Henning Prüser

Long-range Kondo signature of a single magnetic impurity

The Kondo effect describes electrons scattering off a magnetic impurity, which affects the resistivity of a metal at low temperatures. In the case of buried iron or cobalt atoms, the correlations are longer ranged than studies of adatoms have shown.

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.

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.

Harmonic Oscillator Wave Functions of a Self-Assembled InAs Quantum Dot Measured by Scanning Tunneling Microscopy

InAs quantum dots embedded in an AlAs matrix inside a double barrier resonant tunneling diode are investigated by cross-sectional scanning tunneling spectroscopy. The wave functions of the bound quantum dot states are spatially and energetically resolved. These bound states are known to be responsible for resonant tunneling phenomena in such quantum dot diodes. The wave functions reveal a textbook-like one-dimensional harmonic oscillator behavior showing up to five equidistant energy levels of 80 meV spacing. The derived effective oscillator mass of m* = 0.24m0 is 1 order of magnitude higher than the effective electron mass of bulk InAs that we attribute to the influence of the surrounding AlAs matrix. This underlines the importance of the matrix material for tailored QD devices with well-defined properties.

Erratum: Interplay between Kondo effect and Ruderman-Kittel-Kasuya-Yosida interaction.

Nature Communications 5: Article number:5417 (2014); Published 11 November 2014; Updated 13 February 2015. This Article contains typographical errors in equation (1), in which a number of mathematical operators were inadvertently removed during the production process. The correct version of equation(1) appears below.

Signatures of Non-magnetic Atoms

The last chapter was devoted to single-impurity Kondo physics of magnetic iron and cobalt atoms. In this chapter non-magnetic silver (Ag) impurities below the Cu(100) surface are considered.

Experimental Setup and Background

The experimental task of this thesis is to prepare magnetic and non-magnetic bulk impurities and to investigate these systems by scanning tunneling microscopy (STM). In the first part of this chapter the sample preparation will be described. The main experimental instrumentation, the low temperature STM as well as the standard theory for the interpretation are introduced in the second part.

Kondo Physics of Single Sub-surface Atoms

The Kondo effect has regained interest due to scanning tunneling spectroscopy (STS) experiments carried out on single magnetic impurities, which were adsorbed on a noble metal surface [1, 2].

Investigation of Sub-surface Atoms by STM

Scanning tunneling microscopy (STM) and spectroscopy (STS) has shown to be a universal tool to study the electronic properties of surfaces. A first clue that STM is not only restricted to surface physics (atoms or nanostructures on a surface) on metallic systems but also give access to extended sub-surface defects in metals was presented by Schmidt and colleagues in 1996.

Two-Impurity Kondo Physics

In small nanostructures containing only a few magnetic atoms as well as in periodically ordered systems the effective interaction between magnetic impurities leads to various magnetic ground states. In a metallic system, the magnetic spin orientation can be affected by direct exchange interaction due to an overlap of the localized impurity orbitals or by indirect exchange interaction mediated by the surrounding conduction electrons.