Quantum Transport

3.1 Quantum transport through clusters and molecules on surfaces. I-V characteristics, conductance maps, Theoretical description of STM measurements

Questions addressed:  How to extract information from STS-spectra of clusters on surfaces? Theoretical development? Explanation for the experimentally observed disappearance of cluster states? Validity of the density-of-states approach? Influence of the shape of the STM-tip on the I-V-characteristics?
We performed a detailed theoretical study of scanning tunneling imaging and spectroscopy of C60 on silver and gold surfaces, motivated by recent experiments. The surface/sample/tip system is described within a self-consistent density-functional-theory-based tight-binding model. The topographic and conductance images are computed at constant current from a full self-consistent transport theory based on non-equili- brium Green's functions and compared with those simulated from the local density of states. The molecular orbitals of C60 are clearly identified in the energy resolved maps, in close correspondence with the experi- mental results. We show how the tip structure and orientation can affect the images. In particular, we consider the effects of truncated tips on the energy-resolved maps.
Low-temperature scanning tunneling spectroscopy (STS) allows us to probe electronic properties of clusters at surfaces with unprecedented accuracy. Recent experimental determination of the differential conductance of supported clusters yield considerable deviations with respect to the expected density of states and suggest that many cluster states are invisible to STS measurements. By means of fully self- consistent quantum transport calculations, using realistic tunneling tips, we show that, depending on the tip shape, only a small fraction of the electronic states contribute to the STS spectra, thus explaining the experimental findings. We demonstrate that the unambiguous characterization of the states on the supported clusters can be achieved with energy-resolved images, obtained from a theoretical analysis which mimics the experimental imaging procedure.
The method has been also applied to the specific problems of  a Si29H24 deposited cluster on a Si(111) surface, and for a Styrene molecule. In the first case, the influence of the bonding between cluster and surface on the I-V-characteristics was analyzed. In the second case negative differential resistance was found.
We have also performed calculations of the distortions of STM images produced by different tip shapes. In the case of carbon nanotubes on metallic surfaces we obtained excellent agreement with experiments carried out at the University of Dortmund (by H. Hövel and coworkers).
Methods: Non-equilibrium Greens functions. Density functional calculations. Surface-Science techniques.

Publications: [58], [64], [70], [71], [74], [80] (see list of publications

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