Quantum interference in off-resonant transport through single molecules

Abstract

We provide a simple set of rules for predicting interference effects in off-resonant transport through single molecule junctions. These effects fall into two classes, showing, respectively, an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diagonalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with $GW$ calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions interference causes no transmission nodes, but we argue that it may lead to a nonstandard gate dependence of the zero-bias Kondo resonance.