we calculate to leading order in the Coulomb interaction strength. In contrast, we do not observe a pure power-law (but more complex) current-voltage-relation if the energy epsilon of the resonant level is pinned close to either one of the chemical potentials +/- V/2.