Phd defence by Kristen Kaasbjerg – Niels Bohr Institute - University of Copenhagen

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Phd defence by Kristen Kaasbjerg

Over the past decade the progress in experimental techniques has resulted in the realization of three-terminal single-molecule transistors. Due to the small size of molecules the transport mechanism of these junctions is dominated by Coulomb blockade. The so-called charge stability diagram which plots the conductance as a function of gate and source-drain voltage, provides rich information on the role of the molecular degrees of freedom in the transport through the junction. Recent experiments have indicated that the junction environment also play an important role for the properties of singlemolecule junctions. So far, however, only few theoretical studies have addressed the subject.

In the present work a theoretical framework for this purpose is set up. Via a numerical implementation it is demonstrated that the polarization of the junction environment that follows charging of the molecule has a large influence on the molecular states and thereby also on the transport characteristics.

Similar effects can be expected to affect the electronic structure of metalmolecule interfaces as occurring in e.g. self-assembled monolayers and singlemolecule junctions operating in the phase coherent regime. The many-body GW approximation for the electronic self-energy has been demonstrated to describe polarization effects of such interfaces. Due to its non-perturbative nature, the accuracy of this method may vary from system to system. The second part of this work presents a benchmark study of the GW method on small molecules described by the semi-empirical Pariser-Parr-Pople Hamiltonian. By taking into account screening effects not included in Hartree-Fock, GW is demonstrated to give an overall good description of the molecular ionization potential and electron affinity.

Phd defence by Kristen Kaasbjerg