Rectification by charging: Contact-induced current asymmetry in molecular conductors

We outline the qualitatively different physics behind charging-induced current asymmetries in molecular conductors operating in the weakly interacting self-consistent field (SCF) and the strongly interacting Coulomb blockade (CB) regimes. The SCF conductance asymmetry originates in the asymmetric shifts of the closed-shell molecular conducting levels, driven by unequal mean-field potentials for positive and negative biases. A very different current asymmetry arises for CB due to the unequal number of open-shell excitation channels at opposite bias voltages. The CB regime, dominated by single charge effects, typically requires a computationally demanding many-electron or Fock-space description to do justice to its complex excitation spectrum. However, our analysis of molecular CB measurements reveals that many novel signatures can be explained using a simpler orthodox model that involves an incoherent sum of Fock-space excitations and hence treats the molecule as a metallic dot. This also reduces the complexity of the Fock-space description by including charge configurations alone, somewhat underscoring the richness of its electronic structure while retaining the essential single charge nature of the transport process. The inclusion of electronic structure with well-resolved Fock space excitations is, however, crucial in some notable examples.