Experimentelle Physik VII


    Electron spectroscopy in the laboratory and at synchrotron radiation facilities, shedding light on...

    • electronic structure of solid state systems, surfaces and thin films
    • multi-particle-physics: Kondo-systems, heavy fermions, superconductors, quantum phase transitions
    • strongly correlated electronic systems, topological insulators
    • reactions on surfaces, interactions at interfaces, organic electronics, nanoanalytics
    • basic research on condensed matter physics

    Some examples of current research activities are given below.

    Besides other surface characterizing techniques (LEED, STM, AFM, NEXAFS, RIXS), the most important analytical method of the EP7 is photoemission spectroscopy (PES). PES has a broad range of applications in chemistry and condensed matter physics, in particular in surface science and solid state physics.
    Depending on the used photon source, one gets e.g. information about the chemical composition of the sample (x-rays: XPS or ESCA, electron spectroscopy for chemical analysis) or the valence band structure of a single crystal (ultraviolet: UPS). The group focuses on fundamental research using angle resolved experiments (ARPES) with high energy resolution.

    An overview can be found in New J. Phys 7 (2005) 97.

    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

    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., Hybridization of Organic Molecular Orbitals with Substrate States at Interfaces: PTCDA on Silver.  Phys. Rev. Lett. 104, 2010.

    M. Wießner et al., Electronic and geometric structure of the PTCDA/Ag(110) interface probed by angle-resolved photoemission. Physical Review B 86, 2012

    Omicron: Result of the month

    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).

    Under certain conditions electronic states can form quasi-two dimensional states, e.g. inside a thin metallic layer (quantum well state) or at the surface of a metal (e.g. Shockley-type surface states.) In particular the latter can be used as sensitive probes for the modification of surface topology or for the adsorption of atoms and molecules, due to their presence at the surface. On the other hand, these states can play an active role for adsorption or cataclytical properties of a surface. As an example for such a surface state, the figure shows a complete ARUPS data set (He I) of the Shockley state of Cu(111) covered with one atomic layer of Ag-layer. This figure displays the band dispersion vs. the parallel momentum as cuts through the two dimensional k-space.

    Forster et al., Phys. Rev. B 84, 075412 (2011).

    The broken inversion symmetry at crystal surfaces and interfaces allows for a lifting of the spin-degeneracy and the occurence of spin-polarized states in the electronic structure of two-dimensional systems through the spin-orbit interaction (Rashba-effect). This effect is discussed in the context of electronic applications (e.g. new types of transistors) which make use of the electron spin in addition to the charge, but it is also of high interest for fundamental questions concerning the role of the spin in condensed matter systems. In our group we investigate the Rashba-effect in surface and thin film systems (such as Bi/Cu(111) and Bi/Ag(111)) using ARPES and spin-resolved PES in order to explore its microscopic origin as well as its dependence on geometrical (film thickness, lattice constant, etc.) and chemical (adatoms, adsorbate-substrate combination, etc.) sample parameters.

    H. Bentmann et al., Europhys. Lett. 87, 37003 (2009).
    H. Bentmann et al., Phys. Rev. Lett. 108, 196801 (2012).

    A particular interesting type of two-dimensional electron system are the surface states of topological insulators which, similar to the cases mentioned above, are spin-polarized as a result of the Rashba-effect. These surface states occur within the global energy gap of the bulk insulator and thus open metallic conduction channels in the near-surface region. Unlike conventional surface states, such as e.g. dangling bonds on semiconductor surfaces, topological surface states (TSS) are expected to particularly robust against defects or adatoms as their existence is a consequence of the bulk band topology. We use ARPES, spin-resolved PES, synchrotron-based core level spectroscopy as well as STM to study the influence of adsorbate atoms on TSS and the interface formation between topological insulators, such as Bi2Se3 or Sb2Te3, and metallic overlayers. The figure on the right shows an ARPES spectrum of Bi2Se3(0001) revealing a TSS bridging the energy gap between the conduction band minimum (CBM) and the valence band maximum (VBM) of the bulk.

    C. Seibel et al., Phys. Rev. B 86, 161105(R) (2012).