Universal Nonequilibrium I-V Curve at an Interacting Impurity Quantum Critical Point

The nonlinear I-V curve at an interacting quantum critical point (QCP) is typically out of reach theoretically. Here, however, we provide an analytical calculation of the I-V curve at a QCP under nonequilibrium conditions and, furthermore, present experimental results to which the theory is compared. The system is a quantum dot coupled to resistive leads: a spinless resonant level interacting with an ohmic electromagnetic environment. A two channel Kondo like QCP occurs when the level is on resonance and symmetrically coupled to the leads. Using a bosonized representation, we calculate the nonlinear I-V curve at this QCP. We then show that it has a physically intuitive interpretation in terms of weak backscattering of non-interacting fermions coupled to a modified environment, thus arriving at the same I-V through dynamical Coulomb blockade theory. The agreement between our theoretical and experimental results is remarkable.