Novel Quantum Transport Phenomena in Molecular Junctions

Prof. Dr. Felix v. Oppen, Freie Universität Berlin [Homepage]

By virtue of their enormous variability, single-molecule devices offer a fascinating
arena for realizing novel effects in quantum transport through nanostructures. A
natural source for such effects lies in the coupling of the electronic degrees of freedom
to few, well-defined molecular vibrations. This coupling is specific to molecules
as opposed to conventional nanostructures such as quantum dots, where one
commonly deals with a continuum of phonon modes. One expects new regimes of
quantum transport for at least three reasons: (i) Strong electron-vibron coupling
(Franck-Condon blockade); (ii) nonequilibrium molecular vibrations induced by the
transport current (avalanche transport and vibrational absorption sidebands); and
(iii) renormalization of the molecular charging energy to negative (attractive) values
(electron-pair tunneling and charge Kondo physics).
Our efforts during the second funding period of the priority program will be
geared towards pushing the theoretical description beyond the high-temperature
limit which can be appropriately described by rate equations. Two aspects will be
at the center of our attention: Effects of broadening of the vibrational sidebands due
to the molecule-lead coupling in the regime of vibrational nonequilibrium, and the
role of coherences in systems with (quasi)degenerate vibrational modes, especially
Jahn-Teller systems. In addition, we also propose to analyze recent experiments
on suspended carbon-nanotube quantum dots which seem to exhibit the Franck-
Condon blockade and have the potential for becoming a paradigm of investigations
into this fundamental effect of molecular electronics.