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  • 251.
    Brumboiu, Iulia Emilia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ericsson, Leif
    Hansson, Rickard
    Moons, Ellen
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    The influence of oxygen adsorption on the NEXAFS and core-level XPS spectra of the C-60 derivative PCBM2015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 5, article id 054306Article in journal (Refereed)
    Abstract [en]

    Fullerenes have been a main focus of scientific research since their discovery due to the interesting possible applications in various fields like organic photovoltaics (OPVs). In particular, the derivative [6,6]-phenyl-C-60-butyric acid methyl ester (PCBM) is currently one of the most popular choices due to its higher solubility in organic solvents compared to unsubstituted C-60. One of the central issues in the field of OPVs is device stability, since modules undergo deterioration (losses in efficiency, open circuit voltage, and short circuit current) during operation. In the case of fullerenes, several possibilities have been proposed, including dimerization, oxidation, and impurity related deterioration. We have studied by means of density functional theory the possibility of oxygen adsorption on the C-60 molecular moiety of PCBM. The aim is to provide guidelines for near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) measurements which can probe the presence of atomic or molecular oxygen on the fullerene cage. By analysing several configurations of PCBM with one or more adsorbed oxygen atoms, we show that a joint core level XPS and O1s NEXAFS investigation could be effectively used not only to confirm oxygen adsorption but also to pinpoint the bonding configuration and the nature of the adsorbate.

  • 252.
    Brumboiu, Iulia Emilia
    et al.
    KTH Royal Inst Technol, Dept Theoret Chem & Biol, S-10691 Stockholm, Sweden;Korea Adv Inst Sci & Technol, Dept Chem, Daejeon 34141, South Korea.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Norman, Patrick
    KTH Royal Inst Technol, Dept Theoret Chem & Biol, S-10691 Stockholm, Sweden.
    Atomic photoionization cross sections beyond the electric dipole approximation2019In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 150, no 4, article id 044306Article in journal (Refereed)
    Abstract [en]

    A methodology is developed to compute photoionization cross sections beyond the electric dipole approximation from response theory, using Gaussian type orbitals and plane waves for the initial and final states, respectively. The methodology is applied to compute photoionization cross sections of atoms and ions from the first four rows of the periodic table. Analyzing the error due to the plane wave description of the photoelectron, we find kinetic energy and concomitant photon energy thresholds above which the plane wave approximation becomes applicable. The correction introduced by going beyond the electric dipole approximation increases with photon energy and depends on the spatial extension of the initial state. In general, the corrections are below 10% for most elements, at a photon energy reaching up to 12 keV.

  • 253.
    Brumboiu, Iulia Emilia
    et al.
    KTH Royal Inst Technol, Dept Theoret Chem & Biol, S-10691 Stockholm, Sweden;Korea Adv Inst Sci & Technol, Dept Chem, Daejeon 34141, South Korea.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Orebro Univ, Sch Sci & Technol, S-70182 Orebro, Sweden.
    Norman, Patrick
    KTH Royal Inst Technol, Dept Theoret Chem & Biol, S-10691 Stockholm, Sweden.
    Photoelectron Spectroscopy of Molecules Beyond the Electric Dipole Approximation2019In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 15, no 10, p. 5483-5494Article in journal (Refereed)
    Abstract [en]

    A methodology implemented to compute photoionization cross sections beyond the electric dipole approximation using Gaussian type orbitals for the initial state and plane waves for the final state is applied to molecules of various sizes. The molecular photoionization cross sections computed for valence molecular orbitals as a function of photon energy present oscillations due to the wave-like nature of both the outgoing photoelectron and of the incoming photon. These oscillations are damped by rotational and vibrational averaging or by performing a k-point summation for the solid state case. For core orbitals, the corrections introduced by going beyond the electric dipole approximation are comparable to the atomic case. For valence orbitals, nondipole corrections to the total photoinization cross sections can reach up to 20% at photon energies above 1 keV. The corrections to the differential cross sections calculated at the magic angle are larger, reaching values between 30% and 50% for all molecules included. Our findings demonstrate that photoelectron spectroscopy, especially angle-resolved, on, e.g., molecules and clusters on surfaces, using high photon energies, must be accompanied by theories that go beyond the electric dipole approximation.

  • 254.
    Brumboiu, Iulia Emilia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Haldar, Soumyajyoti
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Luder, Johann
    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.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Influence of Electron Correlation on the Electronic Structure and Magnetism of Transition-Metal Phthalocyanines2016In: Journal of Chemical Theory and Computation, Vol. 12, no 4, p. 1772-1785Article in journal (Refereed)
    Abstract [en]

    There exists an extensive literature on the electronic structure of transition-metal phthalocyanines (TMPcs), either as single molecules or adsorbed on surfaces, where explicit intra-atomic Coulomb interactions of the strongly correlated orbitals are included in the form of a Hubbard U term. The choice of U is, to a large extent, based solely on previous values reported in the literature for similar systems. Here, we provide a systematic analysis of the influence of electron correlation on the electronic structure and magnetism of several TMPcs (MnPc, FePc, CoPc, NiPc, and CuPc). By comparing calculated results to valence-band photoelectron spectroscopy measurements, and by determining the Hubbard term from linear response, we show that the choice of U is not as straightforward and can be different for each different TMPc. This, in turn, highlights the importance of individually estimating the value of U for each system before performing any further analysis and shows how this value can influence the final results.

  • 255.
    Brumboiu, Iulia Emilia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Theoret Chem & Biol, S-10691 Stockholm, Sweden; Korea Adv Inst Sci & Technol, Dept Chem, Daejeon 34141, South Korea.
    Haldar, Soumyajyoti
    Lüder, Johann
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ligand effects on the linear response Hubbard U: The case of transition metal phthalocyanines2019In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 123, no 14, p. 3214-3222Article in journal (Refereed)
    Abstract [en]

    It is established that density functional theory (DFT) + U is a better choice compared to DFT for describing the correlated electron metal center in organometallics. The value of the Hubbard U parameter may be determined from linear response, either by considering the response of the metal site alone or by additionally considering the response of other sites in the compound. We analyze here in detail the influence of ligand shells of increasing size on the U parameter calculated from the linear response for five transition metal phthalocyanines. We show that the calculated multiple-site U is larger than the single-site U by as much as 1 eV and the ligand atoms that are mainly responsible for this difference are the isoindole nitrogen atoms directly bonded to the central metal atom. This suggests that a different U value may be required for computations of chemisorbed molecules compared to physisorbed and gas-phase cases.

  • 256.
    Brumboiu, Iulia Emilia
    et al.
    Royal Inst Technol, Sch Biotechnol, Dept Theoret Chem & Biol, S-10691 Stockholm, Sweden..
    Prokopiou, Georgia
    Weizmann Inst Sci, Dept Mat & Interfaces, IL-76100 Rehovot, Israel..
    Kronik, Leeor
    Weizmann Inst Sci, Dept Mat & Interfaces, IL-76100 Rehovot, Israel..
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Valence electronic structure of cobalt phthalocyanine from an optimally tuned range-separated hybrid functional2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 147, no 4, article id 044301Article in journal (Refereed)
    Abstract [en]

    We analyse the valence electronic structure of cobalt phthalocyanine (CoPc) by means of optimally tuning a range-separated hybrid functional. The tuning is performed by modifying both the amount of short-range exact exchange (alpha) included in the hybrid functional and the range-separation parameter (gamma), with two strategies employed for finding the optimal gamma for each alpha. The influence of these two parameters on the structural, electronic, and magnetic properties of CoPc is thoroughly investigated. The electronic structure is found to be very sensitive to the amount and range in which the exact exchange is included. The electronic structure obtained using the optimal parameters is compared to gas-phase photo-electron data and GWcalculations, with the unoccupied states additionally compared with inverse photo-electron spectroscopy measurements. The calculated spectrum with tuned gamma, determined for the optimal value of alpha = 0.1, yields a very good agreement with both experimental results and with GW calculations that well-reproduce the experimental data.

  • 257.
    Brumboiu, Iulia Emilia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Totani, Roberta
    de Simone, Monica
    Coreno, Marcello
    Grazioli, Cesare
    Lozzi, Luca
    Herper, Heike C
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    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.
    Puglia, Carla
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Elucidating the 3d Electronic Configuration in Manganese Phthalocyanine2014In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 118, no 5, p. 927-932Article in journal (Refereed)
    Abstract [en]

    To shed light on the metal 3d electronic structure of manganese phthalocyanine, so far controversial, we performed photoelectron measurements both in the gas phase and as thin film. With the purpose of explaining the experimental results, three different electronic configurations close in energy to one another were studied by means of density functional theory. The comparison between the calculated valence band density of states and the measured spectra revealed that in the gas phase the molecules exhibit a mixed electronic configuration, while in the thin film, manganese phthalocyanine finds itself in the theoretically computed ground state, namely, the b2g1eg3a1g1b1g0 electronic configuration.

  • 258.
    Bryngelsson, Maria
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
    Relations between the performance of a coated cutting tool and the composition and properties of the wear resistant coating: A study including first principles modeling, mechanical properties and technological testing2013Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This thesis work was performed at AB Sandvik Coromant and aimed to enhance the knowledge about the relationships between the performance of TiN and TiAlN-coated cutting tools in metal turning and their mechanical and chemical properties.

    Measurements of coating material properties and turning wear tests in annealed tool steel Sverker 21, stainless steel 316L, grey cast iron V314 and nodular cast iron SS0727 were performed. The cutting temperatures were estimated from FEM-simulations. To find the dominant wear mechanism and identify the properties that are most important for the resistance against that particular wear, a correlation analysis was performed together with a wear study using LOM, SEM and EDS.

    The results show that relations between cutting performance and mechanical properties and/or composition of the coatings can be established. The FEM-simulations suggested that the peak tool temperature was highest, ~750°C, for turning in 316L and lowest for turning in Sverker 21, ~300°C. Turning in cast iron resulted in temperatures around 500-550°C.

    A mechanism for the growth of the crater on inserts tested in stainless steel 316L is proposed. Wear due to thermo-mechanical load and adhesion are believed to be the dominating wear mechanisms. The performance of the tool showed a high correlation to the composition of the coatings, with a decreased tool life for higher Al-contents. The reason for this might lie in an increased brittleness of these coatings, accelerating formation of lateral cracks above the crater. Calculated ratios of bulk and shear modulus suggests an increased brittleness for higher Al-contents. A higher tendency to stick to the work piece material might also contribute to a decrease in tool life. An Increased Al-content could also drive the formation of c-AlN to h-AlN, causing even higher wear rates. The coatings with higher substrate bias showed an enhanced performance, even though the crack pattern was worsened for these variants. The reason for the enhanced performance seen for these variants might instead originate in an enhanced adhesion to the substrate.

    In the flank wear resistance test in Sverker 21 the Al-content proved to be important, with an improved performance for higher Al-contents. In contrast to the test in 316L, a change in bias or hardness had no effect on the performance in this test. Scratch patterns on the flank supports that an abrasive wear mechanism is present, but no correlation between hardness and tool life could be obtained. Either some other material property than hardness is of importance for the abrasive resistance in this test, or another wear mechanism, occurring simultaneously with abrasion, is the wear rate deciding.

    The second part of this thesis work was to evaluate the ability of a quantum mechanical computational method, density functional theory, to predict material properties. The method predicts the lattice parameters and bulk moduli in excellent agreement with experimental values. The method also well predicts other elastic properties, with results consistent with reference values. There seems to be a constant shift of about 50-100 GPa between the calculated elastic modulus and the experimentally measured values, probably originating in contributions from grain boundaries, texture, stresses and defects present in the real coatings, and possibly also in errors in the experimental method due to an influence from the substrate. The calculated hardness values did not follow the trend of an increased hardness for TiAlN compared to TiN, which is seen in experiments.

  • 259. Buendia, E.
    et al.
    Galvez, F. J.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sarsa, A.
    Dynamical correlation effects in the transition probability: A study for the atoms Li to Ar2012In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 548, p. 1-6Article in journal (Refereed)
    Abstract [en]

    Explicitly correlated wave functions constructed as a Jastrow correlation factor times a model function have been obtained for the ground and the first excited state of opposite parity of the atoms Li to Ar. Single and restricted multi-configuration model functions are employed. Line strength, oscillator strength and transition probabilities have been obtained. An analysis of the different correlation mechanisms considered, single particle excitations and dynamical correlations, on these quantities is carried out. All calculations have been done by using Variational Monte Carlo, except when no Jastrow is involved where calculations have been performed using the Optimized Effective Potential method. (C) 2012 Elsevier B. V. All rights reserved.

  • 260. Buendia, E.
    et al.
    Galvez, F. J.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sarsa, A.
    Explicitly correlated wave functions for atoms and singly charged ions from Li through Sr: Variational and Diffusion Monte Carlo results2014In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 615, p. 21-25Article in journal (Refereed)
    Abstract [en]

    Total energies calculated from explicitly correlated wave functions for the ground state of the atoms Li to Sr and their singly charged anions and cations are obtained. Accurate all electron, non-relativistic Variational and Diffusion Monte Carlo energies are reported. The quality of the results, when comparing with exact estimations and experimental electron affinities and ionization potential is similar for all of the atoms studied. The parameterization of the explicitly correlated wave functions for all of the atomic systems studied is provided.

  • 261. Buendia, E.
    et al.
    Galvez, F. J.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sarsa, A.
    Quantum Monte Carlo ionization potential and electron affinity for transition metal atoms2013In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 559, p. 12-17Article in journal (Refereed)
    Abstract [en]

    Non-relativistic all-electron Quantum Monte Carlo ground state energies of the neutral atoms K to Zn and positive and negative ions are calculated starting from explicitly correlated wave functions. The accuracy obtained for these atoms and ions in the fourth period is similar to that reached for those in the second and third periods. For the atoms and ions for which the 4s-4p near degeneracy effect can be important a restricted multi-configuration expansion has been employed. Ionization potentials and electron affinities have been calculated showing a good agreement with the experimental values.

  • 262.
    Butorin, Sergei
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Kvashnina, K. O.
    European Synchrotron, Grenoble, France; HZDR, Inst Resource Dresden, Germany.
    Klintenberg, Mattias
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kavcic, M.
    Joief Stefan Inst, Ljubljana, Slovenia.
    Zitnik, Mihael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Joief Stefan Inst, Ljubljana, Slovenia; Univ Ljubljana, Fac Math & Phys, Ljubljana, Slovenia.
    Bucar, K.
    Joief Stefan Inst, Ljubljana, Slovenia.
    Gougeon, P.
    Univ Rennes 1, INSA Rennes, Inst Sci Chim Rennes, Rennes, France.
    Gall, P.
    Univ Rennes 1, INSA Rennes, Inst Sci Chim Rennes, Rennes, France.
    Candolfi, C.
    Univ Lorraine, CNRS, UMR 7198, Inst Jean Lamour, Nancy, France; Univ Lorraine, UMR 7198 CNRS, Inst Jean Lamour, Nancy, France.
    Lenoir, B.
    Univ Lorraine, CNRS, UMR 7198, Inst Jean Lamour, Nancy, France; Univ Lorraine, UMR 7198 CNRS, Inst Jean Lamour, Nancy, France.
    Effect of Ag Doping on Electronic Structure of Cluster Compounds AgxMo9Se11 (x = 3.4, 3.9)2018In: ACS APPLIED ENERGY MATERIALS, ISSN 2574-0962, Vol. 1, no 8, p. 4032-4039Article in journal (Refereed)
    Abstract [en]

    The electronic structure of AgxMo9Se11 as a potential material for thermoelectric applications was studied using high-energy-resolution fluorescence-detection X-ray absorption spectroscopy (HERFD-XAS) and the resonant inelastic X-ray scattering (RIXS) technique. The experiments were supported by first-principle calculations using density functional theory (DFT). The analysis of obtained spectra indicate the presence of subvalent (less than 1+) Ag in AgxMo9Se11. The advanced HERFD-XAS measurements allowed us to resolve the contribution of the electronic states at the Fermi level of AgxMo9Se11 and to monitor its dependence on the x value. A comparison of the experimental data with the results of the DFT calculations suggests the importance of the Ag2-type sites with the shortest Ag–Se distance for affecting the properties of AgxMo9Se11.

  • 263.
    Butorin, Sergei M.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Modin, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Vegelius, Johan R.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Suzuki, Michi-To
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics. RIKEN Ctr Emergent Matter Sci, 2-1 Hirosawa, Wako, Saitama 3510198, Japan..
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Andersson, David A.
    Los Alamos Natl Lab, Div Mat Sci & Technol, Mat Sci Radiat & Dynam Extremes, Los Alamos, NM 87545 USA..
    Shuh, David K.
    Lawrence Berkeley Natl Lab, Div Chem Sci, MS 70A1150,One Cyclotron Rd, Berkeley, CA 94720 USA..
    Local Symmetry Effects in Actinide 4f X-ray Absorption in Oxides2016In: Analytical Chemistry, ISSN 0003-2700, E-ISSN 1520-6882, Vol. 88, no 8, p. 4169-4173Article in journal (Refereed)
    Abstract [en]

    A systematic X-ray absorption study at actinide N-6,(7) (4f -> 6d transitions) edges was performed for light-actinide oxides including data obtained for the first time for NpO2, PuO2 and UO3. The measurements were supported by ab initio calculations based on local-density approximation. with added 5f-5f Coulomb interaction (LDA+U). Improved energy resolution compared to common experiments at actinide L-2,L-3 (2p -> 6d transitions) edges allowed us to resolve the major structures of the 13 unoccupied 6d density of states (DOS) and estimate the crystal-field 116) splittings in the 6d shell directly from the spectra of light-actinide dioxides. The measurements demonstrated an enhanced sensitivity of the N-6,N-7, spectral shape to changes in the compound crystal structure. For nonstoichiometric NpO2-x, the filling of the entire band gap with Np 6d states was observed thus supporting a phase coexistence of Np metal and stoichiometric NpO2 which is in agreement with the tentative Np-O phase diagram.

  • 264.
    Buzzi, Michele
    et al.
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Makita, Mikako
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Howald, Ludovic
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Kleibert, Armin
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Vodungbo, Boris
    Ecole Polytech, CNRS, UMR 7639, Lab Opt Appl,ENSTA ParisTech, Chemin Huniere, F-91761 Palaiseau, France.;UPMC Univ Paris 06, Sorbonne Univ, CNRS, LCPMR, F-75005 Paris, France..
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Raabe, Jörg
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Jaouen, Nicolas
    Synchrotron SOLEIL, BP 48, F-91192 Gif Sur Yvette, France..
    Redlin, Harald
    DESY, HASYLAB, Notkestr 85, D-22607 Hamburg, Germany..
    Tiedtke, Kai
    DESY, HASYLAB, Notkestr 85, D-22607 Hamburg, Germany..
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    David, Christian
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Nolting, Frithjof
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Luning, Jan
    UPMC Univ Paris 06, Sorbonne Univ, CNRS, LCPMR, F-75005 Paris, France.;Synchrotron SOLEIL, BP 48, F-91192 Gif Sur Yvette, France..
    Single-shot Monitoring of Ultrafast Processes via X-ray Streaking at a Free Electron Laser2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 7253Article in journal (Refereed)
    Abstract [en]

    The advent of x-ray free electron lasers has extended the unique capabilities of resonant x-ray spectroscopy techniques to ultrafast time scales. Here, we report on a novel experimental method that allows retrieving with a single x-ray pulse the time evolution of an ultrafast process, not only at a few discrete time delays, but continuously over an extended time window. We used a single x-ray pulse to resolve the laser-induced ultrafast demagnetisation dynamics in a thin cobalt film over a time window of about 1.6 ps with an excellent signal to noise ratio. From one representative single shot measurement we extract a spin relaxation time of (130 +/- 30) fs with an average value, based on 193 single shot events of (113 +/- 20) fs. These results are limited by the achieved experimental time resolution of 120 fs, and both values are in excellent agreement with previous results and theoretical modelling. More generally, this new experimental approach to ultrafast x-ray spectroscopy paves the way to the study of non-repetitive processes that cannot be investigated using traditional repetitive pump-probe schemes.

  • 265.
    Bylin, Johan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Analysis of a spin-particle tunnelling junction2015Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    This project is to analyse the energy spectrum of a spin-molecular tunnelling junction which is composed of molecules confined between two conducting metallic leads. By letting a continuous stream of electrons flow across the junction the molecules can interact with each other with an indirect force called exchange interaction, and those exchange interactions which are of interest in this project are described by models called the Heisenberg, the Ising and the Dzyaloshinski-Moriya models. The molecules may also interact with themselves anisotropically and if there is an external magnetic field there will be yet another kind of interaction. The goal of this project is to see the contribution of all these spin interactions and how they affect the resulting energy spectrum under the variation of the junction's chemical potential and the voltage bias between the leads.

    This project is of a theoretical nature where the models are analytically adapted for a restricted scenario and is later on numerically calculated to be graphed and analysed. The models are restricted to only consider molecules of same spin and approximated to only consider interactions between closest neighbouring molecules.

    The results are composed of both analytically derived energy values and numerical computed values which show that there exists certain critical values of the variation parameters which naturally splits the ground state of the system and that the self-interaction may further split the degenerate ground state. A possible outcome of these result could be the possibility to control the magnetic order of the molecules to either be locked in an anti-ferromagnetic configuration or be easily mixed by manipulating the chemical potential or the voltage bias.

  • 266.
    Bylin, Johan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Best practice of extracting magnetocaloric properties in magnetic simulations2019Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In this thesis, a numerical study of simulating and computing the magnetocaloric properties of magnetic materials is presented. The main objective was to deduce the optimal procedure to obtain the isothermal change in entropy of magnetic systems, by evaluating two different formulas of entropy extraction, one relying on the magnetization of the material and the other on the magnet's heat capacity. The magnetic systems were simulated using two different Monte Carlo algorithms, the Metropolis and Wang-Landau procedures. The two entropy methods proved to be comparably similar to one another. Both approaches produced reliable and consistent results, though finite size effects could occur if the simulated system became too small. Erroneous fluctuations that invalidated the results did not seem stem from discrepancies between the entropy methods but mainly from the computation of the heat capacity itself. Accurate determination of the heat capacity via an internal energy derivative generated excellent results, while a heat capacity obtained from a variance formula of the internal energy rendered the extracted entropy unusable. The results acquired from the Metropolis algorithm were consistent, accurate and dependable, while all of those produced via the Wang-Landau method exhibited intrinsic fluctuations of varying severity. The Wang-Landau method also proved to be computationally ineffective compared to the Metropolis algorithm, rendering the method not suitable for magnetic simulations of this type.

  • 267.
    Béland, Laurent Karim
    et al.
    MIT, MIT CNRS Joint Lab, Multiscale Mat Sci Energy & Environm.;Oak Ridge Natl Lab, Mat Sci & Technol Div. .
    Tamm, Artur
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Univ Tartu, Inst Technol, Intelligent Mat & Syst Lab..
    Mu, Sai
    Oak Ridge Natl Lab, Mat Sci & Technol Div..
    Samolyuk, German D.
    Oak Ridge Natl Lab, Mat Sci & Technol Div..
    Osetsky, Yuri N.
    Oak Ridge Natl Lab, Mat Sci & Technol Div..
    Aabloo, Alvo
    Univ Tartu, Inst Technol, Intelligent Mat & Syst Lab..
    Klintenberg, Mattias
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Caro, Alfredo
    Los Alamos Natl Lab, Mat Sci & Technol Div..
    Stoller, Roger E.
    Oak Ridge Natl Lab, Mat Sci & Technol Div..
    Accurate classical short-range forces for the study of collision cascades in Fe-Ni-Cr2017In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 219, p. 11-19Article in journal (Refereed)
    Abstract [en]

    The predictive power of a classical molecular dynamics simulation is largely determined by the physical validity of its underlying empirical potential. In the case of high-energy collision cascades, it was recently shown that correctly modeling interactions at short distances is necessary to accurately predict primary damage production. An ab initio based framework is introduced for modifying an existing embedded atom method FeNiCr potential to handle these short-range interactions. Density functional theory is used to calculate the energetics of two atoms approaching each other, embedded in the alloy, and to calculate the equation of state of the alloy as it is compressed. The pairwise terms and the embedding terms of the potential are modified in accordance with the ab initio results. Using this reparametrized potential, collision cascades are performed in Ni50Fe50, Ni80Cr20 and Ni33Fe33Cr33. The simulations reveal that alloying Ni and NiCr to Fe reduces primary damage production, in agreement with some previous calculations. Alloying Ni and NiFe to Cr does not reduce primary damage production, in contradiction with previous calculations.

  • 268.
    Callini, Elsa
    et al.
    EPFL Valais Wallis Swiss Fed Inst Technol, LMER, Rue Ind 17, CH-1950 Sion, Switzerland.;Swiss Fed Lab Mat Sci & Technol, Empa, Ueberlandstr 129, CH-8600 Dubendorf, Switzerland..
    Aguey-Zinsou, Kondo-Francois
    Univ New S Wales, Sch Chem Engn, MERLin Grp, Sydney, NSW 2052, Australia..
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, S-10044 Stockholm, Sweden..
    Ramon Ares, Jose
    Univ Autonoma Madrid, Fac Ciencias, Dpto Fis Mat, Grp Mire, E-28049 Madrid, Spain..
    Bals, Sara
    Univ Antwerp, Dept Phys, EMAT, Groenenborgerlaan 171, B-2020 Antwerp, Belgium..
    Biliskov, Nikola
    Rudjer Boskovic Inst, Bijenicka Cesta 54, Zagreb 10000, Croatia..
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Charalambopoulou, Georgia
    Natl Ctr Sci Res Demokritos, Athens 15341, Greece..
    Chaudhary, Anna-Lisa
    Helmholtz Zentrum Geesthacht, Inst Mat Res, Dept Nanotechnol, Max Planck Str 1, Geesthacht, Germany..
    Cuevas, Fermin
    UPEC, CNRS, ICMPE, UMR7182, 2-8 Rue Henri Dunant, F-94320 Thiais, France..
    Dam, Bernard
    Delft Univ Technol, Chem Engn, Julianalaan 136, NL-2628 BL Delft, Netherlands..
    de Jongh, Petra
    Univ Utrecht, Debye Inst Nanomat Sci, Inorgan Chem & Catalysis, Univ Weg 99, NL-3584 CG Utrecht, Netherlands..
    Dornheim, Martin
    Helmholtz Zentrum Geesthacht, Inst Mat Res, Dept Nanotechnol, Max Planck Str 1, Geesthacht, Germany..
    Filinchuk, Yaroslav
    Catholic Univ Louvain, Inst Condensed Matter & Nanosci, B-1348 Louvain, Belgium..
    Novakovic, Jasmina Grbovic
    Univ Belgrade, Vinca Inst Nucl Sci, POB 522, Belgrade 1000, Serbia..
    Hirscher, Michael
    Max Planck Inst Intelligent Syst Stuttgart, Heisenbergstr 3, D-70569 Stuttgart, Germany..
    Jensen, Torben R.
    Aarhus Univ, Dept Chem, Ctr Mat Crystallog, Langelandsgade 140, DK-8000 Aarhus C, Denmark.;Aarhus Univ, iNANO, Langelandsgade 140, DK-8000 Aarhus C, Denmark..
    Jensen, Peter Bjerre
    Tech Univ Denmark, Dept Energy Convers & Storage, Fysikvej,Bldg 309, DK-2800 Lyngby, Denmark..
    Novakovic, Nikola
    Univ Belgrade, Vinca Inst Nucl Sci, POB 522, Belgrade 1000, Serbia..
    Lai, Qiwen
    Univ New S Wales, Sch Chem Engn, MERLin Grp, Sydney, NSW 2052, Australia..
    Leardini, Fabrice
    Univ Autonoma Madrid, Fac Ciencias, Dpto Fis Mat, Grp Mire, E-28049 Madrid, Spain..
    Gattia, Daniele Mirabile
    Res Ctr Casaccia, ENEA, Dept Phys Methods & Mat, Via Anguillarese 301, Rome, Italy..
    Pasquini, Luca
    Alma Mater Studiorum Univ Bologna, Dept Phys & Astron, I-40127 Bologna, Italy..
    Steriotis, Theodore
    Natl Ctr Sci Res Demokritos, Athens 15341, Greece..
    Turner, Stuart
    Univ Antwerp, Dept Phys, EMAT, Groenenborgerlaan 171, B-2020 Antwerp, Belgium..
    Vegge, Tejs
    Tech Univ Denmark, Dept Energy Convers & Storage, Fysikvej,Bldg 309, DK-2800 Lyngby, Denmark..
    Zuttel, Andreas
    EPFL Valais Wallis Swiss Fed Inst Technol, LMER, Rue Ind 17, CH-1950 Sion, Switzerland..
    Montone, Amelia
    Res Ctr Casaccia, ENEA, Dept Phys Methods & Mat, Via Anguillarese 301, Rome, Italy..
    Nanostructured materials for solid-state hydrogen storage: A review of the achievement of COST Action MP11032016In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 41, no 32, p. 14404-14428Article in journal (Refereed)
    Abstract [en]

    In the framework of the European Cooperation in Science and Technology (COST) Action MP1103 Nanostructured Materials for Solid-State Hydrogen Storage were synthesized, characterized and modeled. This Action dealt with the state of the art of energy storage and set up a competitive and coordinated network capable to define new and unexplored ways for Solid State Hydrogen Storage by innovative and interdisciplinary research within the European Research Area. An important number of new compounds have been synthesized: metal hydrides, complex hydrides, metal halide ammines and amidoboranes. Tuning the structure from bulk to thin film, nanoparticles and nanoconfined composites improved the hydrogen sorption properties and opened the perspective to new technological applications. Direct imaging of the hydrogenation reactions and in situ measurements under operando conditions have been carried out in these studies. Computational screening methods allowed the prediction of suitable compounds for hydrogen storage and the modeling of the hydrogen sorption reactions on mono-, bi-, and three-dimensional systems. This manuscript presents a review of the main achievements of this Action.

  • 269. Calmels, Lionel
    et al.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Atomic site sensitivity of the energy loss magnetic chiral dichroic spectra of complex oxides2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 109, no 7, p. 07D328-Article in journal (Refereed)
    Abstract [en]

    The quantitative analysis of magnetic oxide core level spectra can become complicated when the magnetic atoms are located at several nonequivalent atomic sites in the crystal. This is, for instance, the case for Fe atoms in magnetite, which are located in tetrahedral and octahedral atomic sites; in this case, the x-ray magnetic circular dichroic (XMCD) spectra recorded at the L-2,L-3 edge of Fe contain contributions from the different nonequivalent atomic sites, which unfortunately cannot be separated. Energy loss magnetic chiral dichroic (EMCD) spectra are the transmission electron microscope analogies of the XMCD spectra. One of the important differences between these two techniques of magnetic analysis is that EMCD uses a fast electron beam instead of polarized light. The fast electrons behave like Bloch states in the sample, and the fine structure of the EMCD spectra is strongly influenced by channeling and dynamical diffraction effects. These effects can be adjusted by changing the experimental configuration. We use theoretical calculations, which include dynamical diffraction effects and in which electronic transitions are treated in the atomic multiplet formalism, to show that the relative weight of the Fe atoms in different nonequivalent atomic sites can be changed by a proper choice of the position of the detector and of the magnetite sample orientation and thickness. We conclude that EMCD spectra could be used to isolate the magnetic contribution of atoms in each of the nonequivalent atomic sites, which would not be possible with XMCD techniques. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3556769]

  • 270. Calmels, Lionel
    et al.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Momentum-resolved EELS and EMCD spectra from the atomic multiplet theory: application to magnetite2010In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 110, no 8, p. 1042-1045Article in journal (Refereed)
    Abstract [en]

    While the energy loss near edge structures of metallic crystals can be calculated with a good accuracy using density functional theory based codes, core-level spectra of transition metal oxides show pronounced multiplet effects which are better described by atomic multiplet codes. We describe the formalism which allows to calculate momentum-resolved electron energy loss spectra in the electric dipole approximation from the atomic multiplet theory, and we apply this formalism to the calculation of energy loss magnetic chiral dichroic spectra of magnetic transition metal oxides. Explicit results are given for magnetite Fe3O4.

  • 271. Cao, Peiyu
    et al.
    Ni, Xiaodong
    Tian, Fuyang
    Varga, Lajos K.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab initio study of AlxMoNbTiV high-entropy alloys2015In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 27, no 7, article id 075401Article in journal (Refereed)
    Abstract [en]

    The AlxMoNbTiV (x = 0-1.5) high-entropy alloys (HEAs) adopt a single solid-solution phase, having the body centered cubic (bcc) crystal structure. Here we employ the ab initio exact muffin-tin orbitals method in combination with the coherent potential approximation to investigate the equilibrium volume, elastic constants, and polycrystalline elastic moduli of AlxMoNbTiV HEAs. A comparison between the ab initio and experimental equilibrium volumes demonstrates the validity and accuracy of the present approach. Our results indicate that Al addition decreases the thermodynamic stability of the bcc structure with respect to face-centered cubic and hexagonal close packed lattices. For the elastically isotropic Al0.4MoNbTiV HEAs, the valence electron concentration (VEC) is about 4.82, which is slightly different from VEC similar to 4.72 obtained for the isotropic Gum metals and refractory-HEAs.

  • 272.
    Cardias Alves de Almeida, Ramon
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Electronic structure and exchange interactions from ab initio theory: New perspectives and implementations2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis, the magnetic properties of several materials were investigated using first principle calculations. The ab initio method named real space linear muffin-tin orbitals atomic sphere approximation (RS-LMTO-ASA) was used to calculate the electronic structure and magnetic properties of bulk systems, surface and nanostructures adsorbed on surfaces.

    We have implemented new features in the RS-LMTO-ASA method, such as the calculation of (a) Bloch Spectral Function (BSF), (b) orbital resolved Jij and (c) Dzyaloshinskii-Moriya interaction (DMI). Using (a), we have shown that one can calculate the dispersion relation for bulk systems using a real space method. Furthermore, the dispersion relation was revealed to be existent even for finite one-dimensional structures, such as the Mn chain on Au(111) and Ag(111) surfaces. With (b), we have investigated the orbital resolved exchange coupling parameter Jij for 3d metals. It is demonstrated that the nearest neighbor (NN) interaction for bcc Fe has intriguing behavior, however, the contribution coming from the T2g orbitals favours the anti-ferromagnetic coupling behavior. Moreover, the Fermi surface for bcc Fe is formed mostly by the T2g orbitals and these are shown to be highly Heisenberg-like, i.e. do not depend significantly on the magnetic configuration. Later, the same approach was used to study other transition metals, such as Cr, Mn, Co and Ni. In the end, we have presented the results obtained with the implementation (c). Our results have shown the large dependence of the DMI values, both the strength and direction, with respect to which magnetic configuration they are calculated from. We argue that, for the investigated systems, the non-collinearity induces currents (spin and charge) that will influence directly the DMI vectors.

    List of papers
    1. Magnetic and electronic structure of Mn nanostructures on Ag(111) and Au(111)
    Open this publication in new window or tab >>Magnetic and electronic structure of Mn nanostructures on Ag(111) and Au(111)
    Show others...
    2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 1, article id 014438Article in journal (Refereed) Published
    Abstract [en]

    We present results of the electronic and magnetic structure of Mn nanowires adsorbed on Ag(111) and Au(111) surfaces. For finite Mn nanowires on Ag(111) and Au(111) surfaces, our ab initio results show that the large difference between the spin-orbit splitting of these two surfaces leads to completely different magnetic configurations. The magnetic ordering for Mn nanowires adsorbed on Ag(111) is governed by the strong exchange interaction between Mn adatoms. For Mn nano-chains on Au(111), the competition between Heisenberg and Dzyaloshinskii-Moriya interactions leads to a complex magnetic structure of the clusters considered here. Among the more conspicuous results we note a spin-spiral helical type for the nanowire with seven atoms, and a complex magnetic configuration incommensurate with the substrate lattice for a double-sized Mn wire. The effect of the structural relaxation is also investigated, showing sensitivity of the exchange interactions to the bond distance to the substrate. We also demonstrate that small changes in the band filling of these Mn chains results in drastically different changes of the interatomic exchange. Finally, we show that dispersion of the electronic energy spectrum is possible even in nanostructures with bounded spatial extension.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-279629 (URN)10.1103/PhysRevB.93.014438 (DOI)000369216100007 ()
    Funder
    Swedish Research CouncileSSENCE - An eScience CollaborationKnut and Alice Wallenberg Foundation
    Available from: 2016-03-08 Created: 2016-03-02 Last updated: 2018-04-07Bibliographically approved
    2. The Bethe-Slater curve revisited; new insights from electronic structure theory
    Open this publication in new window or tab >>The Bethe-Slater curve revisited; new insights from electronic structure theory
    Show others...
    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
    3. Microscopic Origin of Heisenberg and Non-Heisenberg Exchange Interactions in Ferromagnetic bcc Fe
    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
    4. Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature
    Open this publication in new window or tab >>Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature
    Show others...
    2017 (English)In: Physical review B, ISSN 2469-9950, Vol. 95, no 21, article id 214417Article in journal (Refereed) Published
    Abstract [en]

    Temperature-dependent magnetic experiments such as pump-probe measurements generated by a pulsed laser have become a crucial technique for switching the magnetization in the picosecond time scale. Apart from having practical implications on the magnetic storage technology, the research field of ultrafast magnetization poses also fundamental physical questions. To correctly describe the time evolution of the atomic magnetic moments under the influence of a temperature-dependent laser pulse, it remains crucial to know if the magnetic material under investigation has magnetic excitation spectrum that is more or less dependent on the magnetic configuration, e.g., as reflected by the temperature dependence of the exchange interactions. In this paper, we demonstrate from first-principles theory that the magnetic excitation spectra in Co in fcc, bcc, and hcp structures are nearly identical in a wide range of noncollinear magnetic configurations. This is a curious result of a balance between the size of the magnetic moments and the strength of the Heisenberg exchange interactions, that in themselves vary with configuration, but put together in an effective spin Hamiltonian results in a configuration-independent effective model. We have used such a Hamiltonian, together with ab initio calculated damping parameters, to investigate the magnon dispersion relationship as well as ultrafast magnetization dynamics of Co and Co-rich CoMn alloys.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-308321 (URN)10.1103/PhysRevB.95.214417 (DOI)000404015400003 ()
    Funder
    Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2013.0020, 2012.0031StandUp
    Available from: 2016-11-24 Created: 2016-11-24 Last updated: 2018-04-07Bibliographically approved
    5. Theory of noncollinear interactions beyond Heisenberg exchange: Applications to bcc Fe
    Open this publication in new window or tab >>Theory of noncollinear interactions beyond Heisenberg exchange: Applications to bcc Fe
    Show others...
    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
    6. First-principles Dzyaloshinskii-Moryia interaction in a non-collinear framework
    Open this publication in new window or tab >>First-principles Dzyaloshinskii-Moryia interaction in a non-collinear framework
    Show others...
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    Including dierent terms in the spin-Hamiltonian, we have derived an expression for theDzyaloshinskii-Moryia vector interaction (DMI) where all the three components of the vector canbe calculated independently of the magnetic conguration. Here, we have chosen the Cr triangulartrimer on Au(111) and Mn triangular trimes on Ag(111) and Au(111) surfaces to study the implementationof the derived DMI into the RS-LMTO-ASA method. Our results have shown thatthe DMI value (module and direction) is drastically dierent for collinear and non-collinear states.Based on relation between the spin and charge currents owing in the system and the non-collinearmagnetic conguration of the triangular trimer, we argued that the drastic change between the DMIcalculated considering a collinear and a non-collinear magnetic conguration can be explained bythe mechanism behind the spin and charge currents owing through the atoms when the spins arealigned in a non-collinear fashion.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-347808 (URN)
    Available from: 2018-04-07 Created: 2018-04-07 Last updated: 2018-04-07
  • 273.
    Cardias, R.
    et al.
    Fed Univ Para, Fac Fis, BR-66059 Belem, PA, Brazil..
    Bezerra-Neto, M. M.
    Fed Univ Para, Fac Fis, BR-66059 Belem, PA, Brazil.;Univ Fed Oeste Para, Inst Engn & Geociencias, Santarem, PA, Brazil..
    Ribeiro, M. S.
    Fed Univ Para, Fac Fis, BR-66059 Belem, PA, Brazil.;Inst Fed Para, Campus Belem, Belem, PA, Brazil..
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Szilva, Attila
    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.
    Klautau, A. B.
    Fed Univ Para, Fac Fis, BR-66059 Belem, PA, Brazil..
    Magnetic and electronic structure of Mn nanostructures on Ag(111) and Au(111)2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 1, article id 014438Article in journal (Refereed)
    Abstract [en]

    We present results of the electronic and magnetic structure of Mn nanowires adsorbed on Ag(111) and Au(111) surfaces. For finite Mn nanowires on Ag(111) and Au(111) surfaces, our ab initio results show that the large difference between the spin-orbit splitting of these two surfaces leads to completely different magnetic configurations. The magnetic ordering for Mn nanowires adsorbed on Ag(111) is governed by the strong exchange interaction between Mn adatoms. For Mn nano-chains on Au(111), the competition between Heisenberg and Dzyaloshinskii-Moriya interactions leads to a complex magnetic structure of the clusters considered here. Among the more conspicuous results we note a spin-spiral helical type for the nanowire with seven atoms, and a complex magnetic configuration incommensurate with the substrate lattice for a double-sized Mn wire. The effect of the structural relaxation is also investigated, showing sensitivity of the exchange interactions to the bond distance to the substrate. We also demonstrate that small changes in the band filling of these Mn chains results in drastically different changes of the interatomic exchange. Finally, we show that dispersion of the electronic energy spectrum is possible even in nanostructures with bounded spatial extension.

  • 274.
    Cardias, R.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Fed Univ Para, Fac Fis, Belem, PA, Brazil.
    Szilva, Attila
    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.
    Di Marco, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Katsnelson, M. I.
    Radboud Univ Nijmegen, Inst Mol & Mat, Heijendaalseweg 135, NL-6525 AJ Nijmegen, Netherlands.;Ural Fed Univ, Theoret Phys & Appl Math Dept, Mira Str 19, Ekaterinburg 620002, Russia..
    Lichtenstein, A. I.
    Ural Fed Univ, Theoret Phys & Appl Math Dept, Mira Str 19, Ekaterinburg 620002, Russia.;Univ Hamburg, Inst Theoret Phys, Jungiusstrasse 9, D-20355 Hamburg, Germany..
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Klautau, A. B.
    Fed Univ Para, Fac Fis, Belem, PA, Brazil..
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kvashnin, Yaroslav O.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    The Bethe-Slater curve revisited; new insights from electronic structure theory2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 4058Article in journal (Refereed)
    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.

  • 275.
    Cardias, Ramon
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bezerra-Neto, M. M.
    Instituto de Engenharia e Geociências, Universidade Federal do Oeste do Pará, Santarém, PA, Brazil.
    Ribeiro, M. S.
    Instituto Federal do Pará, Campus Belém, PA, Brazil.
    Szilva, Attila
    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.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kvashnin, Yaroslav
    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. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Klautau, A. B.
    Faculdade de Física, Universidade Federal do Pará, Belém, Pará, Brazil.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    First-principles Dzyaloshinskii-Moryia interaction in a non-collinear frameworkManuscript (preprint) (Other academic)
    Abstract [en]

    Including dierent terms in the spin-Hamiltonian, we have derived an expression for theDzyaloshinskii-Moryia vector interaction (DMI) where all the three components of the vector canbe calculated independently of the magnetic conguration. Here, we have chosen the Cr triangulartrimer on Au(111) and Mn triangular trimes on Ag(111) and Au(111) surfaces to study the implementationof the derived DMI into the RS-LMTO-ASA method. Our results have shown thatthe DMI value (module and direction) is drastically dierent for collinear and non-collinear states.Based on relation between the spin and charge currents owing in the system and the non-collinearmagnetic conguration of the triangular trimer, we argued that the drastic change between the DMIcalculated considering a collinear and a non-collinear magnetic conguration can be explained bythe mechanism behind the spin and charge currents owing through the atoms when the spins arealigned in a non-collinear fashion.

  • 276.
    Cardoch, Sebastian
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Studying Atomic Vibrations by Transmission Electron Microscopy2016Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    We employ the empirical potential function Airebo to computationally model free-standing Carbon-12 graphene in a classical setting. Our objective is to measure the mean square displacement (MSD) of atoms in the system for different average temperatures and Carbon-13 isotope concentrations. From results of the MSD we aim to develop a technique that employs Transmission Electron Microscopy (TEM), using high-angle annular dark filed (HAADF) detection, to obtain atomic-resolution images. From the thermally diffusive images, produced by the vibrations of atoms, we intent to resolve isotopes types in graphene. For this, we establish a relationship between the full width half maximum (FWHM) of real-space intensity images and MSD for temperature and isotope concentration changes. For the case of changes in the temperature of the system, simulation results show a linear relationship between the MSD as a function of increased temperature in the system, with a slope of 7.858×10-6 Å2/K. We also note a power dependency for the MSD in units of [Å2] with respect to the FWHM in units of [Å] given by FWHM(MSD)=0.20MSD0.53+0.67. For the case of increasing isotope concentration, no statistically significant changes to the MSD of 12C and 13C are noted for graphene systems with 2,000 atoms or more. We note that for the experimental replication of results, noticeable differences in the MSD for systems with approximately 320,000 atoms must be observable. For this, we conclude that isotopes in free-standing graphene cannot be distinguished using TEM.

  • 277.
    Carva, Karel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ultrafast Spintronics: Give It A Whirl2014In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 10, no 8, p. 552-553Article in journal (Other (popular science, discussion, etc.))
  • 278. Carva, Karel
    et al.
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Legut, D.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab initio theory of electron-phonon mediated ultrafast spin relaxation of laser-excited hot electrons in transition-metal ferromagnets2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 18, p. 184425-Article in journal (Refereed)
    Abstract [en]

    We report a computational theoretical investigation of electron spin-flip scattering induced by the electron-phonon interaction in the transition-metal ferromagnets bcc Fe, fcc Co, and fcc Ni. The Elliott-Yafet electron-phonon spin-flip scattering is computed from first principles, employing a generalized spin-flip Eliashberg function as well as ab initio computed phonon dispersions. Aiming at investigating the amount of electron-phonon mediated demagnetization in femtosecond laser-excited ferromagnets, the formalism is extended to treat laser-created thermalized as well as nonequilibrium, nonthermal hot electron distributions. Using the developed formalism we compute the phonon-induced spin lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated demagnetization rate is evaluated for laser-created thermalized and nonequilibrium electron distributions. Nonthermal distributions are found to lead to a stronger demagnetization rate than hot, thermalized distributions, yet their demagnetizing effect is not enough to explain the experimentally observed demagnetization occurring in the subpicosecond regime.

  • 279.
    Carva, Karel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Legut, Dominik
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Theory of femtosecond laser-induced demagnetization2015In: ULTRAFAST MAGNETISM I, 2015, p. 111-115Conference paper (Refereed)
    Abstract [en]

    Using ab initio calculations we computed the ultrafast demagnetization that can be achieved by Elliott-Yafet electron-phonon spin-flip scatterings in laser-excited ferromagnets. Our calculations show that nonequilibrium laser-created distributions contribute mostly to the ultrafast demagnetization. Nonetheless, the total Elliott-Yafet contribution is too small to account for the fs-demagnetization.

  • 280.
    Carva, Karel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Battiato, Marco
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab Initio Investigation of the Elliott-Yafet Electron-Phonon Mechanism in Laser-Induced Ultrafast Demagnetization2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 20, p. 207201-Article in journal (Refereed)
    Abstract [en]

    The spin-flip (SF) Eliashberg function is calculated from first principles for ferromagnetic Ni to accurately establish the contribution of Elliott-Yafet electron-phonon SF scattering to Ni's femtosecond laser-driven demagnetization. This is used to compute the SF probability and demagnetization rate for laser-created thermalized as well as nonequilibrium electron distributions. Increased SF probabilities are found for thermalized electrons, but the induced demagnetization rate is extremely small. A larger demagnetization rate is obtained for nonequilibrium electron distributions, but its contribution is too small to account for femtosecond demagnetization.

  • 281.
    Carva, Karel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Is the controversy over femtosecond magneto-optics really solved?2011In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 7, no 9, p. 665-665Article in journal (Refereed)
  • 282.
    Carva, Karel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplap
    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.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Defect-controlled electronic transport in single, bilayer, and N-doped graphene: Theory2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 24, p. 245405-Article in journal (Refereed)
    Abstract [en]

    We report on a theoretical study of the electronic-structure and transport properties of single and bilayer graphene with vacancy defects, as well as N-doped graphene. The theory is based on first-principles calculations as well as model investigations in terms of real-space Green's functions. We show that increasing the defect concentration increases drastically the conductivity in the limit of zero applied gate voltage, by establishing carriers in originally carrier-free graphene, a fact which is in agreement with recent observations. We calculate the amount of defects needed for a transition from a nonconducting to a conducting regime (i.e., a metal-insulator transition) and establish the threshold of the defect concentration where the increase in impurity scattering dominates over the increase in carrier-induced conductivity.

  • 283.
    Carva, Karel
    et al.