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BETA
Nordström, Lars
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Publications (10 of 127) Show all publications
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, 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
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, 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
Available from: 2017-10-10 Created: 2017-10-10 Last updated: 2017-10-10Bibliographically 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, 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: 2017-11-28Bibliographically 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, 472-485 p.Article 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.

Keyword
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, 1415-1422 p.Article 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, 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, 020404- p.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, 165130- p.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
Szilva, A., Costa, M. J., Bergman, A., Szunyogh, L., Nordström, L. & Eriksson, O. (2013). Interatomic Exchange Interactions for Finite-Temperature Magnetism and Nonequilibrium Spin Dynamics. Physical Review Letters, 111(12), 127204.
Open this publication in new window or tab >>Interatomic Exchange Interactions for Finite-Temperature Magnetism and Nonequilibrium Spin Dynamics
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2013 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, no 12, 127204- p.Article in journal (Refereed) Published
Abstract [en]

We derive ab inito exchange parameters for general noncollinear magnetic configurations, in terms of a multiple scattering formalism. We show that the general exchange formula has an anisotropiclike term even in the absence of spin-orbit coupling, and that this term is large, for instance, for collinear configuration in bcc Fe, whereas for fcc Ni it is quite small. We demonstrate that keeping this term leads to what one should consider a biquadratic effective spin Hamiltonian even in the case of collinear arrangement. In noncollinear systems this term results in new tensor elements that are important for exchange interactions at finite temperatures, but they have less importance at low temperature. To illustrate our results in practice, we calculate for bcc Fe magnon spectra obtained from configuration-dependent exchange parameters, where the configurations are determined by finite-temperature effects. Our theory results in the same quantitative results as the finite-temperature neutron scattering experiments.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-209476 (URN)10.1103/PhysRevLett.111.127204 (DOI)000324542900016 ()
Available from: 2013-10-24 Created: 2013-10-21 Last updated: 2017-12-06Bibliographically approved
Spaldin, N. A., Fechner, M., Bousquet, E., Balatsky, A. & Nordström, L. (2013). Monopole-based formalism for the diagonal magnetoelectric response. Physical Review B. Condensed Matter and Materials Physics, 88(9), 094429.
Open this publication in new window or tab >>Monopole-based formalism for the diagonal magnetoelectric response
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2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 9, 094429- p.Article in journal (Refereed) Published
Abstract [en]

We develop the formalism of the macroscopic monopolization-that is, the magnetoelectric monopole moment per unit volume-in periodic solids, and discuss its relationship to the magnetoelectric effect. For the series of lithium transition metal phosphate compounds, we use first-principles density functional theory to calculate the contributions to the macroscopic monopolization from the global distribution of magnetic moments within the unit cell, as well as from the distribution of magnetization around the atomic sites. We find one example within the series (LiMnPO4) that shows a macroscopic monopolization corresponding to a ferromonopolar ordering consistent with its diagonal magnetoelectric response. The other members of the series (LiMPO4, with M = Co, Fe, and Ni) have zero net monopolization but have antiferromonopolar orderings that should lead to q-dependent diagonal magnetoelectric effects.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-209472 (URN)10.1103/PhysRevB.88.094429 (DOI)000324689900004 ()
Available from: 2013-10-24 Created: 2013-10-21 Last updated: 2017-12-06Bibliographically approved
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