M. Wenderoth

Doping of Graphene by Low-Energy Ion Beam Implantation: Structural, Electronic, and Transport Properties

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, (12)C(+) 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 (11)B(+)- and (12)C(+)-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.

Electronic Transport on the Nanoscale: Ballistic Transmission and Ohm's Law

If a current of electrons flows through a normal conductor (in contrast to a superconductor), it is impeded by local scattering at defects as well as phonon scattering. Both effects contribute to the voltage drop observed for a macroscopic complex system as described by Ohms law. Although this concept is well established, it has not yet been measured around individual defects on the atomic scale. We have measured the voltage drop at a monatomic step in real space by restricting the current to a surface layer. For the Si(111)-( [see text]3 x [see text]3)-Ag surface a monotonous transition with a width below 1 nm was found. A numerical analysis of the data maps the current flow through the complex network and the interplay between defect-free terraces and monatomic steps.

Seeing the Fermi Surface in Real Space by Nanoscale Electron Focusing

The Fermi surface that characterizes the electronic band structure of crystalline solids can be difficult to image experimentally in a way that reveals local variations. We show 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.

Current transport through single grain boundaries: A scanning tunneling potentiometry study

Spatial variations of the local electric fields in current‐carrying thin gold films were studied with a scanning tunneling microscope on a nanometer scale. With a refined potentiometry technique we resolved potential steps at grain boundaries and investigated the potential gradients within each grain. These gradients are caused by nonlocal background scattering of the conduction electrons and are used to measure the local current density. We determine the reflectivity of an individual grain boundary without invoking an averaging procedure over the whole film. We find that the reflectivity of grain boundaries varies between R=0.7 and 0.9 and depends on their orientation.

A versatile high resolution scanning tunneling potentiometry implementation

We have developed a new scanning tunneling potentiometry technique which can-with only minor changes of the electronic setup-be easily added to any standard scanning tunneling microscope (STM). This extension can be combined with common STM techniques such as constant current imaging or scanning tunneling spectroscopy. It is capable of performing measurements of the electrochemical potential with microvolt resolution. Two examples demonstrate the versatile application. First of all, we have determined local variations of the electrochemical potential due to charge transport of biased samples down to angstrom length scales. Second, with tip and sample at different temperatures we investigated the locally varying thermovoltage occurring at the tunneling junction. Aside from its use in determining the chemical identity of substances at the sample surface our method provides a controlled way to eliminate the influence of laterally varying thermovoltages on low-bias constant current topographies.

Connection of anisotropic conductivity to tip-induced space-charge layers in scanning tunneling spectroscopy of p-doped GaAs

The electronic properties of shallow acceptors in p-doped GaAs{110} are investigated with scanning tunneling microscopy at low temperature. Shallow acceptors are known to exhibit distinct triangular contrasts in STM images for certain bias voltages. Spatially resolved I(V)-spectroscopy is performed to identify their energetic origin and behavior. A crucial parameter - the STM tips work function - is determined experimentally. The voltage dependent potential configuration and band bending situation is derived. Ways to validate the calculations with the experiment are discussed. Differential conductivity maps reveal that the triangular contrasts are only observed with a depletion layer present under the STM tip. The tunnel process leading to the anisotropic contrasts calls for electrons to tunnel through vacuum gap and a finite region in the semiconductor.

Spatial extent of a Landauer residual-resistivity dipole in graphene quantified by scanning tunnelling potentiometry.

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.

Interplay between the Kondo effect and the Ruderman–Kittel–Kasuya–Yosida interaction

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.

Luminescence from gold-passivated gallium arsenide surfaces excited with a scanning tunneling microscope

Abstract A scanning tunneling microscope (STM) is used to investigate light emission induced by electron tunneling into GaAs. Reproducible results are obtained in air on cleaved (110) surfaces passivated with a thin (∼ 1.6 nm ) gold film. The luminescence spectrum is dominated by recombination of thermalized electron-hole pairs. In p-type material, the light emission threshold is at 1.45 V bias voltage, close to the band gap at 300 K. The quantum efficiency of this minority-carrier injection process increases quadratically with increasing bias voltage. In n-type material, the current induced edge emission occurs at a much higher threshold of U > 3.2 V , giving evidence for the onset of hole production via impact ionization by the injected hot electrons.

Mapping itinerant electrons around Kondo impurities.

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.