Research Highlights

Nature Communications 2020

In collaboration with the Chemnitz University of Technology and the Physikalisch-Technische Bundesanstalt Braunschweig we have investigated nanoscale resistance variations in epitaxial graphene induced vy the proximity of the graphene sheet to the silicon carbide substrate. The results have been published in Nature Communications.

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Nature Communications 2017

Setting up a new Scanning Tunneling Microscope equipped with a 7 T magnet we have studied the tranport in graphene down to the nanometer scale. The results have been published in Nature Communications.

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Science Advances 2017

The key elements in the steady miniaturization process of cutting-edge semiconductor devices are the understanding and controlling of charge dynamics on the atomic scale. In detail, we address the study of charging processes of individual doping atoms and, especially, the interaction of those atoms with their surroundings. We use pulsed optical excitation in combination with scanning tunneling microscopy at the n-doped gallium arsenide [GaAs (110)] surface to investigate single donor dynamics within a nanoscaled, localized space charge region. Tuning the tunnel rate can drive the system into nonequilibrium conditions, allowing distinction between the decay of optically induced free charge carriers and the decay of donor charge states. The latter process is atomically resolved and discussed with respect to donor-level binding energies and local donor configurations.


Nature Communications 2016

The miniaturization of future electronic devices is intimately connected to the ability to control electric fields on the atomic scale. In a nanoscopic system defined by a limited number of charges, the combined dynamics of bound and free charges become important. Here we present a model system based on the electrostatic interaction between a metallic tip of a scanning tunnelling microscope and a GaAs(110) semiconductor surface. The system is driven out of equilibrium by optical excitation, which provides ambipolar free charge carriers, and by an optically induced unipolar tunnel current. This combination enables the active control of the density and spatial distribution of free and bound charge in the space-charge region, that is, modifying the screening processes. Temporal fluctuations of single dopants are modified, meaning we are able to control the noise of the system. It is found that free charge carriers suppress the noise level in field-controlled, nanoscopic systems.


Nano Letters July 2015

We investigate the structural, electronic, and transport properties of substitutional defects in SiC-graphene by means of scanning tunneling microscopy and magnetotransport experiments. Using ion incorporation via ultralow energy ion implantation, the influence of different ion species (boron, nitrogen, and carbon) can directly be compared. While boron and nitrogen atoms lead to an effective doping of the graphene sheet and can reduce or raise the position of the Fermi level, respectively, 12C+ carbon ions are used to study possible defect creation by the bombardment. For low-temperature transport, the implantation leads to an increase in resistance and a decrease in mobility in contrast to undoped samples. For undoped samples, we observe in high magnetic fields a positive magnetoresistance that changes to negative for the doped samples, especially for 11B+- and 12C+-ions. We conclude that the conductivity of the graphene sheet is lowered by impurity atoms and especially by lattice defects, because they result in weak localization effects at low temperatures.


PRL Editor's Choice April 2015

We investigate low temperature grown, abrupt, epitaxial, nonintermixed, defect-free n-type and p-type Fe/GaAs(110)interfaces by cross-sectional scanning tunneling microscopy and spectroscopy with atomic resolution. The probed local density of states shows that a model of the ideal metal-semiconductor interface requires a combination of metal-induced gap states and bond polarization at the interface which is nicely corroborated by density functional calculations. A three-dimensional finite element model of the space charge region yields a precise value for the Schottky barrier height.


Nature Communications March 2015

Electronic transport on a macroscopic scale is described by spatially averaged electric fields and scattering processes summarized in a reduced electron mobility. That this does not capture electronic transport on the atomic scale was realized by Landauer long ago. Local and non-local scattering processes need to be considered separately, the former leading to a voltage drop localized at a defect, the so-called Landauer residual-resistivity dipole. Lacking precise experimental data on the atomic scale, the spatial extent of the voltage drop remained an open question. Here, we provide an experimental study showing that the voltage drop at a monolayer?bilayer boundary in graphene clearly extends spatially up to a few nanometres into the bilayer and hence is not located strictly at the structural defect. Moreover, different scattering mechanisms can be disentangled. The matching of wave functions at either side of the junction is identified as the dominant process, a situation similar to that encountered when a molecule bridges two contacts.


Nature Communications December 2014

The interplay between the Ruderman-Kittel-Kasuya-Yosida interaction and the Kondo effect is expected to provide the driving force for the emergence of many phenomena in strongly correlated electron materials. Two magnetic impurities in a metal are the smallest possible system containing all these ingredients and define a bottom-up approach towards a long-term understanding of concentrated/dense systems. Here we report on the experimental and theoretical investigation of iron dimers buried below a Cu(100) surface by means of low-temperature scanning tunnelling spectroscopy combined with density functional theory and numerical renormalization group calculations. The Kondo effect, in particular the width of the Abrikosov-Suhl resonance, is strongly altered or even suppressed due to magnetic coupling between the impurities. It oscillates as a function of dimer separation revealing that it is related to indirect exchange interactions mediated by the conduction electrons.


Physical Review Letters April 2012

We investigate single Fe and Co atoms buried below a Cu(100) surface using low temperature scanning tunneling spectroscopy. By mapping the local density of states of the itinerant electrons at the surface, the Kondo resonance near the Fermi energy is analyzed. Probing bulk impurities in this well-defined scattering geometry allows separating the physics of the Kondo system and the measuring process. The line shape of the Kondo signature shows an oscillatory behavior as a function of depth of the impurity as well as a function of lateral distance. The oscillation period along the different directions reveals that the spectral function of the itinerant electrons is anisotropic.


Nature Physics January 2011

In collaboration with the Institute of theoretical physics, Göttingen we have investigated the Kondo physics of magnetic impurities embedded in bulk copper.


Science Februar 2009

In collaboration with the Forschungszentrum Jülich and the Universitity of Halle we have shown that Fermi surfaces can be imaged in real space with a low-temperature scanning tunneling microscope when subsurface point scatterers are present: in this case, cobalt impurities under a copper surface. Even the very simple Fermi surface of copper causes strongly anisotropic propagation characteristics of bulk electrons that are confined in beamlike paths on the nanoscale. The induced charge density oscillations on the nearby surface can be used for mapping buried defects and interfaces and some of their properties.


Nature Materials: RESEARCH HIGHLIGHT October 2008

In collaboration with the University of Eindhoven (AG Prof. Koenraad) we have studied the ionization process of single donors and multiple donor complexes.