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  • 1.
    Bhandary, Sumanta
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Szunyogh, Laszlo
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Route towards finding large magnetic anisotropy in nanocomposites: Application to a W(1-x)Re(x)/Fe multilayer2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 9, p. 092401-Article in journal (Refereed)
    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.

  • 2.
    Björkman, Torbjörn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Adaptive smearing for Brillouin zone integrationManuscript (preprint) (Other (popular science, discussion, etc.))
    Abstract [en]

    We suggest a simple scheme for automatically determining the witdth 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. 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.

  • 3.
    Björkman, Torbjörn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Adaptive Smearing for Brillouin Zone Integration2011In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 111, no 5, p. 1025-1030Article in journal (Refereed)
    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.

  • 4.
    Bultmark, Fredrik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Cricchio, Francesco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Multipole decomposition of LDA+U energy and its application to actinides compounds2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 3, p. 035121-Article in journal (Refereed)
    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.

  • 5.
    Costa, Marcio
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Venezuela, P.
    Instituto de Física, Universidade Federal Fluminense, Rio de Janeiro, Brasilien.
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Klintenberg, Mattias
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Large magnetic anisotropy of Fe2P investigated via ab initio density functional theory calculations2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 8, p. 085125-Article in journal (Refereed)
    Abstract [en]

    We present an investigation of the large magnetic anisotropy of Fe2P, based on ab initio density functional theory calculations, with a full-potential linear muffin-tin orbital basis. We obtain a uniaxial magnetic anisotropy energy (MAE) of 664 mu eV/f.u., which is in decent agreement with experimental observations. Based on a band structure analysis the microscopic origin of the large magnetic anisotropy is explained. We also show that by straining the crystal structure, the MAE can be enhanced further.

  • 6.
    Cricchio, Francesco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Bultmark, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Itinerant Magnetic Multipole Moments of Rank Five as the Hidden Order in URu2Si22009In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 103, no 10, p. 107202-Article in journal (Refereed)
    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.

  • 7.
    Cricchio, Francesco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Low spin moment due to hidden multipole order from spin-orbital ordering in LaFeAsO2010In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 81, no 14, p. 140403-Article in journal (Refereed)
    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.

  • 8.
    Cricchio, Francesco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Multipolar and orbital ordering in ferro-pnictidesManuscript (preprint) (Other academic)
    Abstract [en]

    The stability of different solutions corresponding to various arrangement of magnetic rank-4 multipoles are evaluated for the ferro-pnictides LaOFeAs, BaFe2As2 and CaFe2As2 compounds. For LaOFeAs, the ground-state solution is identified with a combination of large w4120 and w4120 multipole tensors of the magnetization density which integrates to the correct value of experimental moment. The same procedure is applied to BaFe2As2 and CaFe2 As2 where the lowest energy solution corresponds again to a combination of w4120 and w4120 multipoles, but with a magnetic moment lower than the experimental value. However, for these compounds, our calculated moment constitutes a significant improvement over previous ab-initio calculations that largely overestimate the Fe moment. Moreover for BaFe2As2 and CaFe2As2 , in the total energy curves as function of constrained spin moment, we identify a local minima close to the experimental moment.

  • 9.
    Cricchio, Francesco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Multipolar magnetic ordering in actinide dioxides from first-principles calculationsManuscript (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.

  • 10.
    Cricchio, Francesco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Polarization of an open shell in the presence of spin-orbit coupling2011In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 94, no 5, p. 57009-Article in journal (Refereed)
    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.

  • 11.
    Cricchio, Francesco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    The role of triakontadipoles in uranium-based magnetic superconductor materialsManuscript (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.

  • 12.
    di Marco, Igor
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Thunström, P
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Pourovskii, L
    Centre de Physique Théorique (CPHT) École Polytechnique.
    Katsnelson, M I
    Inst. for Molecules and Materials, Radboud University of Nijmegen.
    Nordström, L
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, O
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    An LDA+DMFT study of the orbital magnetism and total energy properties of the late transition metals: conserving and non-conserving approximationsManuscript (preprint) (Other academic)
  • 13.
    Droulias, Sotirios A.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Hartmann, Ola
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Komander, Kristina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Wolff, Max
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Pálsson, Gunnar K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Tailoring thermodynamic properties of metal hydrides with interfacesManuscript (preprint) (Other academic)
    Abstract [en]

    We report profound changes in the enthalpy of solution and critical temperature of nano-sizedmetal hydrides in proximity to dierent non-absorbing metals. We use Cr/V and Fe/V (001) superlatticesas model systems to map out these eects since the superlattices can be made singlecrystalline,have identical strain states and are fully reversible. The thermodynamic propertiesare determined using isothermal pressure and optical transmission measurements. We determinethe mechanism underlying the changes in the thermodynamic properties as well as the ecacyof optical transmission by utilizing simultaneous neutron reectometry and optical transmissionmeasurements, combined with rst principle calculations. The underlying mechanism, which issupported by quantitative agreement between theory and experiment is found to be rooted in aradically dierent volume expansion in the two systems. The commonly used optical transmissionmethod is found to be linear with concentration in both types of superlattices and is attributed tochanges in electron density, rather than a direct inuence from a redistribution of electronic states.The conclusions are generalized to other metal hydride systems and can open up for exploration ofproximity induced eects in metal hydrides.

  • 14.
    Droulias, Sotirios A.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Hartmann, Ola
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Komander, Kristina
    Wolff, Max
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Pálsson, Gunnar K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Using interfaces to influence thermodynamic properties of metal hydridesManuscript (preprint) (Other academic)
    Abstract [en]

    We report profound proximity effects on the enthalpy of solution and critical temperature ofnano-sized vanadium hydrides. We use single crystalline Cr/V and Fe/V (001) superlattices forthese studies, in which the V layers are under close to identical strain. The thermodynamic propertiesare determined using isothermal pressure and optical transmission measurements, utilisingthe fully reversibel hydrogen uptake and release.The underlying mechanism is argued to be rootedin a radically different volume expansion in the two systems, arising from different site occupancyof hydrogen in Fe/V(001) and Cr/V(001). The optical transmission is found to be linear withconcentration in both types of superlattices, while exhibiting different slopes. The differences areattributed to changes in electron density arising from the volume expansion, rather than a directinfluence from a redistribution of electronic states.

  • 15.
    Ganguly, Shreemoyee
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Granas, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordstrom, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nontrivial order parameter in Sr2IrO42015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 2, p. 020404-Article in journal (Refereed)
    Abstract [en]

    A thorough analysis of the ground state of the relativistic magnetic insulator Sr2IrO4 is performed. The results are in accordance with the small antiferromagnetic moment and gapped state found in experiment. The solution, obtained using the DFT+SO+U methodology, is thoroughly analyzed in terms of Landau theory. We find that the ordered magnetic moment only forms a secondary order parameter while the primary order parameter is a higher order magnetic multipole of rank five. It is further observed that the electronic structure in the presence of this order parameter is related to the earlier proposed j(eff) = 1/2 model, but in contrast to that model, the present picture can exactly explain the small magnitude of the ordered magnetic moments.

  • 16.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Theoretical Studies of Magnetism and Electron Correlation in Solids2012Doctoral 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].

    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
    Show others...
    (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
    Show others...
    2012 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 55, p. 295-302Article 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.

    Keywords
    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: 2017-12-07Bibliographically 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, p. 1025-1030Article 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.

    Keywords
    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: 2017-12-11Bibliographically 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, p. 035121-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: 2017-12-13Bibliographically 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, p. 57009-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.

    Keywords
    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: 2017-12-12Bibliographically 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.

    Keywords
    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, E-ISSN 1079-7114, Vol. 103, no 10, p. 107202-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: 2017-12-13Bibliographically 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.

    Keywords
    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, p. 140403-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.

    Keywords
    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: 2017-12-12Bibliographically approved
    12. Microscopic picture of Co clustering in ZnO
    Open this publication in new window or tab >>Microscopic picture of Co clustering in ZnO
    Show others...
    2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 12, p. 125202-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
    Keywords
    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: 2017-12-12Bibliographically 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
    Show others...
    (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
    Show others...
    2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 9, p. 092401-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: 2017-12-08Bibliographically approved
    15. On the large magnetic anisotropy of Fe2P
    Open this publication in new window or tab >>On the large magnetic anisotropy of Fe2P
    Show others...
    (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
    Show others...
    2012 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 23, p. 236006-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: 2017-12-07Bibliographically approved
  • 17. Grånäs, Oscar
    et al.
    Bultmark, F.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Cricchio, F.
    Nordström, Lars
    Analysis of dynamical exchange and correlation in terms of coupled multipolesManuscript (Other academic)
  • 18.
    Grånäs, Oscar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Cricchio, F
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, L
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Systematic study of the hidden order in URu2Si2 as a multipolar orderManuscript (preprint) (Other academic)
  • 19.
    Grånäs, Oscar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Di Marco, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Etz, Corina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Electronic structure, cohesive properties, and magnetism of SrRuO32014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 16, p. 165130-Article in journal (Refereed)
    Abstract [en]

    We have performed an extensive test of the ability of density functional theory within several approximations for the exchange-correlation functional, local density approximation + Hubbard U, and local density approximation + dynamic mean field theory to describe magnetic and electronic properties of SrRuO3. We focus on the ferromagnetic phase, illustrating differences between the orthorhombic low-temperature structure versus the cubic high-temperature structure. We assess how magnetism, spectral function, and cohesive properties are affected by methodology, onsite Hubbard U, and double-counting corrections. Further, we compare the impact of the impurity solver on the quasiparticle weight Z, which is in turn compared to experimental results. The spectral functions resulting from the different treatments are also compared to experimental data. Finally, the impact of spin-orbit coupling is studied, allowing us to determine the orbital moments. In the orthorhombic phase, the orbital moments are found to be tilted with respect to the spin moments, emphasizing the importance of taking into account the distortion of the oxygen octahedra.

  • 20.
    Grånäs, Oscar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Di Marco, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Thunström, Patrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Björkman, T.
    Wills, J. M.
    Charge self-consistent dynamical mean-field theory based on the full-potential linear muffin-tin orbital method: Methodology and applications2012In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 55, p. 295-302Article in journal (Refereed)
    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.

  • 21.
    Grånäs, Oscar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Dutta, Biswanath
    Ghosh, Subhradip
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    A new first principles approach to calculate phonon spectra of disordered alloys2012In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 1, p. 015402-Article in journal (Refereed)
    Abstract [en]

    The lattice dynamics in substitutional disordered alloys with constituents having large size differences is driven by strong disorder in masses, inter-atomic force constants and local environments. In this paper, a new first principles approach based on special quasirandom structures and an itinerant coherent potential approximation to compute the phonon spectra of such alloys is proposed and applied to Ni(0.5)Pt(0.5) alloy. The agreement between our results and experiments is found to be much better than for previous models of disorder due to an accurate treatment of the interplay of inter-atomic forces among various pairs of chemical species. This new formalism serves as a potential solution to the longstanding problem of a proper microscopic understanding of lattice dynamical behavior of disordered alloys.

  • 22.
    Grånäs, Oscar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Harvard University, School of Engineering and Applied Sciences.
    Kolesov, Grigory
    Harvard University, School of Engineering and Applied Sciences.
    Kaxiras, Efthimios
    Harvard University, School of Engineering and Applied Sciences.
    Impact of Vibrations and Electronic Coherence on Electron Transfer in Flat Molecular Wires2017In: MRS Advances, ISSN 2316-7858, E-ISSN 1610-191X, Vol. 2, no 14, p. 811-816Article in journal (Refereed)
    Abstract [en]

    Electron transfer in molecular wires are of fundamental importance for a range of optoelectronic applications. The impact of electronic coherence and ionic vibrations on transmittance are of great importance to determine the mechanisms, and subsequently the type of wires that are most promising for applications. In this work, we use the real-time formulation of time-dependent density functional theory to study electron transfer through oligo-pphenylenevinylene (OPV) and the recently synthesized carbon bridged counterpart (COPV). A system prototypical of organic photovoltaics is setup by bridging a porphyrin-fullerene dyad, allowing a photo-excited electron to flow between the Zn-porphyrin (ZnP) chromophore and the C60 electron acceptor through the molecular wire. The excited state is described using the fully self-consistent.-SCF method. The state is then propagated in time using the real-time TD-DFT scheme, while describing ionic vibrations with classical nuclei. The charge transferred between porphyrin and C60 is calculated and correlated with the velocity autocorrelation functions of the ions. This provides a microscopic insight to vibrational and tunneling contributions to electron transport in linked porphyrin-fullerene dyads. We elaborate on important details in describing the excited state and trajectory sampling.

  • 23.
    Grånäs, Oscar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Timneanu, Nicusor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Eliah Dawod, Ibrahim
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Ragazzon, Davide
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Trygg, Sebastian
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Souvatzis, Petros
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
    Caleman, C
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Femtosecond bond breaking and charge dynamics in ultracharged amino acids2019In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, no 14Article in journal (Refereed)
    Abstract [en]

    Historically, structure determination of nanocrystals, proteins, and macromolecules required the growth of high-quality crystals sufficiently large to diffract X-rays efficiently while withstanding radiation damage. The development of the X-ray free-electron laser has opened the path toward high resolution single particle imaging, and the extreme intensity of the X-rays ensures that enough diffraction statistics are collected before the sample is destroyed by radiation damage. Still, recovery of the structure is a challenge, in part due to the partial fragmentation of the sample during the diffraction event. In this study, we use first-principles based methods to study the impact of radiation induced ionization of six amino acids on the reconstruction process. In particular, we study the fragmentation and charge rearrangement to elucidate the time scales involved and the characteristic fragments occurring.

  • 24.
    Iusan, Diana
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Kabir, Mukul
    Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Microscopic picture of Co clustering in ZnO2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 12, p. 125202-Article in journal (Refereed)
    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.

  • 25.
    Kolesov, Grigory
    et al.
    Harvard Univ, John A Paulson Sch Engn & Appl Sci, Cambridge, MA 02138 USA..
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Harvard Univ, John A Paulson Sch Engn & Appl Sci, Cambridge, MA 02138 USA..
    Hoyt, Robert
    Harvard Univ, Dept Phys, Cambridge, MA 02138 USA..
    Vinichenko, Dmitry
    Harvard Univ, Dept Chem & Chem Biol, Cambridge, MA 02138 USA..
    Kaxiras, Efthimios
    Harvard Univ, John A Paulson Sch Engn & Appl Sci, Cambridge, MA 02138 USA.;Harvard Univ, Dept Phys, Cambridge, MA 02138 USA..
    Real-Time TD-DFT with Classical Ion Dynamics: Methodology and Applications2016In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 12, no 2, p. 466-476Article in journal (Refereed)
    Abstract [en]

    We present a method for real-time propagation of electronic wave functions, within time-dependent density functional theory (RT-TDDFT), coupled to ionic motion through mean-field classical dynamics. The goal of our method is to treat large systems and complex processes, in particular photocatalytic reactions and electron transfer events on surfaces and thin films. Due to the complexity of these processes, computational approaches are needed to provide insight into the underlying physical mechanisms and are therefore crucial for the rational design of new materials. Because of the short time step required for electron propagation (of order similar to 10 attoseconds), these simulations are computationally very demanding. Our methodology is based on numerical atomic-orbital-basis sets for computational efficiency. In the computational package, to which we refer as TDAP-2.0 (Time-evolving Deterministic Atom Propagator), we have implemented a number of important features and analysis tools for more accurate and efficient treatment of large, complex systems and time scales that reach into a fraction of a picosecond. We showcase the capabilities of our method using four different examples: (i) photodissociation into radicals of opposite spin, (ii) hydrogen adsorption on aluminum surfaces, (iii) optical absorption of spin-polarized organic molecule containing a metal ion, and (iv) electron transfer in a prototypical dye sensitized solar cell.

  • 26.
    Kvashnin, Yaroslav O.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Di Marco, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Katsnelson, M. I.
    Lichtenstein, A. I.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Exchange parameters of strongly correlated materials: Extraction from spin-polarized density functional theory plus dynamical mean-field theory2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 12, article id 125133Article in journal (Refereed)
    Abstract [en]

    In this paper we present an accurate numerical scheme for extracting interatomic exchange parameters (J(ij)) of strongly correlated systems, based on first-principles full-potential electronic structure theory. The electronic structure is modeled with the help of a full-potential linear muffin-tin orbital method. The effects of strong electron correlations are considered within the charge self-consistent density functional theory plus dynamical mean-field theory. The exchange parameters are then extracted using the magnetic force theorem; hence all the calculations are performed within a single computational framework. The method allows us to investigate how the Jij parameters are affected by dynamical electron correlations. In addition to describing the formalism and details of the implementation, we also present magnetic properties of a few commonly discussed systems, characterized by different degrees of electron localization. In bcc Fe, treated as a moderately correlated metal, we found a minor renormalization of the Jij interactions once the dynamical correlations are introduced. However, generally, if the magnetic coupling has several competing contributions from different orbitals, the redistribution of the spectral weight and changes in the exchange splitting of these states can lead to a dramatic modification of the total interaction parameter. In NiO we found that both static and dynamical mean-field results provide an adequate description of the exchange interactions, which is somewhat surprising given the fact that these two methods result in quite different electronic structures. By employing the Hubbard-I approximation for the treatment of the 4f states in hcp Gd we reproduce the experimentally observed multiplet structure. The calculated exchange parameters result in being rather close to the ones obtained by treating the 4f electrons as noninteracting core states.

  • 27.
    Panda, Swarup K.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Di Marco, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Harvard, Sch Engn & Appl Sci, 29 Oxford St, Cambridge, MA 02138 USA.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Electronic and magnetic properties of single Fe atoms on a CuN surface: Effects of electron correlations2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 14, article id 140101Article in journal (Refereed)
    Abstract [en]

    The electronic structure and magnetic properties of a single Fe adatom on a CuN surface have been studied using density functional theory in the local spin density approximation (LSDA), the LSDA+U approach, and the local density approximation plus dynamical mean-field theory (LDA+DMFT). The impurity problem in LDA+DMFT is solved through exact diagonalization and in the Hubbard-I approximation. The comparison of the one-particle spectral functions obtained from LSDA, LSDA+U, and LDA+DMFT show the importance of dynamical correlations for the electronic structure of this system. Most importantly, we focused on the magnetic anisotropy and found that neither LSDA nor LSDA+U can explain the measured high values of the axial and transverse anisotropy parameters. Instead, the spin excitation energies obtained from our LDA+DMFT approach with exact diagonalization agree significantly better with experimental data. This affirms the importance of treating fluctuating magnetic moments through a realistic many-body treatment when describing this class of nanomagnetic systems. Moreover, it facilitates insight to the role of the hybridization with surrounding orbitals.

  • 28.
    Peters, L.
    et al.
    Radboud Univ Nijmegen, Inst Mol & Mat, NL-6525 AJ Nijmegen, Netherlands..
    Di Marco, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sasioglu, E.
    Forschungszentrum Julich, Peter Grunberg Inst, D-52425 Julich, Germany.;Forschungszentrum Julich, Inst Adv Simulat, D-52425 Julich, Germany.;JARA, D-52425 Julich, Germany..
    Altun, A.
    Fatih Univ, Dept Phys, TR-34500 Istanbul, Turkey..
    Rossen, S.
    Radboud Univ Nijmegen, Inst Mol & Mat, NL-6525 AJ Nijmegen, Netherlands.;Forschungszentrum Julich, Peter Grunberg Inst, D-52425 Julich, Germany.;Forschungszentrum Julich, Inst Adv Simulat, D-52425 Julich, Germany.;JARA, D-52425 Julich, Germany..
    Friedrich, C.
    Forschungszentrum Julich, Peter Grunberg Inst, D-52425 Julich, Germany.;Forschungszentrum Julich, Inst Adv Simulat, D-52425 Julich, Germany.;JARA, D-52425 Julich, Germany..
    Bluegel, S.
    Forschungszentrum Julich, Peter Grunberg Inst, D-52425 Julich, Germany.;Forschungszentrum Julich, Inst Adv Simulat, D-52425 Julich, Germany.;JARA, D-52425 Julich, Germany..
    Katsnelson, M. I.
    Radboud Univ Nijmegen, Inst Mol & Mat, NL-6525 AJ Nijmegen, Netherlands..
    Kirilyuk, A.
    Radboud Univ Nijmegen, Inst Mol & Mat, NL-6525 AJ Nijmegen, Netherlands..
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Correlation effects and orbital magnetism of Co clusters2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 22, article id 224428Article in journal (Refereed)
    Abstract [en]

    Recent experiments on isolated Co clusters have shown huge orbital magnetic moments in comparison with their bulk and surface counterparts. These clusters hence provide the unique possibility to study the evolution of the orbital magnetic moment with respect to the cluster size and how competing interactions contribute to the quenching of orbital magnetism. We investigate here different theoretical methods to calculate the spin and orbital moments of Co clusters, and assess the performances of the methods in comparison with experiments. It is shown that density-functional theory in conventional local density or generalized gradient approximations, or even with a hybrid functional, severely underestimates the orbital moment. As natural extensions/corrections, we considered the orbital polarization correction, the LDA+U approximation as well as the LDA+DMFT method. Our theory shows that of the considered methods, only the LDA+DMFT method provides orbital moments in agreement with experiment, thus emphasizing the importance of dynamic correlations effects for determining fundamental magnetic properties of magnets in the nanosize regime.

  • 29.
    Sanyal, Biplab
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Knut, Ronny
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Coleman, V. A
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Thunström, Patrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Iusan, Diana M
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Karis, Olof
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Westin, Gunnar
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry.
    Electronic structure of Co doped ZnO: Theory and experiment2008In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 103, no 7, p. 07D130-Article in journal (Refereed)
    Abstract [en]

    We have studied the electronic structure of Co doped ZnO by theory and experiment. Photoelectron and x-ray absorption spectroscopy experiments were performed on 5% Co doped ZnO thin films. The results show that Co-3d states lie in the valence band and hybridize considerably with O-2p states. These results compare qualitatively with our density functional calculations combined with a Hubbard approach to include strong electron correlations within the Co-3d state.

  • 30.
    Sanyal, Biplab
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Knut, Ronny
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Iusan, Diana
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Karis, Olof
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Inhomogeneity in Co Doped ZnO Diluted Magnetic Semiconductor2008In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 103, no 7, p. 07D131-Article in journal (Refereed)
    Abstract [en]

    Here, we have studied the chemical and magnetic interactions in Co doped ZnO diluted magnetic semiconductor by ab initio density functional calculations. The calculated chemical pair interaction parameters suggest a strong tendency of clustering between Co atoms. Both chemical and magnetic pair interaction parameters are short ranged with a large first neighbor interaction. Monte Carlo simulations show that the Curie temperature for a homogeneous sample is low, whereas allowing for an inhomogeneous growth results in very high Curie temperatures. It is argued that these inhomogeneities may be the reason for the observation of high Curie temperatures in Co doped ZnO. It is also demonstrated that the electronic structure is modified significantly in presence of Co clusters.

  • 31.
    Schött, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Locht, Inka L. M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Radboud Univ Nijmegen, Inst Mol & Mat, Heyendaalseweg 135, NL-6525 AJ Nijmegen, Netherlands..
    Lundin, Elin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Harvard Univ, Sch Engn & Appl Sci, Cambridge, MA 02138 USA..
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Di Marco, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Analytic continuation by averaging Pade approximants2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 7, article id 075104Article in journal (Refereed)
    Abstract [en]

    The ill-posed analytic continuation problem for Green's functions and self-energies is investigated by revisiting the Pade approximants technique. We propose to remedy the well-known problems of the Pade approximants by performing an average of several continuations, obtained by varying the number of fitted input points and Pade coefficients independently. The suggested approach is then applied to several test cases, including Sm and Pr atomic self-energies, the Green's functions of the Hubbard model for a Bethe lattice and of the Haldane model for a nanoribbon, as well as two special test functions. The sensitivity to numerical noise and the dependence on the precision of the numerical libraries are analyzed in detail. The present approach is compared to a number of other techniques, i.e., the nonnegative least-squares method, the nonnegative Tikhonov method, and the maximum entropy method, and is shown to perform well for the chosen test cases. This conclusion holds even when the noise on the input data is increased to reach values typical for quantum Monte Carlo simulations. The ability of the algorithm to resolve fine structures is finally illustrated for two relevant test functions.

1 - 31 of 31
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