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Contains fulltext : 75264.pdf (Author’s version preprint ) (Open Access) 6 p.

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates., Comment: 4 pages, 3 figures

This thesis describes the efforts to stabilize chromium monoxide (CrO) as a thin epitaxial film on various substrates. The most stable and common oxidation phase for chromium is in chromium sesquioxide (Cr2O3). CrO does not exist as a bulk material in nature, therefore we have to grow it in a artificial manner as a single crystal film using molecular beam epitaxy (MBE). The motivation behind this project is that in CrO, Cr ion has the same electronic configuration as the manganese ion in the lanthanum manganates (LaMnO3), a material that doped with strontium or calcium (La1-xSrxMnO3) is the so-called colossal magneto resistance material. One may expect that CrO has equally fascinating magnetic and electrical properties as the manganates. ... Zie: Summary

We study the one hole spectral function in a model for LaMnO3 including both the effects of orbital ordering and the quantum decoherence due to the antiferromagnetic coupling between the ferromagnetic layers. We find that the classical picture of a ferromagnetic polaron does not apply and free dispersion is replaced by rigid quasiparticles on the scale of magnetic excitations, while the spectra are dominated by the incoherent spectral weight at high energies. These results have important implications on the in-plane transport and angular resolved photoemission in the manganites.

We report high-resolution x-ray-absorption near-edge structure measurements at the Mn K edge as a function of temperature, for La1−xCaxMnO3 samples, with a focus mainly on the pre-edge region. Small peaks labeled A1–A3 are observed which correspond to 1s−3d dipole transitions, made weakly allowed via a hybridization of Mn 4p states with Mn 3d states on neighboring atoms. Adjusting the parameters in a local spin-density approximation calculation to approximately match the experimental A1−A2 splitting yields U=4 eV and JH=0.7 eV. For colossal magnetoresistance samples, A1 decreases with T while A2 increases with T below Tc, which shows that the 3d bands change significantly as T moves through Tc. There are also small changes in the shape of the main edge (1s−4p transitions).

We have explored spin, charge and orbitally ordered states in La1-xSrxMnO3 (0 < x < 1/2) using model Hartree-Fock calculations on d-p-type lattice models. At x=1/8, several charge and orbitally modulated states are found to be stable and almost degenerate in energy with a homogeneous ferromagnetic state. The present calculation indicates that a ferromagnetic state with a charge modulation along the c-axis which is consistent with the experiment by Yamada et al. might be responsible for the anomalous behavior around x = 1/8., 5 pages, 5 figures

We have studied the interplay between orbital ordering, Jahn-Teller and GdFeO3-type lattice distortions in perovskite-type transition-metal oxides using model Hartree-Fock calculations. It has been found that the covalency between A-site cations and oxygens causes interaction between the Jahn-Teller and GdFeO3-type distortions. The present calculations explain why the d-type Jahn-Teller distortion and orbital ordering compatible with it are realized in LaMnO3, YVO3 and YTiO3, Comment: 5 pages, 8 figures

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.

A local-density-approximation (LDA) and LDA + U band structure study of various phases of V 2 O 3 shows that the most prominent (spin- 1 2 ) models used to describe the semiconductor-metal transition are not valid. We find that the large on-site Coulomb and exchange interactions result in a total spin of 1 rather than 1 2 and especially an orbital occupation which largely removes the orbital degeneracy. The calculated magnetic structure is consistent with experiment, again without the need for orbital ordering. These results strongly suggest that the phase transition in V 2 O 3 which is so often quoted as the example of a spin- 1 2 Mott-Hubbard system have a different origin.

We have perfrmed second-harmonic generation (SHG) and sum-frequency generation (SFG) measurements on C60,NiO, and CuCl, in which Frenkel, charge-transfer, and Wannier excitons were probed, respectively. From these experiments we learn that the excitons can be clearly observed and no contribution from the electron-hole continuum can be found in our spectra. This is very different from spectra obtained by SPA, TPA and EELS. In these experiments, excitons are also observed, but are often overshadowed by the strong contributions from the optically allowed transitions which form the electron-hole continuum. From our measurements follows that SHG and SFG can be used to discriminate exciton absorptions from those of free electron-hole pairs. ... Zie: Conclusions and discussion.

K-doped C60 monolayers on polycrystalline Cu, Ag, and Au are studied by means of ultraviolet photoelectron spectroscopy. The polycrystalline character of the substrate allows an accurate subtraction procedure to isolate the photoemission signal of the monolayer. Charge transfers of 1.0–1.8 electrons per molecule are found for the undoped monolayer. On high K doping the hybridization with the substrate is shown to practically disappear, and the overlayer almost totally suppresses the substrate signal.

We report the measurement of the local Ni 3d spin polarization, not only below but also above the Curie temperature (T-C), using the newly developed spin-resolved circularly polarized 2p (L-3) resonant photoemission technique. The experiment identifies the presence of 3d(8) singlets at high energies and 3d(8) triplets at low energies extending all the way to the Fermi energy, both below and above T-C, 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.

Using a combination of photoelectron and inverse photoelectron spectroscopy, we show that the band gap in a monolayer of C-60 on a Ag surface is strongly reduced from the solid C-60 surface value. We argue that this is a result of the reduction of the on-site molecular Coulomb interaction due to the influence of image charges in the metal substrate. This result suggests that the physical properties of correlated insulators and semiconductors will be strongly modified if prepared in ultra thin form on metal substrates or sandwiched between metal layers.

We present temperature-dependent V-2p and O-1s x-ray-absorption spectra of LiVO2. The aim of this study is to monitor changes in electronic structure on going through the phase transition. The spectral changes turn out to be very small: the V-3d-O-2p hybridization does not change considerably, and the symmetry of the V-ion ground state (high-spin T-3(1)) is retained. To explain our results, together with the anomalously low magnetic susceptibility below the transition temperature, we propose a model in which a three-orbital sublattice is formed in the V (111) planes, which results in a total singlet state.