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Theoretical studies of lattice- and spin-polarons
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Theoretical studies of lattice- and spin-polarons are presented in this thesis, where the primary tool is ab-initio electronic structure calculations. The studies are performed with employment of a variety of analytical and computational methods. For lattice-polarons, we present an analytical study where multipolaron solutions were found in the framework of the Holstein 1D molecular crystal model. Interestingly, we found a new periodic, dnoidal, solution for the multipolaron system. In addition to it, we examined the stability of multipolaron solutions, and it was found that cnoidal and dnoidal solutions stabilize in different ranges of the parameter space. Moreover, we add to the model nonlocal effects and described dynamics in terms of internal solitonic modes.

Hole-polaron localization accompanying the formation of a cation vacancy in bulk MgO and CaO and at the (100) MgO/CaO interfaces is presented. We show that the ground state is found to be the O1-O1 bipolaronic configuration both in bulk oxides and at their interfaces. Moreover, the one-centered O2-O0 bipolaron was found to be metastable with its stability being enhanced at the interfaces compared to that in bulk oxides. Also, for several bipolaronic configurations, we analyzed possible transitions from O1-O1 to O2-O0. On the same line of reasoning, electron localization and polaron mobility in oxygen-deficient and Li-doped monoclinic tungsten trioxide has been studied. It is shown for WO3, that small polarons formed in the presence of oxygen vacancy prefer bipolaronic W5+-W5+ configuration rather than W6+-W4+ configuration, which is found to be metastable state. Also, it is demonstrated that the bipolarons are tightly bound to vacancies, and consequently exhibit low mobility in the crystal. On the other hand, we show that polarons formed as a result of Li intercalation are mobile and that they are being responsible for electrochromic properties discovered in the compound.

Spin-polaron formation in La-doped CaMnO3, with G-type antiferromagnetic structure, was also studied. We found that for this material, spin-polarons are stabilized due to the interplay of magnetic and lattice-effects at lower La concentrations and mostly due to the lattice contribution at larger concentrations. We show that the formation of SP is unfavourable in the C- and A-type antiferromagnetic phase, in agreement with previously reported experimental studies. We have also studied dynamical and temperature dependent properties of spin-polarons in this compound. We estimated material specific exchange parameters from density functional theory and found that 3D magnetic polarons in the Heisenberg lattice stabilize at slightly higher temperatures than in the case of 2D magnetic polarons. Next, we have proposed a method to calculate magnetic polaron hopping barriers and studied spin-polaron mobility CaMnO3 using additional methods such as atomistic spin dynamics and kinetic Monte Carlo. We make a suggestion of using this system in nano-technological applications.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. , p. 103
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1631
Keywords [en]
Polaron, Nonlinear Schrödinger Equation, Nonlocality, Solitons, Integrable systems, Quantum field theory (low energy), Electron-phonon interaction, Density functional theory, Electronic structure of atoms and molecules, Spin-polaron, Langevin equation, Transport properties, Hubbard model, Heisenberg lattice
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-340947ISBN: 978-91-513-0235-5 (print)OAI: oai:DiVA.org:uu-340947DiVA, id: diva2:1180455
Public defence
2018-03-27, Häggsalen, Ångströmlaboratoriet, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2018-03-06 Created: 2018-02-05 Last updated: 2018-05-08
List of papers
1. Multi-polaron solutions, nonlocal effects and internal modes in a nonlinear chain
Open this publication in new window or tab >>Multi-polaron solutions, nonlocal effects and internal modes in a nonlinear chain
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Multipolaron solutions were studied in the framework of the Holstein one-dimensional molecular crystal model. The study was performed in the continuous limit where the crystal model maps into the nonlinear Schrödinger equation for which a new periodic dnoidal solution was found for the multipolaron system. In addition, the stability of the multi-polaron solutions was examined, and it was found that  cnoidal and dnoidal solutions stabilize in different ranges of the parameter space. Moreover, the model was studied under the influence of nonlocal effects and the polaronic dynamics was described in terms of internal solitonic modes.

National Category
Natural Sciences
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-340881 (URN)
Funder
Swedish Research Council, 2014-5993
Available from: 2018-02-05 Created: 2018-02-05 Last updated: 2018-05-28Bibliographically approved
2. Hole bipolaron formation at (100) MgO/CaO epitaxial interface
Open this publication in new window or tab >>Hole bipolaron formation at (100) MgO/CaO epitaxial interface
2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 12, p. 125118-Article in journal (Refereed) Published
Abstract [en]

Hole localization accompanying the formation of a cation vacancy in bulk MgO and CaO and at the (100) MgO/CaO interface is described using the Heyd-Scuseria-Ernzerhof hybrid functionals and DFT + U method. The ground state is found to be the O-1-O-1 bipolaronic configuration both in bulk oxides and at their interfaces. The ground-state magnetic configuration is a triplet, which has an energy only about 1-2 meV lower than that of the singlet state. The one-centered O-2-O-0 bipolaron was found to be metastable with its stability being enhanced at the interfaces compared to that in bulk oxides. Possible transition between different biplaronic configurations at the interface are analyzed for chosen configurations.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-223890 (URN)10.1103/PhysRevB.89.125118 (DOI)000333013900002 ()
Available from: 2014-05-05 Created: 2014-04-28 Last updated: 2018-02-05Bibliographically approved
3. Polaron mobility in oxygen-deficient and lithium-doped tungsten trioxide
Open this publication in new window or tab >>Polaron mobility in oxygen-deficient and lithium-doped tungsten trioxide
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 16, article id 165119Article in journal (Refereed) Published
Abstract [en]

Electron localization and polaron mobility in oxygen-deficient as well as Li-doped monoclinic tungsten trioxide have been studied in the adiabatic limit in the framework of density functional theory. We show that small polarons formed in the presence of oxygen vacancy prefer the bipolaronicW(5+)-W5+ configuration, whereas the W6+-W4+ configuration is found to be metastable. Our calculations suggest that bipolarons are tightly bound by the vacancy and therefore largely immobile. On the contrary, polarons formed as a result of Li intercalation can be mobile; the activation energy for polaron jumping in this case varies between 98 and 124 meV depending on the crystallographic direction. The formation of W5+-W5+ bipolarons in Li-WO3 is possible. When situated along [001] the bipolaronic configuration is 8 meV lower in energy than two separate W5+ polarons.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-267211 (URN)10.1103/PhysRevB.92.165119 (DOI)000363013600002 ()
Funder
eSSENCE - An eScience CollaborationStandUpSwedish Research CouncilSwedish National Infrastructure for Computing (SNIC)National Supercomputer Centre (NSC), Sweden
Available from: 2015-11-19 Created: 2015-11-19 Last updated: 2018-02-05Bibliographically approved
4. Spin-polaron formation and magnetic state diagram in La-doped CaMnO3
Open this publication in new window or tab >>Spin-polaron formation and magnetic state diagram in La-doped CaMnO3
Show others...
2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 22, article id 220401Article in journal (Refereed) Published
Abstract [en]

LaxCa1-xMnO3 (LCMO) has been studied in the framework of density functional theory (DFT) using Hubbard-U correction. We show that the formation of spin polarons of different configurations is possible in the G-type antiferromagnetic phase. We also show that the spin-polaron (SP) solutions are stabilized due to an interplay of magnetic and lattice effects at lower La concentrations and mostly due to the lattice contribution at larger concentrations. Our results indicate that the development of SPs is unfavorable in the C- and A-type antiferromagnetic phases. The theoretically obtained magnetic state diagram is in good agreement with previously reported experimental results.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-327218 (URN)10.1103/PhysRevB.95.220401 (DOI)000402799700002 ()
Available from: 2017-08-17 Created: 2017-08-17 Last updated: 2018-02-05Bibliographically approved
5. Static and dynamical properties of spin-polarons in La-doped CaMnO3
Open this publication in new window or tab >>Static and dynamical properties of spin-polarons in La-doped CaMnO3
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The static and dynamic properties of spin-polarons in La-doped CaMnO3 are explored theoretically,

by means of an eective low energy Hamiltonian. All parameters from the Hamiltoniain

are evaluated from rst principles theory, without adjustable parameters. We compare dierent

geometries, like spin-polarons in bulk, surface and as single two-dimensional layers. The low energy

Hamiltonian is used to investigate the temperature stability of the spin-polaron, as well as the

response to an external applied electric eld. Technically this involves atomistic spin-dynamics simulations

in combination with kinetic Monte Carlo simulations. Where a comparison can be made,

our simulations exhibit an excellent agreement with available experimental data and previous theory.

Remarkably, we nd that excellent control of the mobility of spin-polarons in this material can

be achieved, and that the critical parameters deciding this is the temperature and strength of the

applied electrical eld. We outline dierent technological implications of our ndings.

National Category
Natural Sciences
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-340942 (URN)
Available from: 2018-02-05 Created: 2018-02-05 Last updated: 2018-02-05

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