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Keshavarz, S., Kvashnin, Y. O., Rodrigues, D. C. M., Pereiro, M., Di Marco, I., Autieri, C., . . . Eriksson, O. (2017). Exchange interactions of CaMnO3 in the bulk and at the surface. Physical Review B Condensed Matter, 95, Article ID 115120.
Open this publication in new window or tab >>Exchange interactions of CaMnO3 in the bulk and at the surface
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2017 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 95, article id 115120Article in journal (Refereed) Published
Abstract [en]

We present electronic and magnetic properties of CaMnO3 (CMO) as obtained from ab initio calculations. We identify the preferable magnetic order by means of density functional theory plus Hubbard U calculations and extract the effective exchange parameters (Jij ' s) using the magnetic force theorem. We find that the effects of geometrical relaxation at the surface as well as the change of crystal field are very strong and are able to influence the lower-energymagnetic configuration. In particular, our analysis reveals that the exchange interaction between the Mn atoms belonging to the surface and the subsurface layers is very sensitive to the structural changes. An earlier study [A. Filippetti and W. E. Pickett, Phys. Rev. Lett. 83, 4184 (1999)] suggested that this coupling is ferromagnetic and gives rise to the spin-flip (SF) process on the surface of CMO. In our work, we confirm their finding for an unrelaxed geometry, but once the structural relaxations are taken into account, this exchange coupling changes its sign. Thus, we suggest that the surface of CMO should have the same G-type antiferromagnetic order as in the bulk. Finally, we show that the suggested SF can be induced in the system by introducing an excess of electrons.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-319934 (URN)10.1103/PhysRevB.95.115120 (DOI)000396008300003 ()
Funder
Swedish Research CouncileSSENCE - An eScience CollaborationKnut and Alice Wallenberg Foundation
Available from: 2017-04-11 Created: 2017-04-11 Last updated: 2017-11-29Bibliographically approved
Fransson, J., Thonig, D., Bessarab, P. F., Bhattacharjee, S., Hellsvik, J. & Nordström, L. (2017). Microscopic theory for coupled atomistic magnetization and lattice dynamics. Physical Review Materials, 1(7), Article ID 074404.
Open this publication in new window or tab >>Microscopic theory for coupled atomistic magnetization and lattice dynamics
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2017 (English)In: Physical Review Materials, ISSN 2475-9953, Vol. 1, no 7, article id 074404Article in journal (Refereed) Published
Abstract [en]

A coupled atomistic spin and lattice dynamics approach is developed which merges the dynamics of these two degrees of freedom into a single set of coupled equations of motion. The underlying microscopic model comprises local exchange interactions between the electron spin and magnetic moment and the local couplings between the electronic charge and lattice displacements. An effective action for the spin and lattice variables is constructed in which the interactions among the spin and lattice components are determined by the underlying electronic structure. In this way, expressions are obtained for the electronically mediated couplings between the spin and lattice degrees of freedom, besides the well known interatomic force constants and spin-spin interactions. These former susceptibilities provide an atomistic ab initio description for the coupled spin and lattice dynamics. It is important to notice that this theory is strictly bilinear in the spin and lattice variables and provides a minimal model for the coupled dynamics of these subsystems and that the two subsystems are treated on the same footing. Questions concerning time-reversal and inversion symmetry are rigorously addressed and it is shown how these aspects are absorbed in the tensor structure of the interaction fields. By means of these results regarding the spin-lattice coupling, simple explanations of ionic dimerization in double-antiferromagnetic materials, as well as charge density waves induced by a nonuniform spin structure, are given. In the final parts, coupled equations of motion for the combined spin and lattice dynamics are constructed, which subsequently can be reduced to a form which is analogous to the Landau-Lifshitz-Gilbert equations for spin dynamics and a damped driven mechanical oscillator for the ionic motion. It is important to notice, however, that these equations comprise contributions that couple these descriptions into one unified formulation. Finally, Kubo-like expressions for the discussed exchanges in terms of integrals over the electronic structure and, moreover, analogous expressions for the damping within and between the subsystems are provided. The proposed formalism and types of couplings enable a step forward in the microscopic first principles modeling of coupled spin and lattice quantities in a consistent format.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:uu:diva-339791 (URN)10.1103/PhysRevMaterials.1.074404 (DOI)000418772500005 ()
Funder
Swedish Research Council, 2016-06955Wenner-Gren FoundationsStiftelsen Olle Engkvist Byggmästare
Available from: 2018-02-23 Created: 2018-02-23 Last updated: 2018-02-23Bibliographically approved
Cardias, R., Szilva, A., Bergman, A., Di Marco, I., Katsnelson, M. I., Lichtenstein, A. I., . . . Kvashnin, Y. O. (2017). The Bethe-Slater curve revisited; new insights from electronic structure theory. Scientific Reports, 7, Article ID 4058.
Open this publication in new window or tab >>The Bethe-Slater curve revisited; new insights from electronic structure theory
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2017 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 4058Article in journal (Refereed) Published
Abstract [en]

The Bethe-Slater (BS) curve describes the relation between the exchange coupling and interatomic distance. Based on a simple argument of orbital overlaps, it successfully predicts the transition from antiferromagnetism to ferromagnetism, when traversing the 3d series. In a previous article [Phys. Rev. Lett. 116, 217202 (2016)] we reported that the dominant nearestneighbour (NN) interaction for 3d metals in the bcc structure indeed follows the BS curve, but the trends through the series showed a richer underlying physics than was initially assumed. The orbital decomposition of the inter-site exchange couplings revealed that various orbitals contribute to the exchange interactions in a highly non-trivial and sometimes competitive way. In this communication we perform a deeper analysis by comparing 3d metals in the bcc and fcc structures. We find that there is no coupling between the E-g orbitals of one atom and T-2g orbitals of its NNs, for both cubic phases. We demonstrate that these couplings are forbidden by symmetry and formulate a general rule allowing to predict when a similar situation is going to happen. In gamma-Fe, as in alpha-Fe, we find a strong competition in the symmetry-resolved orbital contributions and analyse the differences between the high-spin and low-spin solutions.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-330728 (URN)10.1038/s41598-017-04427-9 (DOI)000403874900041 ()28642615 (PubMedID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

Erratum

doi:10.1038/s41598-017-09611-5

In the original version of this Article, Y. O. Kvashnin was incorrectly affiliated with ‘Faculdade de Fisica, Universidade Federal do Para, Belem, PA, Brazil’. The correct affiliation is listed below.

Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Box 516, SE-75120, Uppsala, Sweden.

This error has now been corrected in the PDF and HTML versions of the Article.

Available from: 2017-10-10 Created: 2017-10-10 Last updated: 2018-04-07Bibliographically approved
Szilva, A., Thonig, D., Bessarab, P. F., Kvashnin, Y., Rodrigues, D. C. M., Cardias, R., . . . Eriksson, O. (2017). Theory of noncollinear interactions beyond Heisenberg exchange: Applications to bcc Fe. Physical Review B, 96(14), Article ID 144413.
Open this publication in new window or tab >>Theory of noncollinear interactions beyond Heisenberg exchange: Applications to bcc Fe
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 14, article id 144413Article in journal (Refereed) Published
Abstract [en]

We show for a simple noncollinear configuration of the atomistic spins (in particular, where one spin is rotated by a finite angle in a ferromagnetic background) that the pairwise energy variation computed in terms of multiple-scattering formalism cannot be fully mapped onto a bilinear Heisenberg spin model even in the absence of spin-orbit coupling. The non-Heisenberg terms induced by the spin-polarized host appear in leading orders in the expansion of the infinitesimal angle variations. However, an E-g - T-2g symmetry analysis based on the orbital decomposition of the exchange parameters in bcc Fe leads to the conclusion that the nearest-neighbor exchange parameters related to the T-2g orbitals are essentially Heisenberg-like: they do not depend on the spin configuration, and can, in this case, be mapped onto a Heisenberg spin model even in extreme noncollinear cases.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-340146 (URN)10.1103/PhysRevB.96.144413 (DOI)000412699400003 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2012.0031Knut and Alice Wallenberg Foundation, 2013.0020EU, FP7, Seventh Framework Programme, 600382
Available from: 2018-01-26 Created: 2018-01-26 Last updated: 2018-04-07Bibliographically approved
Kvashnin, Y. O., Cardias, R., Szilva, A., Di Marco, I., Katsnelson, M. I., Lichtenstein, A. I., . . . Eriksson, O. (2016). Microscopic Origin of Heisenberg and Non-Heisenberg Exchange Interactions in Ferromagnetic bcc Fe. Physical Review Letters, 116(21), Article ID 217202.
Open this publication in new window or tab >>Microscopic Origin of Heisenberg and Non-Heisenberg Exchange Interactions in Ferromagnetic bcc Fe
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2016 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 116, no 21, article id 217202Article in journal (Refereed) Published
Abstract [en]

By means of first principles calculations, we investigate the nature of exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the basic electronic structure reveals a drastic difference between the 3d orbitals of E-g and T-2g symmetries. The latter ones define the shape of the Fermi surface, while the former ones form weakly interacting impurity levels. We demonstrate that, as a result of this, in Fe the T-2g orbitals participate in exchange interactions, which are only weakly dependent on the configuration of the spin moments and thus can be classified as Heisenberg-like. These couplings are shown to be driven by Fermi surface nesting. In contrast, for the E-g states, the Heisenberg picture breaks down since the corresponding contribution to the exchange interactions is shown to strongly depend on the reference state they are extracted from. Our analysis of the nearest-neighbor coupling indicates that the interactions among E-g states are mainly proportional to the corresponding hopping integral and thus can be attributed to be of double-exchange origin. By making a comparison to other magnetic transition metals, we put the results of bcc Fe into context and argue that iron has a unique behavior when it comes to magnetic exchange interactions.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:uu:diva-298894 (URN)10.1103/PhysRevLett.116.217202 (DOI)000376628800010 ()27284671 (PubMedID)
Funder
EU, European Research Council, 338957 FEMTO/NANOSwedish Research CouncileSSENCE - An eScience CollaborationKnut and Alice Wallenberg Foundation, 2012.0031 2013.0020
Available from: 2016-07-12 Created: 2016-07-12 Last updated: 2018-04-07Bibliographically approved
Sandels, C., Brodén, D., Widén, J., Nordström, L. & Andersson, E. (2016). Modeling office building consumer load with a combined physical and behavioral approach: Simulation and validation. Applied Energy, 162, 472-485
Open this publication in new window or tab >>Modeling office building consumer load with a combined physical and behavioral approach: Simulation and validation
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2016 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 162, p. 472-485Article in journal (Refereed) Published
Abstract [en]

Due to an expanding integration of renewable energy resources in the power systems, mismatches between electricity supply and demand will increase. A promising solution to deal with these issues is Demand Response (DR), which incentives end-users to be flexible in their electricity consumption. This paper presents a bottom up simulation model that generates office building electricity load profiles representative for Northern Europe. The model connects behavioral aspects of office workers with electricity usage from appliances, and physical representation of the building to describe the energy use of the Heating Ventilation and Air Conditioning systems. To validate the model, simulations are performed with respect to two data sets, and compared with real load measurements. The validation shows that the model can reproduce load profiles with reasonable accuracy for both data sets. With the presented model approach, it is possible to define simple portfolio office building models which subsequently can be used for simulation and analysis of DR in the power systems.

Keywords
Office electricity demand, Office building design and architecture, HVAC system, Markov-chain models, Demand response, Holistic
National Category
Energy Systems
Identifiers
urn:nbn:se:uu:diva-269037 (URN)10.1016/j.apenergy.2015.10.141 (DOI)000367631000043 ()
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Available from: 2015-12-12 Created: 2015-12-12 Last updated: 2017-12-01Bibliographically approved
Lejaeghere, K., Bihlmayer, G., Björkman, T., Blaha, P., Blügel, S., Blum, V., . . . Cottenier, S. (2016). Reproducibility in density functional theory calculations of solids.. Science, 351(6280), 1415-1422
Open this publication in new window or tab >>Reproducibility in density functional theory calculations of solids.
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2016 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 351, no 6280, p. 1415-1422Article in journal (Refereed) Published
Abstract [en]

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements.

National Category
Materials Chemistry Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-283301 (URN)10.1126/science.aad3000 (DOI)000372756200038 ()27013736 (PubMedID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2013.0020, 2012.0031EU, FP7, Seventh Framework Programme, 329386eSSENCE - An eScience Collaboration
Available from: 2016-04-12 Created: 2016-04-12 Last updated: 2017-11-30Bibliographically approved
Panda, S. K., Bhowal, S., Li, Y., Ganguly, S., Valenti, R., Nordström, L. & Dasgupta, I. (2015). Electronic structure and spin-orbit driven magnetism in d(4.5) insulator Ba3YIr2O9. Physical Review B. Condensed Matter and Materials Physics, 92(18), Article ID 180403.
Open this publication in new window or tab >>Electronic structure and spin-orbit driven magnetism in d(4.5) insulator Ba3YIr2O9
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 18, article id 180403Article in journal (Refereed) Published
Abstract [en]

We have carried out a detailed first-principles study of a d(4.5) quaternary iridate Ba3YIr2O9 both in its 6H-perovskite- type ambient pressure (AP) phase and also for the high pressure (HP) cubic phase. Our analysis reveals that the AP phase belongs to the intermediate spin-orbit coupling (SOC) regime. This is further supported by the identification of the spin moment as the primary order parameter (POP) obtained from a magnetic multipolar analysis. The large t(2g) bandwidth renormalizes the strength of SOC and the Ir intersite exchange interaction dominates resulting in long-range magnetic order in the AP phase. In addition to SOC and Hubbard U, strong intradimer coupling is found to be crucial for the realization of the insulating state. At high pressure (HP) the system undergoes a structural transformation to the disordered cubic phase. In sharp contrast to the AP phase, the calculated exchange interactions in the HP phase are found to be much weaker and SOC dominates leading to a quantum spin-orbital liquid (SOL) state.

National Category
Physical Sciences Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-268406 (URN)10.1103/PhysRevB.92.180403 (DOI)000364014600002 ()
Funder
German Research Foundation (DFG), TR/SFB 49Swedish Research Council
Available from: 2015-12-09 Created: 2015-12-04 Last updated: 2017-12-01Bibliographically approved
Ganguly, S., Granas, O. & Nordstrom, L. (2015). Nontrivial order parameter in Sr2IrO4. Physical Review B. Condensed Matter and Materials Physics, 91(2), 020404
Open this publication in new window or tab >>Nontrivial order parameter in Sr2IrO4
2015 (English)In: 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) Published
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.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:uu:diva-246367 (URN)10.1103/PhysRevB.91.020404 (DOI)000348473700001 ()
Available from: 2015-03-06 Created: 2015-03-05 Last updated: 2017-12-04Bibliographically approved
Grånäs, O., Di Marco, I., Eriksson, O., Nordström, L. & Etz, C. (2014). Electronic structure, cohesive properties, and magnetism of SrRuO3. Physical Review B. Condensed Matter and Materials Physics, 90(16), 165130
Open this publication in new window or tab >>Electronic structure, cohesive properties, and magnetism of SrRuO3
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2014 (English)In: 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) Published
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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-238564 (URN)10.1103/PhysRevB.90.165130 (DOI)000343944400003 ()
Available from: 2014-12-17 Created: 2014-12-14 Last updated: 2017-12-05Bibliographically approved
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