量子物性分光理論研究室 (Krüger Group)

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X-ray absorption spectra of transition metal L-edges calculated with multichannel multiple scattering theory.

Ti-L23-XAS X-ray absorption spectroscopy at the transition metal L23-edges is a powerful tool for element specific information about local coordination, bonding and magnetism. Due to strong core-hole effects, especially multiplet couplings, one-electron schemes such as density-functional theory fail to reproduce the spectra and atomic model calculations are generally used where all extra-atomic effects are reduced to an empirical ligand field. We have developed a multichannel multiple scattering theory for L23-edge absorption which features the precise electronic structure of the material through density functional theory, but also includes the major final state correlation effet, namely particle-hole multiplet coupling. The L23-edge of spectra of TiO2 was reproduced correctly for the first time. The "D-E" level splitting, fingerprint of the different crystal phases is shown to reflect the stacking of the TiO6 octahedra on a length scale of 1 nm. read article

Electronic structure and core-level spectra of magnetic films. Metamagnetic transition in FeRh.

FeRh The FeRh ordered alloy switches from the antiferromagnetic state at low temperature to the ferromagnetic state above 360 K, which makes it an interesting material for spintronic applications. We have studied the electronic structure changes across this "meta-magnetic" transition of FeRh films using hard x-ray photoemission spectroscopy and first-principles calculations. A significant enhancement of the electronic density of states at the Fermi level are observed for the ferromagnetic state, implying that the electronic entropy partially drives the magnetic transition. read article

New computational methods for angle-resolved photoemission. Transition metal surfaces.

Cu ARPES A novel real-space multiple scattering computational method for ultra-violet angle resolved photoemssion and photoelectron diffraction based on real-space multiple scattering has been developed. The interplay between initial and final state effects in ultra-violet photoemission spectroscopy has been studied for the Cu-3d band from Cu(111) surfaces. read article
A first principles method has been developed for angle- and spin-resolved resonant photoemission. The link between spin-resolution and circular dichroism in transition metal resonant photoemission at the 2p-3d resonance was studied. We show that the strong spin-polarization of the photocurrent observed in certain of antiferromagnetic systems is not a measure of the local magnetic moments but a consequence of the angular momentum transfer from the light to the photoelectron spin through core-level spin-orbit coupling and exchange-scattering (Auger decay). read article

Defect electronic states in wide-gap semiconductors probed by resonant photoelectron diffraction

TiO2 defect charge Oxide semi-conductors such as TiO2 have various applications for opto-electronics, solar energy, catalysis etc. The band gap states play a major role for transport and optical properties. We have investigated the defect states at the n-doped TiO2(110) surface using resonant photoelectron diffraction, that is the photoelectron diffraction from the valence defect state at the Ti-2p-3d resonance. Thereby the charge distribution of the defect state could be revealed. By comparing O-vacancy doped and chemically (Na-adsorption) doped surfaces, it is shown that the defect charge distribution is essentially an intrinsic property of the Ti(110) surface. read article

Organic thin films: Adsorption and decomposition of single layer Mo(CO)6/Cu

MoCO6/Cu(111)Molybdenum carbonyl Mo(CO)6 was adsorbed on the Cu(111) surface at 160 K in the monolayer coverage range and studied by scanning tunneling microscopy. A well-ordered monolayer of hexacarbonyl molecules was observed experimentally for the first time. The monolayer has a hexagonal structure compatible with a (√7 × √7)R19 superlattice on the copper (111) plane. The arrangement and orientation of the molecules on the surface were determined by density functional theory calculations, including van der Waals interactions. The comparison of adsorption and cohesive energies reveals that the molecule−substrate interaction is stronger than the intermolecular one, which explains the observed two-dimensional growth. read article

Vibrational properties of copper oxides

copper oxide modes Copper oxides are used in catalysis and are the basis of most high-temperature superconductor materials. Apart from the abundant tenorite CuO and fcc Cu2O phases, more exotic phases may be stabilized in nanostructures, among them the paramelaconite Cu4O3 phase. Knowledge of the vibrational modes is a prerequiste for the understanding of thermodynamics and electron-phonon coupling. Using Raman spectroscopy and first principles calculations we have studied the phonon modes of CuO, Cu2O2 and Cu4O3. Through a detailed analysis of the displacement eigenvectors, it is shown that a close relationship exists between the Raman modes of CuO and Cu4O3. read article

Density fluctuations in liquids at the nanoscale

Kirkwood-Buff Scheme Various thermodynamics quantities are directly linked to the density fluctuations <n(r) n(r')>-<n(r)><n(r')> and can be computed as volume integrals over the pair-correlation function <g(r-r')>, so-called Kirkwood-Buff integrals. In the traditional approach these integrals converge very badly as a function of the upper integration bound. We have derived exact expressions for Kirkwood-Buff integrals over finite (hyper-spherical) volumes. It is shown that they scale as 1/R with system size and converge much better than the usual cut-off integrals. read article