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Initial and final state effects in the x-ray absorption process of La1-xSrxMnO3
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
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2003 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 23, 235109- p.Article in journal (Refereed) Published
Abstract [en]

We have probed the electronic structure of La1-xSrxMnO3 using x-ray absorption spectroscopy, and compared our results to theory. It is shown that the O projected spectrum reveals a signal that is consistent only with hybridized and delocalized Mn d-states whereas the Mn projected spectrum reveals a signal that can only be reproduced by atomic like theory allowing for multiplet formation between the core hole and the valence electrons of the final state. Our results indicate that the ground state electronic structure has delocalized Mn d-states. This is discussed to be caused by a dynamical screening of the core hole created in the absorption process.

Place, publisher, year, edition, pages
2003. Vol. 68, no 23, 235109- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-91422DOI: 10.1103/PhysRevB.68.235109OAI: oai:DiVA.org:uu-91422DiVA: diva2:164143
Available from: 2004-02-25 Created: 2004-02-25 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Transition Properties of f-electrons in Rare-earth Optical Materials
Open this publication in new window or tab >>Transition Properties of f-electrons in Rare-earth Optical Materials
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main purpose of this thesis is to theoretically study energy levels and intra-electronic transition intensities for various f-electron systems. The f-f electronic dipole transitions are parity-forbidden for a free ion but become non-zero when the ion is subject to a crystal-field. This is commonly described within the framework of Judd-Ofelt theory which accounts for the mixing of odd parity into the wave-functions.

Some refinements and quantitative studies have been made by applying many-body perturbation theory, or the perturbed functions approach, to obtain effective dipole operators due to correlation, spin-orbit and higher order crystal-field effects not included in Judd-Ofelt theory. A software for the computation of f-electron multiplets and Stark levels was implemented and published as well.

The single- and pair-functions used for the evaluation of intensity parameters were obtained by solving various inhomogeneous Schrödinger equations. The wave-functions and energies obtained by diagonalizing an effective Hamiltonian have been used together with the oscillator strength methods to simulate absorption spectrum. Consistent crystal-field parameters applied in some of the papers were obtained by fitting crystal polarizabilities to reflect the experimental Stark levels. The same crystal model was then used to generate odd crystal field parameters needed for the f-f transition intensities. The total effect of these refinements are spectral features that usually agree well with experimental findings. Some of these methods have also been applied and seen to be quite useful for the understanding of optical fiber amplifiers frequently used in today's optical networks.

Finally, a finite-difference approach was applied for the Helium iso-electronic sequence. The exact wave-function was expanded in a sum of partial waves, and accurate ground- and excited state energies were obtained by using the iterative Arnoldi approach.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2004. 50 p.
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 944
Keyword
Physics, rare-earth ions, oscillator strengths, correlation, crystal field, f-electrons, Fysik
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-4014 (URN)91-554-5890-4 (ISBN)
Public defence
2004-03-19, Siegbahnsalen, Ångström laboratory, Lägerhyddsvägen 1, Polacksbacken, Uppsala, 14:00
Opponent
Supervisors
Available from: 2004-02-25 Created: 2004-02-25 Last updated: 2012-04-04Bibliographically approved
2. Theory of X-ray Absorption Spectra and Spin Transfer Torque
Open this publication in new window or tab >>Theory of X-ray Absorption Spectra and Spin Transfer Torque
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The subjects of the thesis are theoretical first principles calculations of X-ray absorption (XA) spectra and current induced spin transfer torque. XA spectra calculated from atomic multiplet theory and from band structure calculations, based on density functional theory for La0.7Sr0.3MnO3 have been compared to experiment. The comparison shows that the effect of the core hole created in the XA process must be considered in the calculation. The theory by Mahan, Nozières and De Dominicis (MND) of dynamical core hole screening is generalised to multiband systems and implemented in first principle calculations. Calculations of the XA spectrum of graphite, including dynamical core hole screening, are shown to better reproduce the relative intensity of the peaks in the experimental spectrum compared to static calculations based on the local density of state of a core excited atom. In combination with experiments the developed method to calculate XA spectra is used to investigate the electronic structure of mixed valent Yb, hydrogen storage in carbon nanotubes and the structure of liquid water. Moreover, a method to calculate the current induced spin transfer torque in materials with a helical spin density wave from first principles has been developed. The method is applied to rare earth metals and it is shown that a current along the axis of spin rotation induces a torque which gives rise to a rotation of the magnetisation direction.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2006. viii + 76 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 158
Keyword
Physics, First principles calculations, X-ray absorption spectroscopy, Carbon nanotubes, Helical spin density waves, Hydrogen storage, Spin transfer torque, Fysik
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-6685 (URN)91-554-6504-8 (ISBN)
Public defence
2006-04-28, Room 4, University main building, Övre Slottsgatan, Uppsala, 10:15
Opponent
Supervisors
Available from: 2006-04-04 Created: 2006-04-04 Last updated: 2012-04-03Bibliographically approved

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Karis, OlofSanyal, BiplabSvedlindh, PeterEriksson, Olle

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