Interplay between Kondo effect and Ruderman-Kittel-Kasuya-Yosida interaction

The interplay between the Ruderman-Kittel-Kasuya-Yosida (RKKY) 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 (STS) combined with density functional theory (DFT) and numerical renormalization group (NRG) 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 function of dimer separation revealing that it is related to the RKKY interaction mediated by the conduction electrons. Simulations based on density functional theory support this concept showing the same oscillation period and trends in the coupling strength as found in the experiment

Theory of real space imaging of Fermi surfaces

A scanning tunneling microscope can be used to visualize in real space Fermi surfaces with buried impurities far below substrates acting as local probes. A theory describing this feature is developed based on the stationary phase approximation. It is demonstrated how a Fermi surface of a material acts as a mirror focusing electrons that scatter at hidden impurities.Comment: 10 pages, 4 figure

Long-range Kondo signature of a single magnetic impurity

The Kondo effect, one of the oldest correlation phenomena known in condensed matter physics, has regained attention due to scanning tunneling spectroscopy (STS) experiments performed on single magnetic impurities. Despite the sub-nanometer resolution capability of local probe techniques one of the fundamental aspects of Kondo physics, its spatial extension, is still subject to discussion. Up to now all STS studies on single adsorbed atoms have shown that observable Kondo features rapidly vanish with increasing distance from the impurity. Here we report on a hitherto unobserved long range Kondo signature for single magnetic atoms of Fe and Co buried under a Cu(100) surface. We present a theoretical interpretation of the measured signatures using a combined approach of band structure and many-body numerical renormalization group (NRG) calculations. These are in excellent agreement with the rich spatially and spectroscopically resolved experimental data.Comment: 7 pages, 3 figures + 8 pages supplementary material; Nature Physics (Jan 2011 - advanced online publication

Graphene-metal contact resistivity on semi-insulating 6H-SiC(0001) measured with Kelvin probe force microscopy

We present Kelvin probe force microscopy measurements and resistance network simulations of the lateral charge transport across few-layer graphene on the semi-insulating 6H-SiC(0001) surface. After preparation of the SiC crystal by thermal decomposition, gold electrodes were prepared on the top of the graphene layers. The transport field is extracted by subtracting measurements of reverse lateral bias applied to the gold electrodes. Graphene sheet resistances as low as 0: 75 k Omega/sq were observed. By comparing the experimental transport measurements with a resistance network simulation the contact resistivity between graphene and a gold electrode can be determined to be < 1 x 10(-6) Omega cm(2). (C) 2013 AIP Publishing LLC

Origin of Schottky barriers in gold contacts on GaAs(110)

Gold contacts on n-type GaAs(110) have been investigated using scanning tunneling microscopy and spectroscopy in cross-sectional configuration. In spatially resolved current voltage spectroscopy the Schottky barrier potential is visible. We find signatures of delocalized gap states at the interface decaying into the semiconductor and observe a defect density at the interface below 3x10(13) cm(-2). Both findings support that the Fermi level pinning at the Au/GaAs(110) interface is dominated by metal-induced gap states

Intrinsic quantum dots in partially ordered bulk (GaIn)P

We present a photoluminescence study with 500 nm lateral resolution on partially ordered bulk (GaIn)P alloys at lattice temperatures 3-60 K, external magnetic fields 0-12 T, and excitation power 0.1-100 mu W. In the known ordering-induced low energy emission band we resolve narrow optical transition lines with 0.3-1.0 meV width. They show no thermal broadening, a diamagnetic shift with pronounced anisotropy, and biexci-tonic states. We demonstrate that the transitions are connected with intrinsic quasi-zero-dimensional electron-hole confinement formed at the antiphase-boundaries in the crystal