Scanning tunneling spectroscopy of magnetic bulk impurities: From a single Kondo atom towards a coupled system

by Henning Prüser
Doctoral thesis
Date of Examination:2013-02-22
Date of issue:2013-03-06
Advisor:Prof. Dr. Rainer G. Ulbrich
Referee:Prof. Dr. Rainer G. Ulbrich
Referee:Prof. Dr. Thomas Pruschke
Referee:Prof. Dr. Richard Berndt
Persistent Address: http://dx.doi.org/10.53846/goediss-3762

 

 

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Abstract

English

This thesis focuses on single impurity Kondo physics and the transition towards coupled Kondo systems in real space. Single magnetic iron and cobalt and non magnetic silver atoms embedded below a copper surface are investigated using low temperature scanning tunneling microscopy and spectroscopy. Spectroscopic information is used to map the conduction electrons at the surface in the vicinity of sub-surface impurities. Iron and cobalt atoms show a spatially extended Kondo signature which allows determining the phase shift caused by the Kondo effect. Both impurity species show similar behavior on completely different energy scales which can be understood by their different Kondo temperatures. A detailed analysis of the spectroscopic information as function of lateral tip position and impurity distance below the surface is presented for both magnetic impurity atoms. The Kondo signature shows an oscillatory behavior as function of distance, which is anisotropic for different directions. This observation is in good agreement with the band structure of the underlying copper crystal. Silver atoms are studied as a non magnetic reference system proving that the signatures are related to the localized magnetic moment of iron and cobalt. Apart from isolated single impurities, geometries where two iron impurities are located in the same monolayer close to each other are investigated. The dimer configurations show a considerably different spectral signature as compared to single iron impurities located in the same monolayer. In particular, the resonance width is strongly altered or even suppressed due to the presence of the second impurity. The experimental findings can be related to the two impurity Kondo model comprising two competing effects: On the one hand the Kondo effect, which tries to screen the impurity spin. On the other hand the Rudermann Kittel Kasuya Yosida (RKKY) interaction promoting a strong correlation between the magnetic moments of the two impurities. The behavior strongly depends on the atomic configuration of the iron dimer and not simply on the geometric distance. The strongest broadening does not occur for the smallest interatomic distance but for an intermediate separation, indicating an oscillatory behavior of the interaction strength. The experimental findings are compared with first principles calculations showing very good agreement for both strength and directionality of the magnetic interaction.
Keywords: Scanning tunneling microscopy and spectroscopy; low temperature; STM; STS; Kondo effect; RKKY interaction; Friedel oscillation; sub-surface impurities; single-impurity Kondo physics; two-impurity Kondo physics
 
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