Palestrante: Prof. Harold Baranger

Resumo: Many-body systems that are either driven far from equilibrium or simply subjected to quantum noise exhibit complex interplay between the many-body correlations and the external variables, and so are attracting increasing attention. I shall discuss a system that is particularly advantageous for studying these effects: it exhibits impurity quantum criticality, it is amenable to detailed experimental study (and initial experiments have been done), and it is simple enough theoretically that analytical results can be obtained. (i) First, I briefly survey the experimental system and initial results. The system consists of a spin-polarized carbon nanotube quantum dot connected to resistive leads via tunable tunnel barriers. A quantum critical point (QCP) occurs when a level in the dot is resonant with the leads and the dot is symmetrically coupled to them. (ii) Second, I present our calculation of the nonlinear I-V curve at the QCP and show remarkable agreement with the experiment. This result has a simple interpretation as an environmental blockade, albeit one involving a strange barrier between two chiral fermion modes and a strange environment that involves a nonlinear combination of the original electrons and environment. (iii) Third, turning to a more complicated structure, I discuss the case of two dots in the Kondo regime connected to leads in series. In this system, we find that the (equilibrium) quantum noise from the resistive leads stabilizes a non-Fermi liquid QCP. While it is natural to suppose that quantum noise will suppress many-body correlations, this is a striking counterexample in which the noise "rescues" the quantum phase transition.