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Electronic structure, magnetism, and exchange integrals in transition-metal oxides: Role of the spin polarization of the functional in DFT+U calculations
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Columbia Univ, Dept Phys, New York, NY USA; Flatiron Inst, Ctr Computat Quantum Phys, New York, NY USA.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 18, article id 184404Article in journal (Refereed) Published
Abstract [en]

Density functional theory augmented with Hubbard-U corrections (DFT+U) is currently one of the most widely used methods for first-principles electronic structure modeling of insulating transition-metal oxides (TMOs). Since U is relatively large compared to bandwidths, the magnetic excitations in TMOs are expected to be well described by a Heisenberg model. However, in practice the calculated exchange parameters J(ij) depend on the magnetic configuration from which they are extracted and on the functional used to compute them. In this work we investigate how the spin polarization dependence of the underlying exchange-correlation functional influences the calculated magnetic exchange constants of TMOs. We perform a systematic study of the predictions of calculations based on the local density approximation plus U (LDA+U) and the local spin density approximation plus U (LSDA+U) for the electronic structures, total energies, and magnetic exchange interactions Jij extracted from ferromagnetic (FM) and antiferromagnetic (AFM) configurations of several transition-metal oxide materials. We report that for realistic choices of Hubbard U and Hund's J parameters, LSDA+U and LDA+U calculations result in different values of the magnetic exchange constants and band gap. The dependence of the band gap on the magnetic configuration is stronger in LDA+U than in LSDA+U and we argue that this is the main reason why the configuration dependence of Jij is found to be systematically more pronounced in LDA+U than in LSDA+U calculations. We report a very good correspondence between the computed total energies and the parametrized Heisenberg model for LDA+U calculations, but not for LSDA+U, suggesting that LDA+U is a more appropriate method for estimating exchange interactions.

Place, publisher, year, edition, pages
2018. Vol. 97, no 18, article id 184404
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-356500DOI: 10.1103/PhysRevB.97.184404ISI: 000431989000004OAI: oai:DiVA.org:uu-356500DiVA, id: diva2:1236026
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationeSSENCE - An eScience CollaborationAvailable from: 2018-07-30 Created: 2018-07-30 Last updated: 2018-10-10Bibliographically approved
In thesis
1. Magnetism in Transition Metal Systems: Interplay between structure, dimensionality and electron correlation
Open this publication in new window or tab >>Magnetism in Transition Metal Systems: Interplay between structure, dimensionality and electron correlation
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, an ab initio study of electronic structures and magnetic properties of transition metal systems has been presented, covering bulk, interface and surface geometries. Among them are Fe, Co, Ni, CaMnO3, Co2MnSi, a ferromagnetic Heusler alloy, as well as double-perovskites oxides such as Sr3(Fe1.25Ni0.75)O6 and Nd2NiMnO6.

Their electronic structures have been obtained within the framework of the density functional theory (DFT) in combination with Hubbard type interaction such as the static correction evaluated within the Hartree-Fock method (DFT+U) or within the more sophisticated method of dynamical mean-field theory (DFT+DMFT). Using many-body approaches enables us to treat the correlation effects such as non-quasiparitcle states above the Fermi level for Co2MnSi and in the half-metallic side of the Co2MnAl/CoMnVAl heterostructure.

Based on the converged electronic structure, the magnetic excitations were mapped onto the Heisenberg Hamiltonian. Among various ways to extract exchange parameters, in this Thesis the method of infinitesimal rotation of the spins has been applied in the framework of the local magnetic force approach. It is shown that the exchange interactions on the surface of transition metals can be substantially different from those in the corresponding bulk. At the same time, the dynamical correlations lead to a slight renormalization of the magnetic couplings. For CaMnO3, we demonstrated the crucial role of the atomic relaxations defining the magnetic order on the surface atoms. We were also able to extract the orbital decompositions, which helped identify the main contributions to the total exchange. For the double-perovskite systems, the extracted exchange parameters were then used to evaluate the ordering temperature using Monte-Carlo simulations, and the calculated critical temperatures were found to be in good agreement with our experimental measurements.

In a more technical investigation, the influence of the spin polarization of the DFT exchange-correlation functional on the extracted exchange parameters has been investigated. We found a very good correspondence between the computed total energies and the parametrized Heisenberg model for LDA+U calculations, but not for LSDA+U. This means that for the extraction of the exchange parameters based on total energy differences, LDA+U is more appropriate.

Finally, a systematic study of the emergence of the local minima in DFT+U calculations has been performed for the bulk of NiO, FeO, CoO and UO2. We extended the use of the occupation matrix control method to randomly generate density matrices which help better monitor the local minima and explore the energy landscape. The effect of the Hubbard U and the double-counting in introducing the local minima are discussed.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 94
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1720
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-357525 (URN)978-91-513-0438-0 (ISBN)
Public defence
2018-10-26, Polhemsalen, 10134, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2018-10-05 Created: 2018-08-24 Last updated: 2018-10-05
2. 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

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Keshavarz, SamaraSchött, JohanKvashnin, Yaroslav

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