Ever since the inception of molecular electronics, molecular rectification has continued to be of great interest. An obvious way to produce pronounced asymmetry (rectification) in the current-voltage (I-V) characteristics is by providing asymmetric contact couplings (resistances). However, Coulomb interaction or charging effects can cause additional features especially in molecular transport. The focus of this talk is to outline the physics of how contact asymmetry works side-by-side with charging effects (Coulomb interaction), and manifests itself directly in several notable experiments, many of which can be identified with Coulomb Blockade.
We first outline the qualitatively different physics involved in the charging-induced current asymmetries in molecular conductors operating in the strongly coupled (weakly interacting) self-consistent field (SCF) and the weakly coupled (strongly interacting) Coulomb Blockade (CB) regimes. The CB regime, dominated by single charge effects, typically requires a computationally demanding many-electron or Fock space description.
Our analysis of several molecular Coulomb Blockade measurements reveal 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, especially the ones involving exotic phenomena such as NDR and hysteresis.
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Burton Morgan Building, Purdue University, West Lafayette, IN