Fermi-surface topologies of strongly correlated materials

As an typical example, the image shows two data sets of CePt₅ and LaPt₅ acquired with ARPES, which allows directly the measurement of the dispersion (energy vs. momentum) and, therewith, the mapping of the Fermi surface topology. The purpose of the experiments in the present case is to understand how the single 4f electron of Cerium induces so strong correlations to change fundamentally the macroscopic properties of the material. In the images, the bright colours (yellow and orange) and dark colours (black and dark red) correspond to positions of the Brillouin zone with higher or lower electron densities, respectively. This momentum-dependent information (from so-called Fermi surface maps) is crucial when one wants to understand the macroscopic properties of Ce-compounds (e.g. the electrical resistivity) in terms of microscopic theories (quantum theory of matter). By comparing the  experimental intensities of CePt₅ to those of LaPt₅ it is possible to learn what are the electron states responsible for the unusual properties of CePt₅ and how they can be described by theoretical models (Kondo, SIAM) or whether more sophisticated theories are requested (Periodic Anderson Model PAM).

(see projects in FOR1162 for more information)

ARPES: Molecular Orbital Mapping

One capability of angular resolved photoelectron spectroscopy (ARPES)  is analyzing the geometric structure of molecular orbitals. It has been shown, that the angular dependent intensity distribution of  the photoemission signal can be understood as the Fourier transform of the molecular orbital. This allows, e.g., a straightforward comparison between the experimental results and quantum mechanic calculations.

As an example, the figure shows the 2D-plot of the theoretical and experimental ARPES intensities for the HOMO and LUMO of the organic prototyp molecule PTCDA. The experiment was performed for a highly ordered monolayer of molecules adsorbed on a Ag(110) single crystal surface. This intensity distribution gives a detailed insight into a change of the spacial distribution of the orbitals due to the interaction with the surface.

J. Ziroff et al., Physical Review Letters, in press, arXiv:1002.2055v1.

Resonant inelastic x-ray scattering: Investigation of the electronic structure of solids and liquids

With resonant inelastic x-ray scattering (RIXS) in the soft x-ray range, the chemical and electronic structure of solids, liquids, and gases can be investigated. The left part of the figure below shows some exemplary spectra of liquid water together with an energy level scheme and the corresponding molecular orbitals. The spectra give information about the hydrogen bonding configuration as well as about proton dynamics caused by the x-ray excitation process. The RIXS experiments generally need a high photon flux and thus have to be performed at a third generation synchrotron light source like the Advanced Light Source at LBNL in Berkeley. The second example on the bottom right shows the RIXS “map” of CdS. With this map, detailed information about the band structure as well as about the wave-functions can be obtained. To investigate the dynamics of molecular matter, we combine RIXS with the related technique of resonant PES (ResPES) in the framework of the planned SFB 879.

L. Weinhardt et al., Phys. Rev B79, 165305 (2009).