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The goal of this thesis is to investigate the electron transport through molecular junctions or wires which have recently attracted much attention experimentally as well as theoretically. Under the influence of a bias voltage and the coupling to the leads which act as electron source and drain, a current through the molecular junction is established. When an external time-dependent field, such as a laser field or an additional AC voltage is applied to the molecular junction, new features can be observed. One phenomenon is the well-known photon-assisted tunneling (PAT) which means that an external field periodic in time with frequency $omega$ can induce inelastic tunneling events when the electrons exchange energy quanta $hbar omega$ with the external field. Another important effect is the famous phenomenon of coherent destruction of tunneling (CDT) which exhibits the unusual effect of quenching the tunneling dynamics at specific values of the field amplitude. In the present thesis the theoretical foundation for these investigations is a density matrix formalism where the full system is partitioned into a relevant part, i.e. the molecular junctions and fermionic reservoirs mimicking the leads. By using a perturbative approach in the system-reservoir coupling strength, it is possible to establish a quantum master equation (QMEs) for the population dynamics of the wire states and an equation for the current through the wire. By combining the theory of optimal control and assuming a predefined target current, a laser field can be obtained which does generate a predefined current pattern. The same technique can be applied to minimize the shot noise. Besides of using QMEs to study electron transport through molecular junctions with the affection of a coupling to leads, it is also possible to apply QMEs to investigate the electron transfer through DNA which is coupled to a phonon-bath.
Klaus von Klitzing Max-Planck-Institut fur ̈ Festk ̈ orperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany Already many Cassandras have prematurely announced the end of the silicon roadmap and yet, conventional semiconductor-based transistors have been continuously shrinking at a pace which has brought us to nowadays cheap and powerful microelectronics. However it is clear that the traditional scaling laws cannot be applied if unwanted tunnel phenomena or ballistic transport dominate the device properties. It is generally expected, that a combination of silicon CMOS devices with molecular structure will dominate the ?eld of nanoelectronics in 20 years. The visionary ideas of atomic- or molecular-scale electronics already date back thirty years but only recently advanced nanotechnology, including e.g. scanning tunneling methods and mechanically controllable break junctions, have enabled to make distinct progress in this direction. On the level of f- damentalresearch,stateofthearttechniquesallowtomanipulate,imageand probechargetransportthroughuni-molecularsystemsinanincreasinglyc- trolled way. Hence, molecular electronics is reaching a stage of trustable and reproducible experiments. This has lead to a variety of physical and chemical phenomena recently observed for charge currents owing through molecular junctions, posing new challenges to theory. As a result a still increasing n- ber of open questions determines the future agenda in this ?eld.
DNA molecules possess high density genetic information in living beings, as well as self-assembly and self-recognition properties that make them excellent candidates for many scientific areas, from medicine to nanotechnology. The process of electron transport through DNA is important because DNA repair occurs spontaneously via the process that restores mismatches and lesions, and furthermore, DNA-based molecular electronics in nano-bioelectronics can be possible through the process. Our work considers a one-dimensional one-channel DNA model, a quasi-one-dimensional one-channel DNA model, and a two-dimensional four-channel DNA model by studying the transport properties such as overall contour plots of transmission, localization lengths, the Lyapunov exponent, and current-voltage characteristics as a function of incoming electron energy and magnetic flux. The behavior of these characteristics is analyzed depending on the system parameters, temperature effects, and magnetic flux effects. The study of quantum mechanical electron transport through DNA molecule will enhance understanding of the electrical properties of DNA, both for assessment and repair of damage, and to explore charge-
These three volumes are intended to shape the field of nanoscience and technology and will serve as an essential point of reference for cutting-edge research in the field.
In conclusion, the electrical characteristics for a molecular device are dictated by the chemical functionality of the molecule bonded to the metal electrodes and the substituents attached. The electron flow can be further modified by variations in the device structure parameters and the molecular configurations, which realign the molecular energy states with respect to the reference metal Fermi levels.