The aim of our research is the description of the electrical transport properties of nanoscale systems. These properties are influenced by fundamental physical properties such as quantum mechanical coherence, dissipation, and interaction among the charge carriers. We develop methods to describe the large variety of nanoscale systems, but also work on the description of new measurable observables like current noise and counting statistics that provide a deeper understanding of the relevant transport processes. The following examples give an overview of our research:Research Topics:
In nanoscale quantum dots electronic interactions play a dominant role, leading, e.g. to the Coulomb blockade. We describe electron transport and current noise using various methods like diagrammatic perturbation theory, full counting statistics, or path integral techniques. Similar questions are relevant for charge transport through molecules. However, in long molecules (e.g. DNA) also inelastic transport processes and dissipation of energy to the molecular environment play an important role. Electron transport through quantum dot systems and molecules
Figure 1: Differential conductance and shot noise (Fano factor) of a quantum dot with two energy levels. Within the so-called "Coulomb diamond" (area at low transport bias Vbias limited by the red lines of high differential conductance) second order processes ("Co-tunneling") are dominant. The sketches (right) show the dominant processes at small transport bias Vbias.
Electronic properties of carbon nanotubes and graphene
Carbon nanotubes and graphene are often considered as ideal examples of coherent quantum transport. In experiments, however, often effects due to disorder, inhomogeneity of the environment, or phonon scattering are of importance. Our work studies observables like current noise in various experimental situations.
Figure 2: Sketch of the structure of a graphene ribbon with binary disorder. The disorder causes local scattering potentials Vs and a shift of the chemical potential μ away from the " Dirac points". The conductivity depends on the length of the graphene ribbon.
Transport through hybrid structures
In hybrid structures, materials of strongly differing (or even complementary) properties are brought together. An example is the combination of superconducting and ferromagnetic metals. Transport through such structures show phenomena that are so far poorly understood, e.g. non-local Andreev reflections, that we describe by Green function methods. Hybrid structures are particularly interesting for applications, as their integration with established (semiconductor) technologies is feasible.
Figure 3: Sketch of a superconducting-ferromagnetic hybrid structure. The behavior of the non-local transport properties is under investigation. 
Atomic contacts
The smallest possible transport system consists of single atoms. Here, the difficulty lies in the manufacturing of stable structure, with the possibility to control them via external "knobs". Our research applies a combination of Monte Carlo simulations and tight binding transport calculations, to make the bridge from a purely statistical description to possible applications of such systems with predictable transport properties.
Figure 4: Monte Carlo simulation of the growth of an atomic contact of silver. In specific configurations the controlled opening and closing of the contact is possible.
Co-workers
Dr. Dmitry Golubev
Dr. Matthias Hettler
Dr. Wolfgang Wenzel
Dr. Andrei Zaikin
Dipl.-Phys. Robert Maul
The group of Priv.-Doz. Dr. Wolfgang Wenzel does extensive work on the prediction of biomolecular structures, such as protein folding, or receptor-ligand docking for drug discovery applications. Further information can be found at http://int.kit.edu/nanosimBiomolecular Structure Prediction
