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  • 1. Bergman, Anders
    et al.
    Hellsvik, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Bessarab, Pavel F.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Univ Iceland, Iceland.
    Delin, Anna
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala Univ, Sweden.
    Spin relaxation signature of colossal magnetic anisotropy in platinum atomic chains2016Ingår i: Scientific Reports, E-ISSN 2045-2322, Vol. 6, artikel-id 36872Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent experimental data demonstrate emerging magnetic order in platinum atomically thin nanowires. Furthermore, an unusual form of magnetic anisotropy-colossal magnetic anisotropy (CMA)-was earlier predicted to exist in atomically thin platinum nanowires. Using spin dynamics simulations based on first-principles calculations, we here explore the spin dynamics of atomically thin platinum wires to reveal the spin relaxation signature of colossal magnetic anisotropy, comparing it with other types of anisotropy such as uniaxial magnetic anisotropy (UMA). We find that the CMA alters the spin relaxation process distinctly and, most importantly, causes a large speed-up of the magnetic relaxation compared to uniaxial magnetic anisotropy. The magnetic behavior of the nanowire exhibiting CMA should be possible to identify experimentally at the nanosecond time scale for temperatures below 5 K. This time-scale is accessible in e.g., soft x-ray free electron laser experiments.

  • 2.
    Bergqvist, Lars
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Eriksson, Olle
    Bergman, Anders
    Hellsvik, Johan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kondenserade materiens teori.
    Atomistic Spin Dynamics: Foundations and Applications2017Bok (Övrigt vetenskapligt)
  • 3. Fransson, J.
    et al.
    Thonig, D.
    Bessarab, P. F.
    Bhattacharjee, S.
    Hellsvik, Johan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kondenserade materiens teori.
    Nordström, L.
    Microscopic theory for coupled atomistic magnetization and lattice dynamics2017Ingår i: Physical Review Materials, E-ISSN 2475-9953, Vol. 1, nr 7, artikel-id 074404Artikel i tidskrift (Refereegranskat)
    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.

  • 4.
    Hasan, Md Nur
    et al.
    Department of Chemical and Biological Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector-III, SaltLake, Kolkata 700 106, India.
    Bharati, Ritadip
    School of Physical Sciences, National Institute of Science Education and Research HBNI, Jatni - 752050, Odisha, India.
    Hellsvik, Johan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kondenserade materiens teori. KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Delin, Anna
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Pal, Samir Kumar
    Department of Chemical and Biological Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector-III, SaltLake, Kolkata 700 106, India.
    Bergman, Anders
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Sharma, Shivalika
    Asia Pacific Center for Theoretical Physics, Pohang 37673, Republic of Korea.
    Di Marco, Igor
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden; Asia Pacific Center for Theoretical Physics, Pohang 37673, Republic of Korea; Department of Physics, Pohang University of Science and Technology, Pohang 37673, Republic of Korea.
    Pereiro, Manuel
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Thunström, Patrik
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Oppeneer, Peter M.
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Eriksson, Olle
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Karmakar, Debjani
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden; Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India.
    Magnetism in A V3Sb5 (A=Cs, Rb, and K): Origin and Consequences for the Strongly Correlated Phases2023Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 131, nr 19, artikel-id 196702Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The V-based kagome systems AV3Sb5 (A=Cs, Rb, and K) are unique by virtue of the intricate interplay of nontrivial electronic structure, topology, and intriguing fermiology, rendering them to be a playground of many mutually dependent exotic phases like charge-order and superconductivity. Despite numerous recent studies, the interconnection of magnetism and other complex collective phenomena in these systems has yet not arrived at any conclusion. Using first-principles tools, we demonstrate that their electronic structures, complex fermiologies and phonon dispersions are strongly influenced by the interplay of dynamic electron correlations, nontrivial spin-polarization and spin-orbit coupling. An investigation of the first-principles-derived intersite magnetic exchanges with the complementary analysis of q dependence of the electronic response functions and the electron-phonon coupling indicate that the system conforms as a frustrated spin cluster, where the occurrence of the charge-order phase is intimately related to the mechanism of electron-phonon coupling, rather than the Fermi-surface nesting.

  • 5.
    Hellsvik, Johan
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Mentink, J. H.
    Lorenzana, J.
    Ultrafast cooling and heating scenarios for the laser-induced phase transition in CuO2016Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 94, nr 14, artikel-id 144435Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The multiferroic compound CuO exhibits low-temperature magnetic properties similar to antiferromagnetic iron oxides, while the electronic properties have much more in common with the high-Tc cuprate superconductors. This suggests novel possibilities for the ultrafast optical excitation of magnetism. On the basis of atomistic spin dynamics simulations, we study the effect of phonon-assisted multimagnon absorption and photodoping on the spin dynamics in the vicinity of the first-order phase transition from collinear to spin-spiral magnetic order. Similar as in recent experiments, we find that for both excitations the phase transition can proceed on the picosecond timescale. Interestingly, however, these excitation mechanisms display very distinct dynamics. Following photodoping, the spin system first cools down on subpicosecond time scales, which we explain as an ultrafast magnetocaloric effect. Opposed to this, following phonon-assisted multimagnon excitation, the spin systems rapidly heats up and subsequently evolves to the noncollinear phase even under the influence of isotropic exchange interactions alone. 

  • 6.
    Hellsvik, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kondenserade materiens teori.
    Perez, Roberto Diaz
    Nordita SU.
    Geilhufe, Matthias
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Månsson, Martin
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Spin wave excitations of magnetic metalorganic materials2020Ingår i: Physical Review Materials, E-ISSN 2475-9953, Vol. 4, nr 2, artikel-id 024409Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Organic Materials Database (OMDB) is an open database hosting about 22 000 electronic band structures, density of states, and other properties for stable and previously synthesized three-dimensional organic crystals. The web interface of the OMDB offers various search tools for the identification of novel functional materials such as band structure pattern matching and density of states similarity search. In this work, the OMDB is extended to include magnetic excitation properties. For inelastic neutron scattering, we focus on the dynamic structure factor S(q, omega) which contains information on the excitation modes of the material. We introduce a new dataset containing atomic magnetic moments and Heisenberg exchange parameters for which we calculate the spin wave spectra and dynamic structure factor with linear spin wave theory and atomistic spin dynamics. We thus develop the materials informatics tools to identify novel functional organic and metalorganic magnets.

  • 7.
    Hellsvik, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kondenserade materiens teori. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Thonig, Danny
    Modin, Klas
    Iusan, Diana
    Bergman, Anders
    Eriksson, Olle
    Bergqvist, Lars
    Delin, Anna
    General method for atomistic spin-lattice dynamics with first-principles accuracy2019Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, nr 10, artikel-id 104302Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a computationally efficient and general first-principles based method for spin-lattice simulations for solids and clusters. The method is based on a coupling of atomistic spin dynamics and molecular dynamics simulations, expressed through a spin-lattice Hamiltonian, where the bilinear magnetic term is expanded up to second order in displacement. The effect of first-order spin-lattice coupling on the magnon and phonon dispersion in bcc Fe is reported as an example, and we observe good agreement with previous simulations. We also illustrate the coupled spin-lattice dynamics method on a more conceptual level, by exploring dissipation-free spin and lattice motion of small magnetic clusters (a dimer, trimer, and tetramer). The method discussed here opens the door for a quantitative description and understanding of the microscopic origin of many fundamental phenomena of contemporary interest, such as ultrafast demagnetization, magnetocalorics, and spincaloritronics.

  • 8.
    John Mukkattukavil, Deepak
    et al.
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Hellsvik, Johan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kondenserade materiens teori. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Ghosh, Anirudha
    Lund Univ, MAX Lab 4, SE-22100 Lund, Sweden..
    Chatzigeorgiou, Evanthia
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Nocerino, Elisabetta
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Wang, Qisi
    Univ Zurich, Phys Inst, Winterthurerstr 190, CH-8057 Zurich, Switzerland..
    von Arx, Karin
    Univ Zurich, Phys Inst, Winterthurerstr 190, CH-8057 Zurich, Switzerland..
    Huang, Shih-Wen
    Lund Univ, MAX Lab 4, SE-22100 Lund, Sweden..
    Ekholm, Victor
    Lund Univ, MAX Lab 4, SE-22100 Lund, Sweden..
    Hossain, Zakir
    Indian Inst Technol, Dept Phys, Kanpur 208016, Uttar Pradesh, India..
    Thamizhavel, Arumugum
    Tata Inst Fundamental Res, DCMPMS, Mumbai 400005, Maharashtra, India..
    Chang, Johan
    Univ Zurich, Phys Inst, Winterthurerstr 190, CH-8057 Zurich, Switzerland..
    Månsson, Martin
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Nordstrom, Lars
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Sathe, Conny
    Lund Univ, MAX Lab 4, SE-22100 Lund, Sweden..
    Agaker, Marcus
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden.;Lund Univ, MAX Lab 4, SE-22100 Lund, Sweden..
    Rubensson, Jan-Erik
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Sassa, Yasmine
    Chalmers Univ Technol, Dept Phys, SE-41296 Gothenburg, Sweden..
    Resonant inelastic soft x-ray scattering on LaPt2Si22022Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 34, nr 32, s. 324003-, artikel-id 324003Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    X-ray absorption and resonant inelastic x-ray scattering spectra of LaPt2Si2 single crystal at the Si 2p and La 4d edges are presented. The data are interpreted in terms of density functional theory, showing that the Si spectra can be described in terms of Si s and d local partial density of states (LPDOS), and the La spectra are due to quasi-atomic local 4f excitations. Calculations show that Pt d-LPDOS dominates the occupied states, and a sharp localized La f state is found in the unoccupied states, in line with the observations.

  • 9. Kadas, K.
    et al.
    Iusan, D.
    Hellsvik, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Cedervall, J.
    Berastegui, P.
    Sahlberg, M.
    Jansson, U.
    Eriksson, O.
    AlM2B2 (M = Cr, Mn, Fe, Co, Ni): a group of nanolaminated materials2017Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, nr 15, artikel-id 155402Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Combining theory with experiments, we study the phase stability, elastic properties, electronic structure and hardness of layered ternary borides AlCr2B2, AlMn2B2, AlFe2B2, AlCo2B2, and AlNi2B2. We find that the first three borides of this series are stable phases, while AlCo2B2 and AlNi2B2 are metastable. We show that the elasticity increases in the boride series, and predict that AlCr2B2, AlMn2B2, and AlFe2B2 are more brittle, while AlCo2B2 and AlNi2B2 are more ductile. We propose that the elasticity of AlFe2B2 can be improved by alloying it with cobalt or nickel, or a combination of them. We present evidence that these ternary borides represent nanolaminated systems. Based on SEM measurements, we demonstrate that they exhibit the delamination phenomena, which leads to a reduced hardness compared to transition metal mono-and diborides. We discuss the background of delamination by analyzing chemical bonding and theoretical work of separation in these borides.

  • 10.
    Karmakar, Debjani
    et al.
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden; Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India.
    Pereiro, Manuel
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Hasan, Md Nur
    Department of Chemical and Biological Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector-III, SaltLake, Kolkata 700 106, India.
    Bharati, Ritadip
    School of Physical Sciences, National Institute of Science Education and Research, Homi Bhabha National Institute (HBNI), Jatni, 752050 Odisha, India.
    Hellsvik, Johan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kondenserade materiens teori. KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Delin, Anna
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Pal, Samir Kumar
    Department of Chemical and Biological Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector-III, SaltLake, Kolkata 700 106, India.
    Bergman, Anders
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Sharma, Shivalika
    Asia Pacific Center for Theoretical Physics, Pohang 37673, Republic of Korea.
    Di Marco, Igor
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden; Asia Pacific Center for Theoretical Physics, Pohang 37673, Republic of Korea; Department of Physics, Pohang University of Science and Technology, Pohang 37673, Republic of Korea.
    Thunström, Patrik
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Oppeneer, Peter M.
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Eriksson, Olle
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Magnetism in A V3Sb5 (A=Cs, Rb, K): Complex landscape of dynamical magnetic textures2023Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 108, nr 17, artikel-id 174413Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have investigated the dynamical magnetic properties of the V-based kagome stibnite compounds by combining the ab initio-extracted magnetic parameters of a spin-Hamiltonian, like inter-site exchange parameters, magnetocrystalline anisotropy and site projected magnetic moments, with full-fledged simulations of atomistic spin- dynamics. Our calculations reveal that, in addition to a ferromagnetic order along the [001] direction, the system hosts a complex landscape of magnetic configurations comprised of commensurate and incommensurate spin spirals along the [010] direction. The presence of such chiral magnetic textures may be the key toward solving the mystery about the origin of the experimentally observed inherent breaking of the C6 rotational, mirror, and the time-reversal symmetry.

  • 11.
    Nocerino, Elisabetta
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Stuhr, U.
    Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.
    San Lorenzo, I.
    Nanoscience Center, Niels Bohr Institute, University of Copenhagen, Noerre Alle 59, DK-2100 Copenhagen O, Denmark, Nørre Allé 59; Department of Applied Science and Technology, Politecnico di Torino, corso Duca degli abruzzi 24 10129 Torino, Italy.
    Mazza, F.
    Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, 1040 Vienna, Austria, Wiedner Hauptstraße 8–10.
    Mazzone, D. G.
    Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.
    Hellsvik, Johan
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Hasegawa, S.
    Neutron Science Laboratory, Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
    Asai, S.
    Neutron Science Laboratory, Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
    Masuda, T.
    Neutron Science Laboratory, Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan, Chiba; Trans-scale Quantum Science Institute, The University of Tokyo, Tokyo 113-0033, Japan; Institute of Materials Structure Science, High Energy Accelerator Research Organization, Ibaraki 305-0801, Japan.
    Itoh, S.
    Institute of Materials Structure Science, High Energy Accelerator Research Organization, Ibaraki 305-0801, Japan.
    Minelli, A.
    Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, United Kingdom.
    Hossain, Z.
    Department of Physics, Indian Institute of Technology, Kanpur 208016, India.
    Thamizhavel, A.
    DCMPMS, Tata Institute of Fundamental Research, Mumbai 400005, India.
    Lefmann, K.
    Nanoscience Center, Niels Bohr Institute, University of Copenhagen, Noerre Alle 59, DK-2100 Copenhagen O, Denmark.
    Sassa, Y.
    Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden.
    Månsson, Martin
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Q-dependent electron-phonon coupling induced phonon softening and non-conventional critical behavior in the CDW superconductor LaPt2Si22023Ingår i: Journal of Science: Advanced Materials and Devices, ISSN 2468-2284, Vol. 8, nr 4, artikel-id 100621Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper reports the first experimental observation of phonons and their softening on single crystalline LaPt2Si2 via inelastic neutron scattering. From the temperature dependence of the phonon frequency in close proximity to the charge density wave (CDW) q-vector, we obtain a CDW transition temperature of TCDW = 230 K and a critical exponent β = 0.28 ± 0.03. This value is suggestive of a non-conventional critical behavior for the CDW phase transition in LaPt2Si2, compatible with a scenario of CDW discommensuration (DC). The DC would be caused by the existence of two CDWs in this material, propagating separately in the non equivalent (Si1–Pt2–Si1) and (Pt1–Si2–Pt1) layers, respectively, with transition temperatures TCDW−1 = 230 K and TCDW−2 = 110 K. A strong q-dependence of the electron-phonon coupling has been identified as the driving mechanism for the CDW transition at TCDW−1 = 230 K while a CDW with 3-dimensional character, and Fermi surface quasi-nesting as a driving mechanism, is suggested for the transition at TCDW−2 = 110 K. Our results clarify some aspects of the CDW transition in LaPt2Si2 which have been so far misinterpreted by both theoretical predictions and experimental observations and give direct insight into its actual temperature dependence.

  • 12.
    Nocerino, Elisabetta
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Stuhr, Uwe
    San Lorenzo, Irene
    Mazza, Federico
    Mazzone, Daniel
    Hellsvik, Johan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kondenserade materiens teori.
    Hasegawa, Shunsuke
    Asai, Shinichiro
    Masuda, Takatsugu
    Minelli, Arianna
    Hossain, Zakir
    Thamizhavel, Arumugam
    Lefmann, Kim
    Sassa, Yasmine
    Månsson, Martin
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Q-dependent Phonon Renormalization and Non-Conventional Critical Behavior in the CDW Superconductor LaPt2Si2Manuskript (preprint) (Övrigt vetenskapligt)
  • 13.
    Olsthoorn, Bart
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Hellsvik, Johan
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm, Sweden.;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Univ Connecticut, Dept Phys, Storrs, CT 06269 USA..
    Finding hidden order in spin models with persistent homology2020Ingår i: Physical Review Research, E-ISSN 2643-1564, Vol. 2, nr 4, artikel-id 043308Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Persistent homology (PH) is a relatively new field in applied mathematics that studies the components and shapes of discrete data. In this paper, we demonstrate that PH can be used as a universal framework to identify phases of classical spins on a lattice. This demonstration includes hidden order such as spin-nematic ordering and spin liquids. By converting a small number of spin configurations to barcodes we obtain a descriptive picture of configuration space. Using dimensionality reduction to reduce the barcode space to color space leads to a visualization of the phase diagram.

  • 14.
    Pan, Fan
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Chico, Jonathan
    Hellsvik, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Delin, Anna
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Bergman, Anders
    Bergqvist, Lars
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Systematic study of magnetodynamic properties at finite temperatures in doped permalloy from first-principles calculations2016Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, nr 21, artikel-id 214410Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    By means of first-principles calculations, we have systematically investigated how the magnetodynamic properties Gilbert damping, magnetization, and exchange stiffness are affected when permalloy (Py) (Fe0.19Ni0.81) is doped with 4d or 5d transition metal impurities. We find that the trends in the Gilbert damping can be understood from relatively few basic parameters such as the density of states at the Fermi level, the spin-orbit coupling, and the impurity concentration. The temperature dependence of the Gilbert damping is found to be very weak which we relate to the lack of intraband transitions in alloys. Doping with 4d elements has no major impact on the studied Gilbert damping, apart from diluting the host. However, the 5d elements have a profound effect on the damping and allow it to be tuned over a large interval while maintaining the magnetization and exchange stiffness. As regards the spin stiffness, doping with early transition metals results in considerable softening, whereas late transition metals have a minor impact. Our result agree well with earlier calculations where available. In comparison to experiments, the computed Gilbert damping appears slightly underestimated, whereas the spin stiffness shows a general good agreement.

  • 15. Paul, S.
    et al.
    Iuşan, D.
    Thunström, P.
    Kvashnin, Y. O.
    Hellsvik, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Pereiro, M.
    Delin, Anna
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Knut, R.
    Phuyal, D.
    Lindblad, A.
    Karis, O.
    Sanyal, B.
    Eriksson, O.
    Investigation of the spectral properties and magnetism of BiFeO3 by dynamical mean-field theory2018Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, nr 12, artikel-id 125120Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using the local density approximation plus dynamical mean-field theory (LDA+DMFT), we have computed the valence-band photoelectron spectra and magnetic excitation spectra of BiFeO3, one of the most studied multiferroics. Within the DMFT approach, the local impurity problem is tackled by the exact diagonalization solver. The solution of the impurity problem within the LDA+DMFT method for the paramagnetic and magnetically ordered phases produces result in agreement with the experimental data on electronic and magnetic structures. For comparison, we also present results obtained by the LDA+U approach which is commonly used to compute the physical properties of this compound. Our LDA+DMFT derived electronic spectra match adequately with the experimental hard x-ray photoelectron spectroscopy and resonant photoelectron spectroscopy for Fe 3d states, whereas the LDA+U method fails to capture the general features of the measured spectra. This indicates the importance of accurately incorporating the dynamical aspect of electronic correlation among Fe 3d orbitals to reproduce the experimental excitation spectra. Specifically, the LDA+DMFT derived density of states exhibits a significant amount of Fe 3d states at the position of Bi lone pairs, implying that the latter are not alone in the spectral scenario. This fact might modify our interpretation about the origin of ferroelectric polarization in this material. Our study demonstrates that the combination of orbital cross sections for the constituent elements and broadening schemes for the spectral functions are crucial to explain the detailed structures of the experimental electronic spectra. Our magnetic excitation spectra computed from the LDA+DMFT result conform well with the inelastic neutron scattering data.

  • 16.
    Sadhukhan, Banasree
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Bergman, Anders
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Kvashnin, Yaroslav O.
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Hellsvik, Johan
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC. NORDITA, Hannes Alfvens Vag 12, SE-10691 Stockholm, Sweden.;Stockholm Univ, Hannes Alfvens Vag 12, SE-10691 Stockholm, Sweden..
    Delin, Anna
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Spin-lattice couplings in two-dimensional CrI3 from first-principles computations2022Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, nr 10, artikel-id 104418Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Since thermal fluctuations become more important as dimensions shrink, it is expected that low-dimensional magnets are more sensitive to atomic displacement and phonons than bulk systems are. Here we present a fully relativistic first-principles study on the spin-lattice coupling, i.e., how the magnetic interactions depend on atomic displacement, of the prototypical two-dimensional ferromagnet CrI3. We extract an effective measure of the spin-lattice coupling in CrI3, which is up to ten times larger than what is found for bcc Fe. The magnetic exchange interactions, including Heisenberg and relativistic Dzyaloshinskii-Moriya interactions, are sensitive both to the in-plane motion of Cr atoms and out-of-plane motion of ligand atoms. We find that significant magnetic pair interactions change sign from ferromagnetic (FM) to antiferromagnetic (AFM) for atomic displacements larger than 0.16 (0.18) angstrom for Cr (I) atoms. We explain the observed strong spin-lattice coupling by analyzing the orbital decomposition of isotropic exchange interactions, involving different crystal-field-split Cr-3d orbitals. The competition between the AFM t(2g)-t(2g) and FM t(2g)-e(g) contributions depends on the bond angle formed by Cr and I atoms as well as Cr-Cr distance. In particular, if a Cr atom is displaced, the FM-AFM sign changes when the I-Cr-I bond angle approaches 90 degrees. The obtained spin-lattice coupling constants, along with the microscopic orbital analysis, can act as a guiding principle for further studies of the thermodynamic properties and combined magnon-phonon excitations in two-dimensional magnets.

  • 17.
    Shirinyan, Albert A.
    et al.
    TTMO Univ, St Petersburg 197101, Russia..
    Kozin, Valerii K.
    TTMO Univ, St Petersburg 197101, Russia.;Univ Iceland, Sci Inst, Dunhagi 3, IS-107 Reykjavik, Iceland..
    Hellsvik, Johan
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Pereiro, Manuel
    Uppsala Univ, Dept Phys & Astron, Mat Theory Div, Box 516, SE-75120 Uppsala, Sweden..
    Eriksson, Olle
    Uppsala Univ, Dept Phys & Astron, Mat Theory Div, Box 516, SE-75120 Uppsala, Sweden.;Orebro Univ, Sch Sci & Technol, SE-70182 Orebro, Sweden..
    Yudin, Dmitry
    Skolkovo Inst Sci & Technol, Deep Quantum Labs, Moscow 121205, Russia..
    Self-organizing maps as a method for detecting phase transitions and phase identification2019Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, nr 4, artikel-id 041108Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Originating from image recognition, methods of machine learning allow for effective feature extraction and dimensionality reduction in multidimensional datasets, thereby providing an extraordinary tool to deal with classical and quantum models in many-body physics. In this study, we employ a specific unsupervised machine learning technique-self-organizing maps-to create a low-dimensional representation of microscopic states, relevant for macroscopic phase identification and detecting phase transitions. We explore the properties of spin Hamiltonians of two archetype model systems: a two-dimensional Heisenberg ferromagnet and a three-dimensional crystal, Fe in the body-centered-cubic structure. The method of self-organizing maps, which is known to conserve connectivity of the initial dataset, is compared to the cumulant method theory and is shown to be as accurate while being computationally more efficient in determining a phase transition temperature. We argue that the method proposed here can be applied to explore a broad class of second-order phase-transition systems, not only magnetic systems but also, for example, order-disorder transitions in alloys.

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