Free University of Berlin

Collective effects in transport through single-molecule devices

Abstract: In the last decade, there has been considerable experimental progress in contacting single molecules to metallic leads, and performing controlled measurements of electric currents in these systems. A prime difference of such molecular devices as compared to semiconductor quantum dots is the coupling of electronic degrees of freedom to well-defined collective modes such as molecular vibrations. In its simplest form, this coupling leads to the emergence of phonon sidebands in the current-voltage characteristics.

In my talk, I will show that electron-phonon coupling can result in striking physics beyond the simple appearance of sidebands due to two mechanisms: (i) For nonequilibrium vibrations, the mode of electronic transport and the phonon dynamics crucially depend on the electron-phonon coupling strength. In particular, strong electron-phonon coupling leads to electronic transport in form of a hierarchy of self-similar avalanches, resulting in drastically enhanced current shot noise. (ii) The electron-phonon coupling always causes a reduction of the charging energy (polaron shift). Whenever the effective charging energy becomes negative, we find that the high-temperature transport occurs via tunneling of electron pairs. Due to the unconventional behavior of such two-electron processes, the linear conductance and nonlinear current-voltage characteristics differ significantly from the usual Coulomb blockade scenario.