Spin and orbital fluctuations in non-equilibrium transport through quantum dots: A renormalisation-group analysis

¹ Institut für Theorie der statistischen Physik and JARA-Fundamentals of Future Information Technology, RWTH Aachen - 52056 Aachen, Germany, EU
² Nano-Science Center, Niels Bohr Institute - Universitetsparken 5, 2100 Copenhagen, Denmark, EU
³ Niels Bohr International Academy, Niels Bohr Institute - Blegdamsvej 17, 2100 Copenhagen, Denmark, EU

^a s.andergassen@physik.rwth-aachen.de

Received: 21 July 2010
Accepted: 16 September 2010

Abstract

We study non-equilibrium current and occupation probabilities of a two-orbital quantum dot. The couplings to the leads are allowed to be asymmetric and orbital dependent as is generically the case in transport experiments on molecules and nanowires. Starting from a two-orbital Anderson model, we perform a generalised Schrieffer-Wolff transformation to derive an effective Kondo model. This generates an orbital potential scattering contribution which is of the same order as the spin exchange interaction. In a first perturbative analysis we identify a regime of negative differential conductance and a cascade resonance in the presence of an external magnetic field, which both originate from the non-equilibrium occupation of the orbitals. We then study the logarithmic enhancement of these signatures by means of a renormalisation-group treatment. We find that the orbital potential scattering qualitatively changes the renormalisation of the spin exchange couplings and strongly affects the differential conductance for asymmetric couplings.