Spectroscopy on the Atomic Scale

Our group focusses on the development of state of the art scanning probe tools for challenging questions of fundamental research in nano scale characterization of transport and correlation effects in electronic and magnetic properties of semiconductors and metals. More specific, we investigate

Charge Transport in graphene - Scanning tunneling potentiometry
Access to local transport properties Electronic transport on a macroscopic scale is often described by spatially averaged electric fields and scattering processes. To capture electronic transport on the atomic scale, local and non-local scattering processes need to be considered separately. An experimental study based on low-temperature scanning tunneling potentiometry has allowed us to separate different scattering mechanisms in graphene. Most importantly, we are able to show that the voltage drop at a monolayer/bilayer boundary is not located strictly at the structural defect.
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Dynamic processes studied atom by Atom - Combining pulsed optical exciation and STM/STS
Pump-probe experiments combined with SPM allow to resolve dynamic processes on the nanometer scale. We have utilized this to study the charging process of single donors in GaAs. Our experiments show that the combined dynamics of bound and free charges become important to better understand the physics of nano-scaled systems.
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Correlated electron systems  - Kondo effect of single and coupled magnetic bulk impurities  
Scanning tunneling spectroscopy allows to study single magnetic impurities in bulk Crystals.This surprising finding has opened up a new way to investigate the interplay between the Ruderman-Kittel-Kasuya-Yosida interaction and the Kondo effect, which is expected to provide the driving force for the emergence of many phenomena in strongly correlated electron materials. We have investigated iron dimers buried below a Cu(100) surface by means of low-temperature scanning tunneling spectroscopy. 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 under-standing of dense Systems.
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New material classes: hexaborides and iridates
We have recently started two new projects.
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Metal-semiconductor Interfaces
In order to check the validity of theoretical models describing the microscopic process of Schottky barrier formation, the objective has been to study an ideal metal-semiconductor interface on the atomic scale. The low-temperature grown Fe/GaAs{110} interface serves as an ideal model system that is studied by means of atomically resolved cross-sectional scanning tunneling microscopy (XSTM) and spectroscopy (XSTS). For the first time, this approach yields a spatial and energetic map of the local density of states that covers both the band gap and the valence and conduction bands at the interface. In combination with density functional calculations performed by Dr. Ali Al-Zubi and Prof. Stefan Blügel from the FZ Jülcih this allows a better understanding of the relevance of metal-induced gap states and bond polarization at the interface.

Electronic properties of dopant atoms in semiconductors
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