Quantum transport through single-molecule junctions with orbital degeneracies

We consider electronic transport through a weakly coupled single-molecule junction, where the molecule has a degenerate electronic spectrum being linearly coupled to an internal vibrational mode. As for such molecules, a rate-equation description in the weak-coupling limit is not possible, we investigate the interplay of the coherent physics caused by the off-diagonal elements of the reduced density matrix with the molecular vibrations. We identify two distinct molecular models and provide an in-depth discussion of their properties on the Hamiltonian level, their transport characteristics for weak and strong electron-phonon coupling and different parameter regimes. We finally extend our reasoning to the cases, when the orbital degeneracy is slightly broken and the leads have more than one electronic mode.