Authors: M. Hettler, W. Wenzel, M. Wegewijs and H. Schoeller
Affilation: Forschungszentrum Karlsruhe, Germany
Pages: 101 - 104
Keywords: molecular electronics, tunnel junction, iv-characteristic, blocking state
Single molecule electronic devices offer exciting perspectives for further minituarization of electronic circuits with a potentially large impact in applications. Several experiments have demonstrated the possibility to attach individual molecules to leads and to measure the electrical transport. In contrast to single electron transistors based on quantum dots the electronic structure of molecular devices can be chemically designed for specific applications. When the molecule is coupled weakly to the electrodes, i.e. via electron tunnelling, charging effects, semi-classically determined by the small capacitance of the molecule, become important. The interplay of charging effects with the specific structure of the molecular orbitals leads to nontrivial current voltage I-V characteristics. Very recent experiments demonstrated both Coulomb blockade and the Kondo effect in three terminal transport through a single molecular level. Using benzene (see Fig 1) as a prototypical example we investigate novel effects hat arise when transport through several competing electronic configurations becomes possible. We derive a semi-quantitative model for the conducting many-body states of the system from electronic structure calculations. For weak coupling to the electrode we compute transport within the golden rule approximation (sequential tunnelling) and include screening of the applied electric field as well as radiative transitions between the electronic states of the molecule. We predict a current collapse in the current voltage characteristic (IV) and strong negative differential conductance (NDC) (see Fig 2) due to the occurrence of a ``blocking state when the molecule is coupled to the electrode at the ortho-position (see Fig 3). For coupling at the meta-position, the IV displays a series of steps, but no NDC. We demonstrate how the specific spatial structure of the molecular orbitals qualitatively determines electronic transport. Finally, we discuss the limits of the model and the impact of disorder and symmetry breaking effects likely encountered in an experimental realization.
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