University of Basel  > Condensed Matter Theory

Verena Koerting, Dr.

Address:  Office 4.10
Department of Physics
University of Basel
Klingelbergstrasse 82
CH-4056 Basel, Switzerland
e-mail:  verena.koerting "at"
phone (office):  +41 61 267 3751 
fax:  +41 61 267 1349

Transport properties of a superconducting single-electron transistor coupled to a nanomechanical oscillator
V. Koerting, T. L. Schmidt, C. B. Doiron, B. Trauzettel, and C. Bruder

We investigate a superconducting single-electron transistor capacitively coupled to a nanomechanical oscillator and focus on the double Josephson quasiparticle resonance. The existence of two coherent Cooper-pair tunneling events is shown to lead to pronounced back action effects. Measuring the current and the shot noise provides a direct way of gaining information on the state of the oscillator. In addition to an analytical discussion of the linear-response regime, we discuss and compare results of higher-order approximation schemes and a fully numerical solution. We find that cooling of the mechanical resonator is possible and that there are driven and bistable oscillator states at low couplings. Finally, we also discuss the frequency dependence of the charge noise and the current noise of the superconducting single electron transistor.

Nonequilibrium Cotunneling through a Three-Level Quantum Dot
S. Schmaus, V. Koerting, J. Paaske, T. S. Jespersen, J. Nygård, and P. Wölfle

We calculate the nonlinear cotunneling conductance through a quantum dot with 3 electrons occupying the three highest lying energy levels. Starting from a 3-orbital Anderson model, we apply a generalized Schrieffer-Wolff transformation to derive an effective Kondo model for the system. Within this model we calculate the nonequilibrium occupation numbers and the corresponding cotunneling current to leading order in the exchange couplings. We identify the inelastic cotunneling thresholds and their splittings with applied magnetic field, and make a qualitative comparison to recent experimental data on carbon nanotube and InAs quantum-wire quantum dots. Further predictions of the model like cascade resonances and a magnetic-field dependence of the orbital level splitting are not yet observed but within reach of recent experimental work on carbon nanotube and InAs nanowire quantum dots.

Electron transport in the four-lead two-impurity Kondo model: Nonequilibrium perturbation theory with almost degenerate levels
V. Koerting, J. Paaske, P. Wölfle

The eigenstates of an isolated nanostructure may get mixed by the coupling to external leads. This effect is the stronger, the smaller the level splitting on the dot and the larger the broadening induced by the coupling to the leads. We describe how to calculate the nondiagonal density matrix of the nanostructure efficiently in the cotunneling regime. As an example, we consider a system of two quantum dots in the Kondo regime, the two spins coupled by an antiferromagnetic exchange interaction and each dot tunnel coupled to two leads. Calculating the nonequilibrium density matrix and the corresponding current, we demonstrate the importance of the off-diagonal terms in the presence of an applied magnetic field and a finite bias voltage.

Transconductance of a Double Quantum Dot System in the Kondo Regime
V. Koerting, P. Wölfle, and J. Paaske

We consider a lateral double-dot system in the Coulomb blockade regime with a single spin-1/2 on each dot, mutually coupled by an antiferromagnetic exchange interaction. Each of the two dots is contacted by two leads. We demonstrate that the voltage across one of the dots will have a profound influence on the current passing through the other dot. Using poor man's scaling, we find that the Kondo effect can lead to a strong enhancement of this transconductance.

Interface-mediated pairing in field effect devices
V. Koerting, Qingshan Yuan, P. J. Hirschfeld, T. Kopp and J. Mannhart

We consider the pairing induced in a strictly two-dimensional electron gas (2DEG) by a proximate insulating film with polarizable localized excitations. Within a model of interacting 2D electrons and localized two-level systems, we calculate the critical temperature Tc as a function of applied voltage and for different materials properties. Assuming that a sufficient carrier density can be induced in a field-gated device, we argue that superconductivity may be observable in such systems. Tc is found to be a nonmonotonic function of both electric field and the excitation energy of the two-level systems.

Non-Equilibrium Electron Transport through a Double Quantum Dot System: Study of Two Exchange Coupled Quantum Dots in a 4-Terminal Geometry
In this thesis we study two exchange-coupled quantum dots with an emphasis on non-equilibrium physics. Assuming a single electron on each quantum dot, the double quantum dot system is characterized by an interplay between the Kondo spin coupling of the dots with the leads and the spin-exchange coupling between the dots. We find that a finite voltage on one quantum dot drives the other quantum dot out of equilibrium.