The activities of the Max Planck Fellow Group started in 2OO7. We do basic research in the field of solid state theory. We are interested in a material-specific and parameter-free description of nanostructured systems. Our research is based on density functional theory formulated in terms of Green functions. Green functions are very powerful for the consideration of systems with arbitrary geometry like heterostructures, thin films, surfaces, adatoms on surfaces or nanocontacts. The numerical effort of our method scales with the number of atoms. In this respect we are able to treat nanostructures of realistic size.
Our investigations start from the atomic structure of a system which is either known from experiment or can be determined numerically by structural relaxation. The main focus of our work is the microscopic understanding of the electronic, magnetic, ferroelectric, and transport properties on the atomic scale.
A substantial part of our research is dedicated to the emerging field of spintronics. Spintronics has a large potential for future applications in sensor and information technology in which the charge and spin-degree of freedom of the electrons are exploited. A successful application requires achieving control of the materials and processes involved on the atomic scale. To support the experimental developments, to predict new materials and to optimize the effects, first-principles electronic structure calculations based on density functional theory are the method of choice. Our method is applied to gain insight into the microscopic origin of spin-dependent transport of magnetic heterostructures as well as metallic and molecular contacts. The basic effects of spintronics like giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) have been investigated. Charge and heat to spin current conversion by means of the spin Hall effect (SHE) and the spin Nernst effect (SNE) as well as magnetoelectric coupling via multiferroic interfaces are currently focus areas of our research.
Our activities are embedded in the Collaborative Research Centre 762: Functionality of Oxide Interfaces. Here we investigate metal-oxide and oxide-oxide interfaces. Their atomic structure is the key for the resulting electronic and functional properties. Based on our calculations we design new materials with optimized functionalities.
The Max Planck Fellow group under the guidance of Prof. Dr. Wolf Widdra started in July 2010 in the field of experimental surface science. The group focuses on the atomic and electronic structure of oxide surfaces and thin films. Methodologically, the Fellow group focuses on laser-based photoemission and developed high-repetition-rate pulsed laser sources with tunable photon energies between 1.5 and 40 eV. The photoemission experiments are realized either as angle-resolved photoemission (ARPES) , time-resolved two-photon photoemission (2PPE), photoelectron emission microscopy (PEEM) or as double photoemission (DPE). The DPE coincidence technique is directly sensitive to electron-electron correlations and opens access to complex materials that require an electronic description beyond the traditional one-electron picture. Additionally, the combination of ARPES and 2PPE allows a broad spectroscopic characterization of occupied as well as unoccupied electronic states and their dynamic screening. Scanning tunneling spectroscopy (STS) provides an alternative spectroscopy that links directly local electronic and atomic structure.
Highlight of the recent development is a higher-harmonic generation light source that operates at MHz repetition rates in the energy range between 14 and 40 eV and operates reliably over weeks in ARPES and DPE experiments. Another unique development is centered around a fiber-based laser system that provides pairs of ultrashort laser pulses with two freely tunable photon energies in the 1.2 to 5.0 eV range. Again, its high MHz repetition rate enables efficient time-resolved pump-probe spectroscopy.
On the materials side, the research is dedicated to the spectroscopy and the understanding of the electronic structure of transition metal oxide surfaces and epitaxial thin films. Here the interest starts at model systems like NiO(100), but extends also to more complex Perovskite surfaces, including the recently discovered two-dimensional oxide quasicrystal.
All these activities are closely linked to the Collaborative Research Center 762: Functional oxide interfaces.