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

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 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.PACS numbers:

Transition from Strong to Weak Electronic Coupling in a Single-Molecule Junction

“…4(a) reveals that the level alignment in the CB regime takes values around 0.1-0.3 eV, indicating a smaller injection barrier with respect to the strong coupling regime. We do not include the charging energy in the formula for the determination of the level alignment, since an analytical expression that relates the two quantities does not exist [40]. We notice, however, that by neglecting the charging energy U, we are overestimating ϵ 0 , nevertheless this overestimation would not change the conclusions of the Letter.…”

“…3(c) have asymmetric voltage onsets; this can be explained by a different voltage drop in the molecule at positive and negative bias, caused for instance by an spatially asymmetric junction configuration or by charging effects (see Refs. [39,40] for a discussion of the possible sources of asymmetry in Coulomb blockade (CB I-V's). Figure 3(c) illustrates the transition in more detail by displaying four I-V and dI=dV-V curves measured just before and after the transition point d 0 .…”

Performance analysis of an interacting quantum dot thermoelectric setup

“…Our current theoretical treatment, however, is in the limit of weak coupling to the contacts, and symmetric contact coupling. In the regime of asymmetric and strong contact coupling, we expect novel physics that may be introduced by asymmetric charging [34,35] to affect the thermoelectric transport processes. This will be an object of future research and possible extension of the current work.…”

The role of many-particle excitations in Coulomb blockaded transport

“…Our recent work in the area of molecular transport [12,13,14,15], as well as compelling recent experiments triggered towards spin-based quantum computation [5,16,17,18,19], both argue for increased activity in this area. This article focusses on the role of both electron-electron and electron phonon correlations in non-equilibrium transport.…”