Co-tunneling current and shot noise in quantum dots

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