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

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

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

Stabilizing CrO by epitaxial growth

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: Summar

Systematics in band-gaps and optical-spectra of 3d transition-metal compounds

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Systematics in band-gaps and optical-spectra of 3d transition-metal compounds

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe