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Molecular electronics is based on the use of single molecules as elementary functional electronic components. It is considered to be a promising strategy for further miniaturization of electronic circuits and long-term replacement of semiconductor technology. Moreover the quantum mechanical nature of molecules may be used to design new logical functions, beyond the electron-charge based data processing of classical transistors. With the experimental probing and manipulation of electron and nuclear spin, and the observations of quantum interferences in such systems, this area of research has gained increasing attention in the last decade.Contacting individual molecules in order to perform transport experiments at such a small scale is challenging. Various methods based on the creation of metallic nano-contacts into which molecules are deposited have shown promising results, and already lead to the realization of single-molecule transistors.On the other hand, scanning tunneling microscopy(STM) has also proven to be a successful method for studying single molecules on surfaces and probing some of their electronic properties. Due to geometrical constraints, it is extremely challenging to implement a third gate electrode in such an experiment. This is, however, a prerequisite for controlling the energy of the molecular level responsible for charge transport. The aim of this work was to use graphene on SiO_2/Si as a substrate, Si and graphene being independently electrically contacted and serving as drain- and gate-electrode. The very low density of states of graphene makes it screen only a fraction of the electric field generated by the applied potential on the silicon. A molecule deposited on the graphene is thus placed in a controllable electric field, so that the geometrical arrangement STM tip/molecule/graphene/SiO_2/Si forms a molecular transistor,which makes it possible to tune the molecular levels contributing to charge transport in an STM.A newly acquired low temperature STM was first put into operation. Spatial and energy resolution of the device could be tested, before graphene deposited on various substrates was examined. This consists in a preliminary work laying the basis for the later deposition of molecules onto graphene. STM spectroscopy could furthermore demonstrate the emergence of Landau levels in graphene on NbSe_2 when a magnetic field is applied.The molecules investigated in this work are cobalt phthalocyanines (CoPc). They are planar organic molecules, which have found application in various areas. Since they are thermally stable, they can be easily deposited on various substrates and belong to the most studied molecules in surface science. The adsorption of CoPc molecules on metallic surfaces has been studied first, showing a strong interaction between the molecular orbitals and the substrate, leading to a charge transfer. The insertion of graphene between metallic substrate and molecule was then investigated as a possible buffer layer for decoupling of the molecular levels. STM spectroscopy studies indicate that the molecular energy levels of CoPc on graphene do in fact correspond to the ones of the gas phase. Finally, CoPc molecules on graphene on SiO_2 were studied and their electronic properties probed as a function of the potential applied to the gate. A control of the energy of molecular states contributing to the tunneling current could be achieved, with an efficiency similar to classical transport experiments. The possibility to combine the charge transport characterization of molecules by means of a gate electrode with the spatial resolution of a STM was thus demonstrated. This opens a door towards promising future experiments with a combined control over the spatial position of the source electrode in the picometer-range and over the electronic properties of the studied nanostructures.

In this thesis, we model the dynamics of two-dimensional electron systems (TDES) on deformed superfluid liquid helium films that wet a periodically structured surface exhibiting troughs and peaks and report measurements of the admittance of such TDES for different electron densities and temperatures, as a function of a magnetic field perpendicular to the TDES.

We study the coherent transport of heavy holes through a one-dimensional ring in the presence of spin-orbit coupling. Spin-orbit interaction of holes, cubic in the in-plane components of momentum, gives rise to an angular momentum dependent spin texture of the eigenstates and influences transport. We analyze the dependence of the resulting differential conductance of the ring on hole polarization of the leads and the signature of the textures in the Aharonov-Bohm oscillations when the ring is in a perpendicular magnetic field. We find that the polarization-resolved conductance reveals whether the dominant spin-orbit coupling is of Dresselhaus or Rashba type, and that the cubic spin-orbit coupling can be distinguished from the conventional linear coupling by observing the four-peak structure in the Aharonov-Bohm oscillations., Comment: 12 pages, 11 figures

Im Rahmen dieser Arbeit wurde eine Meßapparatur zur Bestimmung des Leitwerts einzelner C60-Moleküle aufgebaut. Dabei werden mittels der Bruchkontakttechnik atomare Kontaktspitzen erzeugt und die Moleküle thermisch in situ auf die Kontakte verdampft.Es konnten dadurch Vibrationsmoden des C60 zwischen Goldelektroden nachgewiesen werden. Bei Messungen mit Aluminiumelektroden und C60 war mit großer Wahrscheinlichkeit der Kondoeffekt zu sehen. Dabei fungiert das C60 als Insel zwischen den Elektroden.Zusätzlich konnte im Rahmen der Charaktersierung der Elektroden Phononmoden des Aluminiums am Einatomkontakt nachgewiesen werden. Diese zeichnen sich durch eine Änderung ihrer Energie mit zunehmender Streckung der Elektroden aus.

Conductance histograms are a valuable tool to study the intrinsic conduction properties of metallic atomic-sized contacts. These histograms show a peak structure, which is characteristic of the type of metal under investigation. Despite the enormous progress in the understanding of the electronic transport in metallic nanowires, the origin of this peak structure is still a basic open problem. In the present work we tackle this issue, extending our theoretical analysis of Au conductance histograms to different types of metals, namely, Ag, Pt and ferromagnetic Ni. We combine classical molecular dynamics simulations of the breaking of nanocontacts with conductance calculations based on a tight-binding model. This combination gives us access to crucial information such as contact geometries, strain forces, minimum cross-sections, the conductance, transmissions of the individual conduction channels, and, in the case of Ni, the spin polarization of the current. We shall also briefly discuss investigations of Al atomic-sized contacts. From our analysis we conclude that the differences in the histograms of these metals are due to (i) the very different electronic structures, which means different atomic orbitals contributing to the transport and (ii) the different mechanical properties, which in a case such as Pt lead to the formation of special structures, namely, monoatomic chains. Of particular interest are results for Ni that indicate the absence of any conductance quantization, and show how the current polarization evolves (including large fluctuations) from negative values in thick contacts to even positive values in the tunneling regime after rupture of the contact. Finally, we also present a detailed analysis of the breaking forces of these metallic contacts, which are compared to the forces predicted from bulk considerations.

We calculate the mode-dependent transmission probability of massless Dirac fermions through an ideal strip of graphene (length $L$, width $W$, no impurities or defects) to obtain the conductance and shot noise as a function of Fermi energy. We find that the minimum conductivity of order ${e}^{2}/h$ at the Dirac point (when the electron and hole excitations are degenerate) is associated with a maximum of the Fano factor (the ratio of noise power and mean current). For short and wide graphene strips the Fano factor at the Dirac point equals $1/3$, 3 times smaller than for a Poisson process. This is the same value as for a disordered metal, which is remarkable since the classical dynamics of the Dirac fermions is ballistic.

The electric conductance of a strip of undoped graphene increases in the presence of a disorder potential, which is smooth on atomic scales. The phenomenon is attributed to impurity-assisted resonant tunneling of massless Dirac fermions. Employing the transfer matrix approach we demonstrate the resonant character of the conductivity enhancement in the presence of a single impurity. We also calculate the two-terminal conductivity for the model with one-dimensional fluctuations of disorder potential by a mapping onto a problem of Anderson localization., Comment: 6 pages, 3 figures, final version, typos corrected, references added

We examine the full counting statistics of quantum dots, which display super-Poissonian shot noise. By an extension to a generic situation with many excited states we identify the underlying transport process. The statistics is a sum of independent Poissonian processes of bunches of different sizes, which leads to the enhanced noise. The obtained results could be useful to determine transport characteristics in molecules and large quantum dots, since the noise (and higher cumulants) allow to identify the internal level structure, which is not visible in the average current., 4 pages, 1 figure, submitted to PRB

The clear separation of the domain wall magnetoresistance from the anisotropic magnetoresistance has been successfully achieved by investigating the transversal magnetoresistance of individual (Co/Pt)10-nanowires. Since the magnetic easy axis of this nanowires is out of plane, the magnetization is always perpendicular to the current direction when applying a magnetic field transversally. Most importantly, even the domain walls do not give rise to an anisotropic resistance contribution by themselves under these conditions, and thus the pure domain wall magnetoresistance can be measured. The interpretation of the resistance behavior is confirmed by Monte Carlo simulations giving the magnetization distribution during the magnetization reversal process. It can be shown that the observed resistance behavior reflects the dependence of the domain wall magnetoresistance on the domain wall structure.

We report on high power terahertz (THz) emission from ErAs-enhanced In0.52Al0.48As-In0.53Ga0.47As superlattices for operation at 1550 nm. ErAs clusters act as efficient recombination centers. The optical power is distributed among a large, microstructured area in order to reduce the local optical intensity. A THz field strength of 0.7 V/cm (1 V/cm peak-to-peak) at 100 mW average optical power has been obtained, with emission up to about 4 THz in air, limited by the detection crystal used in the system.