Rectification by charging -- the physics of contact-induced current asymmetry in molecular conductors

TL;DR

This paper compares the physics of current asymmetry in molecular conductors under weak and strong electron interactions, showing how simpler models can explain many experimental signatures while highlighting the importance of electronic structure in certain cases.

Contribution

It demonstrates that a simplified orthodox model can effectively explain contact-induced current asymmetries in Coulomb blockade regimes, reducing computational complexity.

Findings

Current asymmetry in SCF arises from mean-field potential shifts.

CB regime asymmetry is due to unequal open-shell excitation channels.

Simpler models can explain many experimental signatures without full Fock space calculations.

Abstract

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. A conductance asymmetry arises in SCF because of the unequal mean-field potentials that shift a closed-shell conducting level differently for positive and negative bias. 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. However, our analysis of molecular Coulomb Blockade measurements reveals that many novel signatures can be explained using a {{simpler}} orthodox model that involves an incoherent sum of Fock space excitations and {\it{hence treats the molecule as a metallic dot or an island}}. This also reduces the complexity of the Fock space description by just including various charge configurations only, thus partially underscoring the importance of electronic structure, while retaining the essence of the single charge nature of the transport process. We finally point out, however, that the inclusion of electronic structure and hence well-resolved Fock space excitations is crucial in some notable examples.