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Doping induced site-selective Mott insulating phase in LaFeO3
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.ORCID iD: 0000-0003-0351-3138
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Tailoring transport properties of strongly correlated electron systems in a controlled fashion counts among the dreams of materials scientists. In copper oxides, vary- ing the carrier concentration is a tool to obtain high- temperature superconducting phases. In manganites, dop- ing results in exotic physics such as insulator-metal tran- sitions (IMT), colossal magnetoresistance (CMR), orbital- or charge-ordered (CO) or charge-disproportionate (CD) states. In most oxides, antiferromagnetic order and CD phase is asssociated with insulating behavior. Here we re- port the realization of a unique physical state that can be induced by Mo doping in LaFeO3: the resulting metallic state is a site-selective Mott insulator where itinerant elec- trons evolving on low-energy Mo states coexist with local- ized carriers on the Fe sites. In addition, a local breathing- type lattice distortion induces charge disproportionation on the latter, without destroying the antiferromagnetic order. A state, combining antiferromangetism, metallic- ity and CD phenomena is rather rare in oxides and have utmost significance for future antiferromagnetic memory devices.

National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-364365OAI: oai:DiVA.org:uu-364365DiVA, id: diva2:1258784
Funder
Swedish Research Council, 2014-6019Swedish Energy Agency, P43549-1Knut and Alice Wallenberg Foundation, 2012.0031Available from: 2018-10-25 Created: 2018-10-25 Last updated: 2018-11-01
In thesis
1. Theoretical and Computational Studies of Strongly Correlated Electron Systems: Dynamical Mean Field Theory, X-ray Absorption Spectroscopy and Analytical Continuation
Open this publication in new window or tab >>Theoretical and Computational Studies of Strongly Correlated Electron Systems: Dynamical Mean Field Theory, X-ray Absorption Spectroscopy and Analytical Continuation
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis encompasses theoretical and computational studies of strongly correlated elec-tron systems. Understanding how electrons in solids interact with each other is of great im-portance for future technology and other applications. From a fundamental point of view, the Coulomb interaction in a solid leads to a very challenging many-body problem, encapsulating many physical phenomena, e.g. magnetism. Treating this interaction, with a focus on local contributions, is the subject of this thesis. Both models and materials have been investigated, to obtain insight on the mechanisms determining the macroscopic properties of matter. This thesis is divided in four parts, each corresponding to a different project or topic.

In the first project a many body method called dynamical mean field theory (DMFT) is used to study the paramagnetic phase of the Hubbard model. A stochastic version of the exact di-agonalization technique is developed for solving the effective impurity model arising in DMFT and generating real frequency spectral functions. In the next project, by combining density functional theory (DFT) with a static solution of the DMFT equations (DFT+U), magnetic ex-change interactions in transition metal oxides (TMOs) are investigated. The spin dependence of the functional is shown to be important for mapping magnetic excitations form the quantum mechanical system to a classical model.

The next topic in this thesis concerns the x-ray absorption spectroscopy of TMOs. Spectral functions, in good agreement with experimental data, are calculated by combining DFT with multiplet ligand field theory (MLFT). The effects of the presence of a core-hole are studied in detail for NiO, as well as double counting issues related to higher order terms of the multiple ex-pansion of the Coulomb interaction. A strained induced linearly polarized spectrum is obtained for CaTiO3. Lastly, charge disproportionation is seen in Mo doped LaFeO3.

Finally, a critical step in DMFT, called analytical continuation, to obtain physical observ-ables of interest is investigated. Analytical continuation means a transformation of a function in the complex plane. Several methods for performing this transformation are explained, and in particular steps for improving the robustness and accuracy of the Padé approximant method are described.

Place, publisher, year, edition, pages
Uppala: Acta Universitatis Upsaliensis, 2018. p. 112
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1729
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-362834 (URN)978-91-513-0471-7 (ISBN)
Public defence
2018-11-30, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Note

the opponents university is University of Bremen

Available from: 2018-11-09 Created: 2018-10-10 Last updated: 2018-11-19
2. An X-ray Spectroscopic Study of Perovskites Oxides and Halides for Emerging Devices
Open this publication in new window or tab >>An X-ray Spectroscopic Study of Perovskites Oxides and Halides for Emerging Devices
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis investigates the electronic structures on several perovskite oxide and halide materials with a focus on light harvesting applications. The systematic study of the electronic properties of the transition metal oxides and post-transition metal halides is a key point if one is to understand their properties. The element and site selective nature of several x-ray based spectroscopic techniques are given special emphasis in order to obtain a complete picture of the electronic properties of the compounds in question. Much of the experimental studies are accompanied by ab initio calculations that corroborate with our experimental results.

In the oxide portion of this work, a new class of metallic oxides based on doping of an antiferromagnetic LaFeO3 was synthesized and systematically studied with x-ray absorption, x-ray emission, and photoemission spectroscopies. The compound’s electronic structure is complex, having itinerant as well as localized components that give rise to a unique physical state where antiferromagnetism, metallicity and charge-disproportionation coexist. Our resonant photoemission results establish that the Fe states in both magnetically ordered oxides show insulting properties, while the Mo states provide an itinerant band crossing the Fermi level. An excitation energy-dependent RIXS investigation on LaFe1-xMoxO3 and the double perovskite Sr2FeMoO6 revealed a double peak structure located in proximity to the elastic peak that is identified to purely d-d excitations, attributed to the strongly correlated nature of these transition metal compounds.

The growth of high-quality thin film ferroelectric based on BaTiO3 grown epitaxially by means of pulsed laser deposition were investigated. We systematically reduce the band gap of the ferroelectric thin film while retaining its polarization at ambient conditions in spite of the aliovalent doping. The electronic structure is studied by several x-ray techniques that show how the ferroelectricity persists as well as the effective reduction of the band gap through hybridized states.

In the post-transition metal halides, the valence and conduction bands were mapped using x-ray absorption, emission, and photoemission spectroscopies. The spectroscopic results identify the constituent states that form the valence band as well as the band energy positions, which is an imperative parameter in optoelectronic devices. In addition, x-ray based spectroscopy was used to demonstrate the stereochemical activity of lone-pair states (5s2 and 6s2) for several different halide compounds and their influence on the chemical, structural, and electronic properties of the material. Nanostructured halide perovskites are also explored. The position of iodine p states and valence band states in reduced dimensional lead-based compounds were examined, as their states are found to be confined in one crystallographic direction in contrast to their three-dimensional counterpart. This information highlights the interesting material properties and their use in current third generation solar cell research.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 84
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1740
Keywords
perovskite oxides, halide perovskites, x-ray spectroscopy, electronic structure
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-364407 (URN)978-91-513-0493-9 (ISBN)
Public defence
2018-12-14, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2018-11-21 Created: 2018-10-26 Last updated: 2018-11-30

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Jana, SomnathPhuyal, DibyaKumar, AnkitHedlund, DanielSchött, JohanThunström, PatrikKvashnin, YaroslavRensmo, HåkanKamalakar, M. VenkataSvedlindh, PeterGunnarsson, KlasEriksson, OlleKaris, Olof

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Jana, SomnathPhuyal, DibyaKumar, AnkitHedlund, DanielSchött, JohanThunström, PatrikKvashnin, YaroslavRensmo, HåkanKamalakar, M. VenkataSvedlindh, PeterGunnarsson, KlasEriksson, OlleKaris, Olof
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