Coherent charge transport through molecular wires: "Exciton blocking" and current from electronic excitations in the wire

TL;DR

This paper investigates how excitonic interactions influence charge transport in molecular nanojunctions, revealing phenomena like exciton blocking and conduction channels that depend on Coulomb interactions and molecular symmetry.

Contribution

It introduces a detailed quantum model of exciton effects on current in molecular wires, highlighting the conditions under which exciton blocking occurs and how Coulomb interactions modify conduction.

Findings

Exciton interactions reduce current at high voltage in homodimer bridges.

Exciton blocking effect is absent in strong Coulomb repulsion regimes.

Coulomb interactions can enable conduction even without direct electronic connectivity.

Abstract

We consider exciton effects on current in molecular nanojunctions, using a model comprising a two two-level sites bridge connecting free electron reservoirs. Expanding the density operator in the many-electron eigenstates of the uncoupled sites, we obtain a 16X16 density matrix in the bridge subspace whose dynamics is governed by Liuoville equation that takes into account interactions on the bridge as well as electron injection and damping to and from the leads. Our consideration can be considerably simplified by using the pseudospin description based on the symmetry properties of Lie group SU(2). We study the influence of the bias voltage, the Coulomb repulsion and the energy-transfer interactions on the steady-state current and in particular focus on the effect of the excitonic interaction between bridge sites. Our calculations show that in case of non-interacting electrons this interaction leads to reduction in the current at high voltage for a homodimer bridge. In other words, we predict the effect of \textquotedblleft exciton\textquotedblright blocking. The effect of \textquotedblleft exciton\textquotedblright blocking is modified for a heterodimer bridge, and disappears for strong Coulomb repulsion at sites. In the latter case the exciton type interactions can open new channels for electronic conduction. In particular, in the case of strong Coulomb repulsion, conduction exists even when the electronic connectivity does not exist.