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Raphaël Van Roermund

Theoretical study of non-equilibrium transport in Kondo quantum dots

Published on 20 October 2010

Thesis presented October 20, 2010

In the absence of exact theoretical methods, many questions related to the non-equilibrium Anderson model have remained unsolved and are at the origin of an intense research activity. In this thesis I discuss transport through quantum dots put in the Kondo regime by means of an equations-of-motion method that was developed in order to account for the non-equilibrium effects and in particular the decoherence of the virtual spin-flip processes involved in the Kondo effect. I compare my results to previous approximations and show the improvements brought by the new decoupling scheme, which solves pathologies at the particle-hole symmetric point and enables the description of the system over a wide range of parameters. A decoherence rate is derived for the excitations which is shown to involve a crossover from the strong- to the weak-coupling regime when either the temperature or the bias voltage or the magnetic field is increased. In the light of this result, I conclude on the applicability of the present equations-of-motion scheme out of equilibrium. I also discuss observables out of equilibrium; the differential conductance exhibits a zero-bias peak reaching a maximum value G = 2e2/h. Its low-energy behavior turns out to be universal after the bias voltage is normalized by the Kondo temperature. I finally show that a finite magnetic field splits the zero-bias peak in the differential conductance. The actual distance between the peaks is discussed in the light of recent experiments for which I give a phenomenological explanation. A new experimental setup is proposed in order to verify my assumptions.

quantum dots, non-equilibrium transport, Kondo effect, quantum decoherence

On-line thesis.