Axel Thielmann

Cotunneling current and shot noise in quantum dots

We derive general expressions for the current and the shot noise, taking into account non-Markovian memory effects. In generalization of previous approaches, our theory is valid for an arbitrary Coulomb interaction and coupling strength and is applicable to quantum dots and more complex systems such as molecules. A fully consistent diagrammatic expansion up to second order in the coupling strength, taking into account cotunneling processes, allows for a study of transport in an intermediate coupling strength regime relevant to many current experiments. We discuss a single-level quantum dot as a first example, focusing on the Coulomb-blockade regime where the cotunneling processes dominate. We find super-Poissonian shot noise due to inelastic spin-flip cotunneling processes at an energy scale different from the one expected from first-order calculations.

Super-Poissonian noise, negative differential conductance, and relaxation effects in transport through molecules, quantum dots, and nanotubes

We consider charge transport through a nanoscopic object, e.g., single molecules, short nanotubes, or quantum dots, that is weakly coupled to metallic electrodes. We account for several levels of the molecule/quantum dot with level-dependent coupling strengths, and allow for relaxation of the excited states. The current\char21{}voltage characteristics as well as the current noise are calculated within first-order perturbation expansion in the coupling strengths. For the case of asymmetric coupling to the leads we predict negative-differential-conductance accompanied with super-Poissonian noise. Both effects are destroyed by fast relaxation processes. The nonmonotonic behavior of the shot noise as a function of bias and relaxation rate reflects the details of the electronic structure and level-dependent coupling strengths.

Strongly enhanced shot noise in chains of quantum dots

The authors study charge transport through a chain of quantum dots. The dots are fully coherent among each other and weakly coupled to metallic electrodes. If the Coulomb interaction dominates over the interdot hopping the authors find strongly enhanced shot noise at biases above the sequential tunneling threshold, with an enhancement which can easily reach a factor of ∼100. This strong enhancement may allow direct experimental detection of shot noise, e.g., in a chain of quantum dots formed in semiconductor heterostructures. The current is not enhanced in the region of enhanced noise, thus rendering the shot noise super-Poissonian.

Single Electron Tunneling in Small Molecules

We discuss current- and noise-spectroscopy of small molecules weakly coupled to electrodes using master equations. The Coulomb interaction and size quantization effect on the molecule restrict the transport to the tunneling of single electrons. We consider situations where orbital-, spin- or vibrational excitations localized on the molecule play a role or even dominate the transport. For each case, we analyze mechanisms which lead to negative differential conductance (NDC) and even total suppression of the current. The shot noise is shown to provide additional information due to its sensitivity to asymmetries in state- and electrode- specific tunnel couplings. Combined current-noise spectroscopy could be very useful for characterization of experiments.