Nonequilibrium properties of correlated quantum matter are being intensively investigated because of the rich interplay between the external drive and the many-body correlations. Of particular interest is the nonequilibrium behavior near a quantum critical point (QCP), where the system is delicately balanced between different ground states. Although such effects are typically out of reach, we meet this challenge, providing here both both an analytical calculation of the nonequilibrium
I-
V curve when the system is tuned to be critical and 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 dissipative environment. A two-channel Kondo 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 the critical value of the gate voltage corresponding to 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 result through dynamical Coulomb blockade theory. The quantitative agreement between the experimental data and our theory, with no fitting parameter, is remarkable. As our system is fully accessible to both theory and experiment, it provides an ideal setting for addressing nonequilibrium phenomena in correlated quantum matter.
This thesis explores several fundamental topics in mesoscopic circuitry that incorporates few electronic conduction channels. The first experiments address the quantized character (the discreteness) of charge in circuits. We demonstrate the charge quantization criterion, observe the predicted charge quantization scaling and demonstrate a crossover toward a universal behavior as temperature is increased. The second set of experiments addresses the unconventional quantum critical physics that arises in the multichannel Kondo model. By implementing a Kondo impurity with a pseudo-spin of ½ constituted by two degenerate charge states of a circuit, we explore the two- and three-channel Kondo physics. At the symmetric quantum critical point, we observe the predicted universal Kondo fixed points, scaling exponents and validate the full numerical renormalization group scaling curves. Away from the quantum critical point, we explore the crossover from quantum criticality: direct visualization of the development of a quantum phase transition, the parameter space for quantum criticality, as well as universality and scaling behaviors.