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Theoretical Studies of Magnetism and Electron Correlation in Solids
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. (Materialteori)
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This work presents new development and applications of ab-initio simulation tools for material science. Focus lies on materials with strong electronic correlation and strong spin-orbit coupling. Improvements on methods for solving the impurity problem in LDA+DMFT is presented, as well as a reliant method for charge self-consistency in a LMTO based electronic structure code. A new adaptive scheme for Brillouin zone integration is developed, where we show a strong reduction of numerical noise compared to standard techniques. A reformulation of the standard LDA+U method aiming to reduce the number of free parameters is introduced. Fast and realistic reduction of the number of free parameters provides the possibility of high throughput calculations and enabled us to study a large number of compounds. An analysis method for polarization in terms of coupled multipoles, and their corresponding energy contributions is developed and applied. This led to the formulation of Katt's rules, a set of rules complementary to Hund's rules. Katt's rules applies for occupying the orbitals of an electronic shell with strong spin-orbit coupling. The analysis is also used to investigate the unconventional Uranium based superconductors URu2Si2, UPt3, UPd2Al3 and UNi2Al3, as well as the high temperature superconductor LaOFeAs. We also investigate the non-magnetic delta-phase of Plutonium, providing insight to the electronic structure and the branching ratios of 4d to 5f transitions seen in photo emission spectra.The influence of surface reconstruction on the magneto crystalline anisotropy is investigated in multilayer Fe/ZnSe, showing that Fe deposited on an unreconstructed interface strongly reduces the uniaxial component of the MAE. We provide a detailed understanding of the magnetic properties of Fe2P, opening possible routes for enhancing the MAE in this system. A general route to strong MAE in nano-laminates is presented, we apply this to propose a candidate with extremely strong anisotropy energy density, 5Fe/2W1-xReX for x=[0.6-0.8].

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , xii+70 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 920
Keyword [en]
Magnetism, Superconductivity, Electron Correlation, DMFT, DFT, Actinides, multipoles, hidden order, magnetic anisotropy
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-172334ISBN: 978-91-554-8337-1 (print)OAI: oai:DiVA.org:uu-172334DiVA: diva2:514082
Public defence
2012-05-22, Siegbahnsalen, Lägerhyddsv. 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2012-04-26 Created: 2012-04-04 Last updated: 2012-08-01Bibliographically approved
List of papers
1. An LDA+DMFT study of the orbital magnetism and total energy properties of the late transition metals: conserving and non-conserving approximations
Open this publication in new window or tab >>An LDA+DMFT study of the orbital magnetism and total energy properties of the late transition metals: conserving and non-conserving approximations
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-172345 (URN)
Available from: 2012-04-05 Created: 2012-04-05 Last updated: 2012-08-01
2. Charge self-consistent dynamical mean-field theory based on the full-potential linear muffin-tin orbital method: Methodology and applications
Open this publication in new window or tab >>Charge self-consistent dynamical mean-field theory based on the full-potential linear muffin-tin orbital method: Methodology and applications
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2012 (English)In: Computational materials science, ISSN 0927-0256, Vol. 55, 295-302 p.Article in journal (Refereed) Published
Abstract [en]

Full charge self-consistence (CSC) over the electron density has been implemented into the local density approximation plus dynamical mean-field theory (LDA + DMFT) scheme based on a full-potential linear muffin-tin orbital method (FP-LMTO). Computational details on the construction of the electron density from the density matrix are provided. The method is tested on the prototypical charge-transfer insulator NiO using a simple static Hartree-Fock approximation as impurity solver. The spectral and ground state properties of bcc Fe are then addressed, by means of the spin-polarized T-matrix fluctuation exchange solver (SPTF). Finally the permanent magnet SmCo5 is studied using multiple impurity solvers, SPTF and Hubbard I, as the strength of the local Coulomb interaction on the Sm and Co sites are drastically different. The developed CSC-DMFT method is shown to in general improve on materials properties like magnetic moments, electronic structure and the materials density.

Keyword
Dynamical mean-field theory, Correlated materials, NiO, Fe, SmCo5
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-171397 (URN)10.1016/j.commatsci.2011.11.032 (DOI)000300728600039 ()
Available from: 2012-03-20 Created: 2012-03-19 Last updated: 2012-08-01Bibliographically approved
3. Adaptive Smearing for Brillouin Zone Integration
Open this publication in new window or tab >>Adaptive Smearing for Brillouin Zone Integration
2011 (English)In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 111, no 5, 1025-1030 p.Article in journal (Refereed) Published
Abstract [en]

We suggest a simple scheme for automatically determining the width parameter of smearing methods of the Brillouin zone integration in electronic structure calculation. The scheme retains one free parameter that at any time can be eliminated by choosing a denser k-space mesh until the desired accuracy is obtained. The tests are carried out in the context of Methfessel-Paxton smearing. This adaptive Gaussian smearing (AGS) is easily implemented, variational with respect to partial occupancies and free from spurious occupancies that are negative or larger than one. Its convergence properties are similar to those obtained with the modified tetrahedron method for energy resolution of >= 0.1 meV.

Keyword
Brillouin zone integration, band structure methods, Gaussian smearing, magnetocrystalline anisotropy energy
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-148949 (URN)10.1002/qua.22476 (DOI)000287163800012 ()
Available from: 2011-03-15 Created: 2011-03-14 Last updated: 2012-08-01Bibliographically approved
4. Multipole decomposition of LDA+U energy and its application to actinides compounds
Open this publication in new window or tab >>Multipole decomposition of LDA+U energy and its application to actinides compounds
2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 3, 035121- p.Article in journal (Refereed) Published
Abstract [en]

A general reformulation of the exchange energy of 5f shell is applied   in the analysis of the magnetic structure of various actinides compounds in the framework of LDA + U method. The calculations are   performed in a convenient scheme with essentially only one free   parameter, the screening length. The results are analyzed in terms of  different polarization channels due to different multipoles. Generally   it is found that the spin-orbital polarization is dominating. This can   be viewed as a strong enhancement of the spin-orbit coupling in these   systems. This leads to a drastic decrease in spin polarization in   accordance with experiments. The calculations are able to correctly   differentiate magnetic and nonmagnetic Pu system. Finally, in all   magnetic systems an unusual multipolar order is observed, whose   polarization energy is often larger in magnitude than the one of spin polarization.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-98059 (URN)10.1103/PhysRevB.80.035121 (DOI)000268617800055 ()
Available from: 2009-02-12 Created: 2009-02-12 Last updated: 2012-08-01Bibliographically approved
5. Analysis of dynamical exchange and correlation in terms of coupled multipoles
Open this publication in new window or tab >>Analysis of dynamical exchange and correlation in terms of coupled multipoles
(English)Manuscript (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-98062 (URN)
Available from: 2009-02-12 Created: 2009-02-12 Last updated: 2012-08-01Bibliographically approved
6. Polarization of an open shell in the presence of spin-orbit coupling
Open this publication in new window or tab >>Polarization of an open shell in the presence of spin-orbit coupling
2011 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 94, no 5, 57009- p.Article in journal (Refereed) Published
Abstract [en]

There exist many f transition metal-based materials, with large orbital degeneracy and relatively strong spin-orbit coupling, that exhibits some multipolar order. We perform first-principles calculations on these systems within DFT method plus on-site Coulomb correction and we discuss the results in terms of polarization of the density occupation matrix. The fact that higher multipole polarizations play a larger role than the spin polarization indicates a breakdown of Hund's rules and we suggest a new complementary set of rules -Katt's rules, valid in case of large spin-orbit coupling interaction.

Keyword
Spin-orbit interaction, actinides, Hund's rules, correlated electrons, electronic structure calculations
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-132067 (URN)10.1209/0295-5075/94/57009 (DOI)000291033900033 ()
Available from: 2010-10-13 Created: 2010-10-13 Last updated: 2012-08-01Bibliographically approved
7. Multipolar magnetic ordering in actinide dioxides from first-principles calculations
Open this publication in new window or tab >>Multipolar magnetic ordering in actinide dioxides from first-principles calculations
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The LDA+U method and its multipole decomposition are applied to investigate the ground state of the actinide insulators dioxides UO2 , NpO2 and PuO2 . In case of UO2 , an antifer- romagnetic 3-k type ordering of U moments is identified as one of the lowest energy solutions, in agreement with experiments and recent calculations. For NpO2 , a magnetic multipolar 3-k ordering of triakontadipoles, with a zero net magnetic moment on Np atoms, is found to be one of the most stable solutions. This is in agreement with recent investigations that predict the triakontadipoles to be the hidden order parameter in NpO2 . Also in case of the non-magnetic insulator PuO2 , the ground-state is predicted to be described by a multipolar 1-k ordering of triakontadipoles associated with a magnetization density that integrates to zero on Pu sites.

Keyword
strongly correlated electrons, electronic structure calculations, actinides, dioxides
National Category
Condensed Matter Physics
Research subject
Physics of Matter
Identifiers
urn:nbn:se:uu:diva-132065 (URN)
Available from: 2010-10-13 Created: 2010-10-13 Last updated: 2012-08-01
8. Itinerant Magnetic Multipole Moments of Rank Five as the Hidden Order in URu2Si2
Open this publication in new window or tab >>Itinerant Magnetic Multipole Moments of Rank Five as the Hidden Order in URu2Si2
2009 (English)In: Physical Review Letters, ISSN 0031-9007, Vol. 103, no 10, 107202- p.Article in journal (Refereed) Published
Abstract [en]

A broken symmetry ground state without any magnetic moments has been   calculated by means of the local-density approximation to density   functional theory plus a local exchange term, the so-called LDA+U   approach, for URu2Si2. The solution is analyzed in terms of a multipole   tensor expansion of the itinerant density matrix and is found to be a   nontrivial magnetic multipole. Analysis and further calculations show   that this type of multipole enters naturally in time reversal breaking   in the presence of large effective spin-orbit coupling and coexists   with magnetic moments for most magnetic actinides.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-98061 (URN)10.1103/PhysRevLett.103.107202 (DOI)000269639800053 ()
Available from: 2009-02-12 Created: 2009-02-12 Last updated: 2012-08-01Bibliographically approved
9. Systematic study of the hidden order in URu2Si2 as a multipolar order
Open this publication in new window or tab >>Systematic study of the hidden order in URu2Si2 as a multipolar order
(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-172346 (URN)
Available from: 2012-04-05 Created: 2012-04-05 Last updated: 2012-08-01
10. The role of triakontadipoles in uranium-based magnetic superconductor materials
Open this publication in new window or tab >>The role of triakontadipoles in uranium-based magnetic superconductor materials
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We investigate the ground state of the heavy-fermion actinide compounds UPd2Al3 , UNi2Al3 and UPt3 by means of electronic structure calculations including on-site correlation. The results are analyzed through multipole decomposition. In case of UPd2Al3 and UNi2Al3 we stabilize a solution with an antiferromagnetic arrangement of triakontadipoles with ordering vectors, respectively, q = (0, 0, 1/2 ) and q = ( 1/2 , 0, 1/2 ), whose total magnetic moment agrees with the experimental value. In case of UPt3 we identify a solution with a triakontadipole order whose symmetry is not compatible with the presence of a magnetic moment, in accordance with experiments that detect a very small value of the moment. The Fermi surfaces of the multipole dominated solutions have many common features with those obtained by conventional density functional methods. Finally, for URu2Si2 we are able to stabilize a new solution characterized by a q = (0, 0, 1/2 ) ordering of triakontadipoles and a symmetry that forbids a magnetic moment on uranium site. The Fermi surface of this solution is surprisingly similar to the one of the magnetic AF 1-q case in agreement with recent measurements.

Keyword
Superconductivity, actinides, hidden order, correlated electrons
National Category
Condensed Matter Physics
Research subject
Physics of Matter
Identifiers
urn:nbn:se:uu:diva-132064 (URN)
Available from: 2010-10-13 Created: 2010-10-13 Last updated: 2012-08-01
11. Low spin moment due to hidden multipole order from spin-orbital ordering in LaFeAsO
Open this publication in new window or tab >>Low spin moment due to hidden multipole order from spin-orbital ordering in LaFeAsO
2010 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 81, no 14, 140403- p.Article in journal (Refereed) Published
Abstract [en]

An antiferromagnetic (AF) low-moment solution, 0.35 μB / Fe, is found in the case of LaOFeAs for an intermediately strong Coulomb interaction U of 2.75 eV. This solution is stabilized over a large moment solution due to the gain in exchange energy in the formation of large multipoles of the spin magnetization density. The multipoles are of rank four and can be understood as a type of spin-orbital ordering. Parallels can be drawn to the stabilization of the AF order in, e.g., CaCuO2.

Keyword
Superconductivity, correlated electrons, electronic structure calculations
National Category
Condensed Matter Physics
Research subject
Physics of Matter
Identifiers
urn:nbn:se:uu:diva-132063 (URN)10.1103/PhysRevB.81.140403 (DOI)000277210200013 ()
Available from: 2010-10-13 Created: 2010-10-13 Last updated: 2012-08-01Bibliographically approved
12. Microscopic picture of Co clustering in ZnO
Open this publication in new window or tab >>Microscopic picture of Co clustering in ZnO
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2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 12, 125202- p.Article in journal (Refereed) Published
Abstract [en]

Density functional theory was applied to study the chemical and magnetic interactions between Co atoms doped in ZnO. It was found that the Co impurities tend to form nanoclusters and the interactions between these atoms are antiferromagnetic within the local spin-density approximation (LSDA)+Hubbard U approach. The extracted interatomic exchange parameters agree reasonably well with recent experimental results. We have analyzed and compared the electronic structure obtained using the LSDA and LSDA+U approaches and found that the LSDA+U gives the most reasonable result, highlighting the importance of short-ranged antiferromagnetic interactions due to superexchange.

Place, publisher, year, edition, pages
The American Physical Society, 2009
Keyword
antiferromagnetic materials, cobalt, density functional theory, II-VI semiconductors, magnetic semiconductors, metal clusters, superexchange interactions, wide band gap semiconductors, zinc compounds
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-119891 (URN)10.1103/PhysRevB.79.125202 (DOI)000264769300047 ()
Available from: 2010-03-02 Created: 2010-03-02 Last updated: 2017-01-25Bibliographically approved
13. Assessment of the magnetic properties of SrRuO3 using LDA and LDA+DMFT
Open this publication in new window or tab >>Assessment of the magnetic properties of SrRuO3 using LDA and LDA+DMFT
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-172347 (URN)
Available from: 2012-04-05 Created: 2012-04-05 Last updated: 2012-08-01
14. Route towards finding large magnetic anisotropy in nanocomposites: Application to a W(1-x)Re(x)/Fe multilayer
Open this publication in new window or tab >>Route towards finding large magnetic anisotropy in nanocomposites: Application to a W(1-x)Re(x)/Fe multilayer
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 9, 092401- p.Article in journal (Refereed) Published
Abstract [en]

We suggest here a nanolaminate, 5[Fe]/2[W(x)Re(1-x)] (x = 0.6-0.8), with enhanced magnetic hardness in combination with a large saturation moment. The calculated magnetic anisotropy of this material reaches values of 5.3-7.0 MJ/m(3), depending on alloying conditions. We also propose a recipe in how to identify other novel magnetic materials, such as nanolaminates and multilayers, with large magnetic anisotropy in combination with a high saturation moment.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-159231 (URN)10.1103/PhysRevB.84.092401 (DOI)000294773300001 ()
Available from: 2011-09-27 Created: 2011-09-26 Last updated: 2017-01-25Bibliographically approved
15. On the large magnetic anisotropy of Fe2P
Open this publication in new window or tab >>On the large magnetic anisotropy of Fe2P
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-172349 (URN)
Available from: 2012-04-05 Created: 2012-04-05 Last updated: 2013-03-21
16. Epitaxial Fe films on ZnSe(001): effect of the substrate surface reconstruction on the magnetic anisotropy
Open this publication in new window or tab >>Epitaxial Fe films on ZnSe(001): effect of the substrate surface reconstruction on the magnetic anisotropy
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2012 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 23, 236006- p.Article in journal (Refereed) Published
Abstract [en]

It is well known that Fe films deposited on a c(2 x 2)-reconstructed ZnSe(001) surface show a strong in-plane uniaxial magnetic anisotropy. Here, the effect of the substrate reconstruction on the magnetic anisotropy of Fe has been studied by in situ Brillouin light scattering. We found that the in-plane uniaxial anisotropy is strongly reduced for Fe films grown on a (1 x 1)-unreconstructed ZnSe substrate while the in-plane biaxial one is nearly unaffected by the substrate reconstruction. Calculations of magnetic anisotropy energies within the framework of ab initio density functional theory reveal that the strong suppression of anisotropy at the (1 x 1) interface occurs due to complex atomic relaxations as well as the competing effects originating from magnetocrystalline anisotropy and dipole-dipole interactions. For both sharp and intermixed c(2 x 2) interfaces, the magnetic anisotropy is enhanced compared to the (1 x 1) case due to the further lowering of symmetry. The theoretical results are in agreement with the experimental findings. 

National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-172350 (URN)10.1088/0953-8984/24/23/236006 (DOI)000304873800028 ()
Available from: 2012-04-05 Created: 2012-04-05 Last updated: 2017-01-25Bibliographically approved

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