Local spin-flip spectral distribution obtained by resonant x-ray Raman scattering

We show that resonant x-ray Raman (RXR) scattering can be used to study the local spin-flip excitation spectral distribution in magnetically ordered 3d transition-metal compounds. We demonstrate with realistic calculations on the 3p x-ray absorption edge of a Cu^2+ compound and the 2p edge of a Ni^2+ compound that the scattered x-ray energy and intensity distribution contains the excitation spectrum resulting from a single local spin flip, as well as dd excitations accompanied by local spin flip satellites. We develop the theory describing this effect and discuss the polarization conditions that can be used to observe these effects

Orbital occupation, local spin and exchange interactions in V2O3

We present the results of an LDA and LDA+U band structure study of the monoclinic and the corundum phases of V2O3 and argue that the most prominent (spin 1/2) models used to describe the semiconductor metal transition are not valid. Contrary to the generally accepted assumptions we find that the large on site Coulomb and exchange interactions result in a total local spin of 1 rather than 1/2 and especially an orbital occupation which removes the orbital degeneracies and the freedom for orbital ordering. The calculated exchange interaction parameters lead to a magnetic structure consistent with experiment again without the need of orbital ordering. While the low-temperature monoclinic distortion of the corundum crystal structure produces a very small effect on electronic structure of v2o3, the change of magnetic order leads to drastic differences in band widths and band gaps. The low temperature monoclinic phase clearly favors the experimentally observed magnetic structure, but calculations for corundum crystal structure gave two consistent sets of exchange interaction parameters with nearly degenerate total energies suggesting a kind of frustration in the paramagnetic phase. These results strongly suggest that the phase transitions in V2O3 which is so often quoted as the example of a S=1/2 Mott Hubbard system have a different origin. So back to the drawing board

In situ XPS analysis of various iron oxide films grown by NO2-assisted molecular-beam epitaxy

We report on a systematic analysis of x-ray photoelectron spectroscopy (XPS) core- and valence-level spectra of clean and well-characterized iron oxide films, i.e., a-Fe2O3, y-Fe2O3, Fe3- dO4, and Fe3O4. All iron oxide films were prepared epitaxially by NO2-assisted molecular-beam epitaxy on single crystalline MgO(100) and a-Al2O3(0001) substrates. The phase and stoichiometry of the films were controlled precisely by adjusting the NO2 pressure during growth. The XPS spectrum of each oxide clearly showed satellite structures. These satellite structures were simulated using a cluster-model calculation, which could well reproduce the observed structures by considering the systematic changes in both the Fe 3d to O 2p hybridization and the d-d electron-correlation energy. The small difference in the satellite structures between a-Fe2O3 and y-Fe2O3 resulted mainly from changes in the Fe-O hybridization parameters, suggesting an increased covalency in g-Fe2O3 compared to a-Fe2O3. With increasing reduction in the y-Fe2O3-Fe3O4 system, the satellite structures in XPS became unresolved. This was not only due to the formation of Fe21 ions, but also to nonhomogeneous changes in the hybridization parameters between octahedral and tetrahedral Fe^3+ ions

Density-functional study of the evolution of the electronic structure of oligomers of thiophene:Towards a model Hamiltonian

We present density-functional and time-dependent density-functional studies of the ground, ionic, and excited states of a series of oligomers of thiophene. We show that, for the physical properties, the most relevant highest occupied and lowest unoccupied molecular orbitals develop gradually from monomer molecular orbitals into occupied and unoccupied broad bands in the large length limit. We show that band gap and ionization potentials decrease with size, as found experimentally and from empirical calculations. This gives credence to a simple tight-binding model Hamiltonian approach to these systems. We demonstrate that the length dependence of the experimental excitation spectra for both singlet and triplet excitations can be very well explained with an extended Hubbard-like Hamiltonian, with a monomer on-site Coulomb and exchange interaction and a nearest-neighbor Coulomb interaction. We also study the ground and excited-state electronic structures as functions of the torsion angle between the units in a dimer, and find almost equal stabilities for the transoid and cisoid isomers, with a transition energy barrier for isomerization of only 4.3 kcal/mol. Fluctuations in the torsion angle turn out to be very low in energy, and therefore of great importance in describing even the room-temperature properties. At a torsion angle of 90° the hopping integral is switched off for the highest occupied molecular orbital levels because of symmetry, allowing a first-principles estimate of the on-site interaction minus the next-neighbor Coulomb interaction as it enters in a Hubbard-like model Hamiltonian

Of spin and charge in the cuprates

Centraal in dit proefschrift staan de elektlonische eigenschappen van de cupraten, waarbij we zowel spinexcitaties als ladingsexcitaties bestudcrcn. De cupraten zijn voornamelijk bekend vanwege hun metallische fase met buitengewoon hoge ovelgangstemperaturen naar de supergeleinde toestand, maar de magnetische eigensxappen van de isolerende fase zijn niet minder interessant. Onze voorrraamste interesse is het verband tussen magnetisme en supergeleiding. Eerst geven we een uitgebreid overzicht van de stand van zaken in de wetenschappelijke discussie over de cupraten. Dit overzicht vormt de achtergrond waarin we onze eigen resultaten plaatsen. ... Zie: Samenvatting

Local Electronic and Magnetic Structure of Ni below and above TC: A Spin-Resolved Circularly Polarized Resonant Photoemission Study

We report the measurement of the local Ni 3d spin polarization, not only below but also above the Curie temperature (TC), using the newly developed spin-resolved circularly polarized 2p (L3) resonant photoemission technique. The experiment identifies the presence of 3d8 singlets at high energies and 3d8 triplets at low energies extending all the way to the Fermi energy, both below and above TC, showing that it is the orbital degeneracy of the 3d band and the Hund's rule splitting which is of utmost importance to understand Ni and other 3d ferromagnets