Karl Franzens University Graz
Graz University of Technology
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Karl Franzens University Graz | Graz University of Technology | |
Charge and heat transport at the molecular scale: is theory really predictive? Molecular nanosystems consist of single molecules or molecular monolayers coupled to macroscopic electrodes and exhibit functionality on the single-molecule level. They can be viewed as building blocks of molecular electronics (molecular wires, rectifiers, transistors, switches and memories), can be used to create new functional nanomaterials and represent a unique set-up for investigation of complex and biological molecules. A first key step in development of molecular nanodevices is the understanding of charge and heat transport at single molecule level. At present the paradigmatic situation is that of a single molecule contacted by two metallic or semiconductor electrodes between which a bias voltage is applied. Since few years, several quantum transport experiments employing unimolecular systems have been proven. Transport physics is governed by quantum coherent transmission and by electron-electron interaction effects (Coulomb blockade, Kondo effect). Besides in contrast to metal or semiconductor nanosystems, molecular structures can be flexible and demonstrate strong mechanical (vibrational and conformational) effects. This property, though parasitic in one case, represents excellent possibilities for spectroscopy of molecular states and for the creation of switching and memory elements. Some of related emerging phenomena, however, are still missing sound theoretical explanations. Principal limitations of the today existing theory include the nature of the molecule-to-electrode contacts as well as the commonly used mean-field like single particle description of molecular bridges. Molecules, indeed, are correlated flexible objects and indeed their nuclear dynamics plays in general a fundamental role in charge transfer as well as transport mechanisms. On the other hand, the problems of charge and heat transport at the molecular scale are strongly interconnected. In this talk, we will consider the basic equations governing electron and phonon (vibron) transport and give an overview on the role of contacts, charging, vibrations and inelastic transport in molecular nanosystems. We will show how such phenomena do dramatically affect the overall conduction properties of molecular junctions. Finally, we will discuss some new trends in development of ab initio nonequilibrium many-body theory of molecular transport. |