Max Planck Fellow Group Wolf Widdra

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) [1], 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.