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  • 1.
    Barthel, J.
    et al.
    Rhein Westfal TH Aachen, Cent Facil Electron Microscopy, D-52074 Aachen, Germany.;Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, D-52425 Julich, Germany..
    Mayer, J.
    Rhein Westfal TH Aachen, Cent Facil Electron Microscopy, D-52074 Aachen, Germany.;Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, D-52425 Julich, Germany..
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ho, P. -L
    Beijing National Center for Electron Microscopy, Laboratory of Advanced Materials, The State Key Laboratory of New Ceramics and Fine Processing, and School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China.; Central Facility for Electron Microscopy, RWTH Aachen University, 52074 Aachen, Germany.
    Zhong, X. Y.
    Tsinghua Univ, State Key Lab New Ceram & Fine Proc, Lab Adv Mat, Beijing Natl Ctr Electron Microscopy, Beijing 100084, Peoples R China.;Tsinghua Univ, Sch Mat Sci & Engn, Beijing 100084, Peoples R China..
    Lentzen, M.
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, D-52425 Julich, Germany..
    Dunin-Borkowski, R. E.
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, D-52425 Julich, Germany..
    Urban, K. W.
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, D-52425 Julich, Germany..
    Brown, H. G.
    Monash Univ, Sch Phys & Astron, Clayton, Vic 3800, Australia..
    Findlay, S. D.
    Monash Univ, Sch Phys & Astron, Clayton, Vic 3800, Australia..
    Allen, L. J.
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, D-52425 Julich, Germany.;Univ Melbourne, Sch Phys, Parkville, Vic 3010, Australia..
    Understanding electron magnetic circular dichroism in a transition potential approach2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 14, article id 144103Article in journal (Refereed)
    Abstract [en]

    This paper introduces an approach based on transition potentials for inelastic scattering to understand the underlying physics of electron magnetic circular dichroism (EMCD). The transition potentials are sufficiently localized to permit atomic-scale EMCD. Two-beam and three-beam systematic row cases are discussed in detail in terms of transition potentials for conventional transmission electron microscopy, and the basic symmetries which arise in the three-beam case are confirmed experimentally. Atomic-scale EMCD in scanning transmission electron microscopy (STEM), using both a standard STEM probe and vortex beams, is discussed.

  • 2.
    Bhowmick, Somnath
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Rusz, Jan
    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.
    X-ray absorption spectra: Graphene, h-BN, and their alloy2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 15, p. 155108-Article in journal (Refereed)
    Abstract [en]

    Using first-principles density functional theory calculations, in conjunction with the Mahan-Nozieres-de Dominicis theory, we calculate the x-ray absorption spectra of the alloys of graphene and monolayer hexagonal boron nitride on a Ni (111) substrate. The chemical neighborhood of the constituent atoms (B, C, and N) inside the alloy differs from that of the parent phases. In a systematic way, we capture the change in the K-edge spectral shape, depending on the chemical neighborhood of B, C, and N. Our work also reiterates the importance of the dynamical core-hole screening for a proper description of the x-ray absorption process in sp(2)-bonded layered materials.

  • 3. Biasini, M
    et al.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Mills, A. P., Jr.
    Positron annihilation study of the electronic structure of URu2Si2: Fermi surface and hidden order parameter2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 8, p. 085115-Article in journal (Refereed)
    Abstract [en]

    The Fermi surface (FS) of URu2Si2 in the paramagnetic phase has been reinvestigated via the measurement of the two-dimensional angular correlation of the positron annihilation radiation. Unlike previous measurements our experiment provides evidence that density-functional theory gives a fair qualitative description of the FS, showing a large contribution from the 5f electrons, although the Fermi volume of electron and hole pockets determined experimentally is smaller than the theoretical description. Furthermore, we propose a method to investigate the nature of the hidden order responsible for the 17.5 K transition.

  • 4.
    Bidermane, Ieva
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Lüder, Johann
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Boudet, S.
    Zhang, T.
    Ahmadi, S.
    Grazioli, C.
    Bouvet, M.
    Rusz, Jan
    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.
    Brena, Barbara
    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.
    Witkowski, N.
    Experimental and theoretical study of electronic structure of lutetium bi-phthalocyanine2013In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 23, p. 234701-Article in journal (Refereed)
    Abstract [en]

    Using Near Edge X-Ray Absorption Fine Structure (NEXAFS) Spectroscopy, the thickness dependent formation of Lutetium Phthalocyanine (LuPc2) films on a stepped passivated Si(100)2x1 reconstructed surface was studied. Density functional theory (DFT) calculations were employed to gain detailed insights into the electronic structure. Photoelectron spectroscopy measurements have not revealed any noticeable interaction of LuPc2 with the H-passivated Si surface. The presented study can be considered to give a comprehensive description of the LuPc2 molecular electronic structure. The DFT calculations reveal the interaction of the two molecular rings with each other and with the metallic center forming new kinds of orbitals in between the phthalocyanine rings, which allows to better understand the experimentally obtained NEXAFS results. 

  • 5. 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]

  • 6. 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.

  • 7.
    Cedervall, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Nonnet, Elise
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
    Hedlund, Daniel
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Häggström, Lennart
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Ericsson, Tore
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Werwinski, Miroslaw
    Institute of Molecular Physics, Polish Academy of Sciences.
    Edström, Alexander
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Department of Materials Theory, ETH Zürich.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Svedlindh, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Gunnarsson, Klas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Influence of cobalt substitution on the magnetic properties of Fe5PB22018In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 57, no 2, p. 777-784Article in journal (Refereed)
    Abstract [en]

    In this study the effects of cobalt substitutions in Fe5PB2 have been studied. An increased cobalt content reduces the magnetic exchange interactions. This has been concluded from a large, linear decrease in both the Curie temperature as well as the saturated magnetic moment. At high cobalt concentrations, cobalt prefers to order at the M(2) position in the crystal structure. A tunable Curie transition like this shows some prerequisites for magnetic cooling applications.

    The substitutional effects of cobalt in (Fe1–xCox)5PB2 have been studied with respect to crystalline structure and chemical order with X-ray diffraction and Mössbauer spectroscopy. The magnetic properties have been determined from magnetic measurements, and density functional theory calculations have been performed for the magnetic properties of both the end compounds, as well as the chemically disordered intermediate compounds. The crystal structure of (Fe1–xCox)5PB2 is tetragonal (space group I4/mcm) with two different metal sites, with a preference for cobalt atoms in the M(2) position (4c) at higher cobalt contents. The substitution also affects the magnetic properties with a decrease of the Curie temperature (TC) with increasing cobalt content, from 622 to 152 K for Fe5PB2 and (Fe0.3Co0.7)5PB2, respectively. Thus, the Curie temperature is dependent on composition, and it is possible to tune TC to a temperature near room temperature, which is one prerequisite for magnetic cooling materials.

  • 8.
    Delczeg-Czirjak, Erna K.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Edström, Alexander
    Werwinski, Miroslaw
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    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.
    Stabilization of the tetragonal distortion of Fe chi Co1-chi alloys by C impurities: A potential new permanent magnet2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 14, p. 144403-Article in journal (Refereed)
    Abstract [en]

    We have analyzed by density functional theory calculations the structural and magnetic properties of Fe-Co alloys doped by carbon. In analogy with the formation of martensite in steels we predict that such a structure also forms for Fe-Co alloys in a wide range of concentrations. These alloys are predicted to have a stable tetragonal distortion, which in turn leads to an enhanced magnetocrystalline anisotropy energy of up to 0.75 MJ/m(3) and a saturated magnetization field of 1.9 T.

  • 9. Divis, M.
    et al.
    Peltierova-Vejpravova, J.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Michor, H.
    The electronic structure and crystal field of RPt3Si (R = Pr, Nd, Sm) compounds2007In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 400, no 1-2, p. 114-118Article in journal (Refereed)
    Abstract [en]

    First-principles calculations based on density-functional theory (DFT) were performed on RPt3Si (R = Pr, Nd, Sm) for the first time. The electronic density of states (DOS) and bonding properties were studied using relativistic full-potential APW plus local orbitals calculations. The crystal-field (CF) splitting was derived for NdPt3Si and SMPt3Si by fitting the temperature-dependent magnetic susceptibility. The starting estimate of the CF parameters for fitting procedure was obtained from first-principles calculations based on DFT.

  • 10.
    Edstrom, Alexander
    et al.
    Swiss Fed Inst Technol, Mat Theory, Wolfgang Pauli Str 27, CH-8093 Zurich, Switzerland.
    Lubk, Axel
    IFW Dresden, Inst Solid State Res, Helmholtzstr 20, D-01069 Dresden, Germany;Tech Univ Dresden, Inst Solid State & Mat Phys, Haeckelstr 3, D-01069 Dresden, Germany.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Quantum mechanical treatment of atomic-resolution differential phase contrast imaging of magnetic materials2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 17, article id 174428Article in journal (Refereed)
    Abstract [en]

    Utilizing the Pauli equation based multislice method, introduced in a previous paper [A. Edstrom, A. Lubk, and J. Rusz, Phys. Rev. Lett. 116, 127203 (2016)], we study the atomic-resolution differential phase contrast (DPC) imaging on an example of a hard magnet FePt with in-plane magnetization. Simulated center-of-mass pattern in a scanning transmission electron microscopy experiment carries information about both electric and magnetic fields. The momentum transfer remains curl free, which has consequences for interpretation of the integrated DPC technique. The extracted magnetic component of the pattern is compared to the expected projected microscopic magnetic field as obtained by density functional theory calculation. Qualitative agreement is obtained for low sample thicknesses and a suitable range of collection angles.

  • 11.
    Edström, Alexander
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Chico, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Jakobsson, Adam
    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.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Electronic structure and magnetic properties of L1(0) binary alloys2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 1, p. 014402-Article in journal (Refereed)
    Abstract [en]

    We present a systematic study of the magnetic properties of L1(0) binary alloys FeNi, CoNi, MnAl, and MnGa via two different density functional theory approaches. Our calculations show large magnetocrystalline anisotropies in the order 1 MJ/m(3) or higher for CoNi, MnAl, and MnGa, while FeNi shows a somewhat lower value in the range 0.48-0.77 MJ/m(3). Saturation magnetization values of 1.3 MA/m, 1.0 MA/m, 0.8 MA/m, and 0.9 MA/m are obtained for FeNi, CoNi, MnAl, and MnGa, respectively. Curie temperatures are evaluated via Monte Carlo simulations and show T-C = 916 K and T-C = 1130 K for FeNi and CoNi, respectively. For Mn-based compounds Mn-rich off-stoichiometric compositions are found to be important for the stability of a ferro- or ferrimagnetic ground state with T-C greater than 600 K. The effect of substitutional disorder is studied and found to decrease both magnetocrystalline anisotropies and Curie temperatures in FeNi and CoNi.

  • 12.
    Edström, Alexander
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Lubk, Axel
    Tech Univ Dresden, Triebenberg Lab, Dresden, Germany.
    Rusz, Ján
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Elastic Scattering of Electron Vortex Beams in Magnetic Matter.2016In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 116, no 12, article id 127203Article in journal (Refereed)
    Abstract [en]

    Elastic scattering of electron vortex beams on magnetic materials leads to a weak magnetic contrast due to Zeeman interaction of orbital angular momentum of the beam with magnetic fields in the sample. The magnetic signal manifests itself as a redistribution of intensity in diffraction patterns due to a change of sign of the orbital angular momentum of the electron vortex beam. While in the atomic resolution regime the magnetic signal is most likely under the detection limits of present transmission electron microscopes, for electron probes with high orbital angular momenta, and correspondingly larger spatial extent, its detection is predicted to be feasible.

  • 13.
    Edström, Alexander
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Werwinski, M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Iusan, Diana
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Rusz, Jan
    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.
    Skokov, K. P.
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Radulov, I. A.
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Ener, S.
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Kuz'min, M. D.
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Hong, J.
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Fries, M.
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Karpenkov, D. Yu.
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Gutfleisch, O.
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Toson, P.
    Vienna Univ Technol, Inst Solid State Phys, Adv Magnet Grp, A-1040 Vienna, Austria..
    Fidler, J.
    Vienna Univ Technol, Inst Solid State Phys, Adv Magnet Grp, A-1040 Vienna, Austria..
    Magnetic properties of (Fe1-xCox)(2)B alloys and the effect of doping by 5d elements2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 17, article id 174413Article in journal (Refereed)
    Abstract [en]

    We have explored, computationally and experimentally, the magnetic properties of (Fe1-xCox)(2)B alloys. Calculations provide a good agreement with experiment in terms of the saturation magnetization and the magnetocrystalline anisotropy energy with some difficulty in describing Co2B, for which it is found that both full potential effects and electron correlations treated within dynamical mean field theory are of importance for a correct description. The material exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations between x = 0.1 and x = 0.5. A simple model for the temperature dependence of magnetic anisotropy suggests that the complicated nonmonotonic behavior is mainly due to variations in the band structure as the exchange splitting is reduced by temperature. Using density functional theory based calculations we have explored the effect of substitutionally doping the transition metal sublattice by the whole range of 5d transition metals and found that doping by Re or W elements should significantly enhance the magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed in enhancing the magnetic anisotropy due to formation of other phases. On the other hand, doping by Ir and Re was successful and resulted in magnetic anisotropies that are in agreement with theoretical predictions. In particular, doping by 2.5 at.% of Re on the Fe/Co site shows a magnetocrystalline anisotropy energy which is increased by 50% compared to its parent (Fe0.7Co0.3)(2)B compound, making this system interesting, for example, in the context of permanent magnet replacement materials or in other areas where a large magnetic anisotropy is of importance.

  • 14.
    Elgazzar, S
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Amft, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Mydosh, J. A.
    Hidden order in URu2Si2 originates from Fermi surface gapping induced by dynamic symmetry breaking2009In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 8, no 4, p. 337-341Article in journal (Refereed)
    Abstract [en]

    Spontaneous, collective ordering of electronic degrees of freedom leads to second-order phase transitions that are characterized by an order parameter driving the transition. The notion of a 'hidden order' has recently been used for a variety of materials where a clear phase transition occurs without a known order parameter. The prototype example is the heavy-fermion compound URu2Si2, where a mysterious hidden-order transition occurs at 17.5 K. For more than twenty years this system has been studied theoretically and experimentally without a firm grasp of the underlying physics. Here, we provide a microscopic explanation of the hidden order using density-functional theory calculations. We identify the Fermi surface 'hot spots' where degeneracy induces a Fermi surface instability and quantify how symmetry breaking lifts the degeneracy, causing a surprisingly large Fermi surface gapping. As the mechanism for the hidden order, we deduce spontaneous symmetry breaking through a dynamic mode of antiferromagnetic moment excitations.

  • 15. Elgazzar, S.
    et al.
    Rusz, Jan
    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.
    Mydosh, J. A.
    Electronic structure and Fermi surface of paramagnetic and antiferromagnetic UPt2Si22012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 7, p. 075104-Article in journal (Refereed)
    Abstract [en]

    We report density functional theory-based calculations of the electronic structure and Fermi surface properties of the intermetallic uranium compound UPt2Si2, which orders antiferromagnetically at TN=32 K with a total magnetic moment of 2μB/U-atom and exhibits a moderate mass enhancement in the specific-heat coefficient. Our investigation is carried out using relativistic, full-potential band-structure methods within the framework of the local spin density approximation (LSDA), the LSDA with orbital polarization correction (LSDA+OPC), and the LSDA supplemented with an additional Hubbard U (LSDA+U). We find that the LSDA+OPC scheme predicts the total magnetic moment in best agreement with experiment; from this we infer that the 5f electrons in UPt2Si2 are orbitally polarized, mostly itinerant, and exhibit only a slight tendency toward localization. Our total energy calculations predict UPt2Si2 to form in the CaBe2Si2 (P4/nmm) structure, in contrast to URu2Si2 (ThCr2Si2: I4/mmm). The theoretical Fermi surfaces are also studied for the nonmagnetic and antiferromagnetic phases with the employed computational schemes and are found to be quasi-two-dimensional. At the antiferromagnetic transition, the Fermi surface is found to become more two-dimensional with small regions of gapping.

  • 16.
    Elgazzar, Saad
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Colineau, E.
    Griveau, J. -C
    Magnani, N.
    Rebizant, J.
    Caciuffo, R.
    Ab initio computational and experimental investigation of the electronic structure of actinide 218 materials2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 23, p. 235117-Article in journal (Refereed)
    Abstract [en]

    We report a comprehensive investigation of the electronic structure and magnetic properties of actinide 218 compounds, which crystallize in the tetragonal Ho2CoGa8 crystal structure. Specifically, we study experimentally the group of plutonium-based compounds Pu2MGa8 (with M=Rh, Co, and Fe), which are structurally related to the unconventional superconductors PuCoGa5 and PuRhGa5 and are measured to be nonmagnetic and nonsuperconducting down to 2 K, yet displaying relatively high linear specific-heat coefficients of 61 to 133 mJ/mol K-2. We perform density-functional theory based calculations, in which we apply three different approaches to access the tendency of 5f electron localization, the local spin-density approximation (LSDA) LSDA+U, and the 5f open-core approach. For comparison to the above-mentioned compounds we also investigate computationally the plutonium compounds with M=Ir and Pd, the uranium-based compounds U2MGa8 (with M=Co, Fe, Rh, and Ru), as well as Np2CoGa8, and Am2CoGa8. On the basis of ab initio LSDA calculations we optimize the equilibrium lattice parameters and the internal fractional coordinates within the Ho2CoGa8 crystal structure. The obtained lattice parameters are in relatively good agreement with experimental values, when we assume delocalized 5f states for all compounds except Am2CoGa8. We discuss the computed electronic structures and the theoretical Fermi surfaces. For the Pu-218 compounds we find that LSDA calculations, in which the 5f's are treated as delocalized, predict a magnetically ordered ground state, whereas LSDA+U calculations predict a nonmagnetic ground state in accordance with experiment. For the U-218 compounds the LSDA itinerant 5f approach predicts a nonmagnetic ground state, in accordance with available experimental data. For Am2CoGa8 our calculations are consistent with the scenario of localized 5f electrons. We find that, on account of the elongated tetragonal structure, most of the theoretical Fermi surfaces are quasi-two-dimensional.

  • 17.
    Gang, Seung-gi
    et al.
    Res Ctr Julich, Peter Grunberg Inst PGI 6, D-52425 Julich, Germany..
    Adam, Roman
    Res Ctr Julich, Peter Grunberg Inst PGI 6, D-52425 Julich, Germany..
    Plötzing, Moritz
    Res Ctr Julich, Peter Grunberg Inst PGI 6, D-52425 Julich, Germany..
    von Witzleben, Moritz
    Res Ctr Julich, Peter Grunberg Inst PGI 6, D-52425 Julich, Germany..
    Weier, Christian
    Res Ctr Julich, Peter Grunberg Inst PGI 6, D-52425 Julich, Germany..
    Parlak, Umut
    Res Ctr Julich, Peter Grunberg Inst PGI 6, D-52425 Julich, Germany..
    Bürgler, Daniel E.
    Res Ctr Julich, Peter Grunberg Inst PGI 6, D-52425 Julich, Germany..
    Schneider, Claus M.
    Res Ctr Julich, Peter Grunberg Inst PGI 6, D-52425 Julich, Germany..
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Maldonado, Pablo
    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.
    Element-selective investigation of femtosecond spin dynamics in NiPd magnetic alloys using extreme ultraviolet radiation2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 6, article id 064412Article in journal (Refereed)
    Abstract [en]

    We studied femtosecond spin dynamics in NixPd1-x magnetic thin films by optically pumping the system with infrared (1.55 eV) laser pulses and subsequently recording the reflectivity of extreme ultraviolet (XUV) pulses synchronized with the pump pulse train. XUV light in the energy range from 20 to 72 eV was produced by laser high-harmonic generation. The reflectivity of XUV radiation at characteristic resonant energies allowed separate detection of the spin dynamics in the elemental subsystems at the M-2,M-3 absorption edges of Ni (68.0 and 66.2 eV) and N-2,N-3 edges of Pd (55.7 and 50.9 eV). The measurements were performed in transversal magneto-optical Kerr effect geometry. In static measurements, we observed a magnetic signature of the Pd subsystem due to an induced magnetization. Calculated magneto-optical asymmetries based on density functional theory show close agreement with the measured results. Femtosecond spin dynamics measured at the Ni absorption edges indicates that increasing the Pd concentration, which causes a decrease in the Curie temperature T-C, results in a drop of the demagnetization time tau(M), contrary to the tau(M) similar to 1/T-C scaling expected for single-species materials. This observation is ascribed to the increase of the Pd-mediated spin-orbit coupling in the alloy.

  • 18. Hebert, C.
    et al.
    Schattschneider, P.
    Rubino, S.
    Novak, P.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Stoeger-Pollach, M.
    Magnetic circular dichroism in electron energy loss spectrometry2008In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 108, no 3, p. 277-284Article in journal (Refereed)
    Abstract [en]

    The measurement of circular dichroism in the electron microscope is a new, emerging method and, as such, it is subject to constant refinement and improvement. Different ways can be envisaged to record the signal. We present an overview of the key steps in the energy-loss magnetic chiral dichroism (EMCD) experiment as well as a detailed review of the methods used in the intrinsic way where the specimen is used as a beam splitter. Lateral resolution up to 20-30 nm can be achieved, and the use of convergent beam techniques leads to an improved SIN ratio. Dichroic effects are shown for Ni and Co single crystal; as a counterexample, measurements were carried also for a non-magnetic (Ti) sample, where no dichroic effect was found.

  • 19.
    Hedlund, Daniel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Cedervall, Johan
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Edström, Alexander
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Materials Theory, ETH Zürich.
    Werwinski, Miroslaw
    Polish Academy of Sciences.
    Kontos, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Örebro University.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Svedlindh, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Gunnarsson, Klas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Magnetic properties of the Fe5SiB2−Fe5PB2 system2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 9, article id 094433Article in journal (Refereed)
    Abstract [en]

    The magnetic properties of the compound Fe5Si1−xPxB2 have been studied, with a focus on the Curie temperature TC, saturation magnetization MS, and magnetocrystalline anisotropy. Field and temperature dependent magnetization measurements were used to determine TC(x) and MS(x). The saturation magnetization at 10 K (300 K) is found to monotonically decrease from 1.11MA/m (1.03MA/m) to 0.97MA/m (0.87MA/m), as x increases from 0 to 1. The Curie temperature is determined to be 810 and 615 K in Fe5SiB2 and Fe5PB2, respectively. The highest TC is observed for x=0.1, while it decreases monotonically for larger x. The Curie temperatures have also been theoretically determined to be 700 and 660 K for Fe5SiB2 and Fe5PB2, respectively, using a combination of density functional theory and Monte Carlo simulations. The magnitude of the effective magnetocrystalline anisotropy was extracted using the law of approach to saturation, revealing an increase with increasing phosphorus concentration. Low-field magnetization vs temperature results for x=0,0.1,0.2 indicate that there is a transition from easy-axis to easy-plane anisotropy with decreasing temperature.

  • 20.
    Karis, Olof
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Surface and Interface Science.
    Svensson, Svante
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Surface and Interface Science.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Gorgoi, M.
    Schäfers, F.
    Braun, W.
    Eberhardt, W.
    Mårtensson, Nils
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Surface and Interface Science.
    High-kinetic-energy photoemission spectroscopy of Ni at 1s: 6-eV satellite at 4 eV2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 23, p. 233105-Article in journal (Refereed)
    Abstract [en]

    Electron correlations are responsible for many profound phenomena in solid-state physics. A classical example is the 6-eV satellite in the photoelectron spectrum of Ni. Until now the satellite structure has only been investigated at the L shell and more shallow levels. Here we report a high-kinetic-energy photoemission spectroscopy (HIKE) investigation of Ni metal. We present 1s and 2p photoelectron spectra, obtained using excitation energies up to 12.6 keV. Our investigation demonstrates that the energy position of the satellite relative to the main line is different for the 1s and the 2p levels. In combination with electronic structure calculations, we show that this energy shift is attributed to unique differences in the core-valence coupling for the K and L2,3 shells in 3d transition metals, resulting in different screening of the core holes.

  • 21. Kislitsyn, Dmitry A
    et al.
    Kocevski, Vancho
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Mills, Jon M
    Chiu, Sheng-Kuei
    Gervasi, Christian F
    Taber, Benjamen N
    Rosenfield, Ariel E
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Rusz, Ján
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Goforth, Andrea M
    Nazin, George V
    Mapping of Defects in Individual Silicon Nanocrystals Using Real-Space Spectroscopy.2016In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 7, no 6, p. 1047-1054Article in journal (Refereed)
    Abstract [en]

    The photophysical properties of silicon semiconductor nanocrystals (SiNCs) are extremely sensitive to the presence of surface chemical defects, many of which are easily produced by oxidation under ambient conditions. The diversity of chemical structures of such defects and the lack of tools capable of probing individual defects continue to impede understanding of the roles of these defects in SiNC photophysics. We use scanning tunneling spectroscopy to study the impact of surface defects on the electronic structures of hydrogen-passivated SiNCs supported on the Au(111) surface. Spatial maps of the local electronic density of states (LDOS) produced by our measurements allowed us to identify locally enhanced defect-induced states as well as quantum-confined states delocalized throughout the SiNC volume. We use theoretical calculations to show that the LDOS spectra associated with the observed defects are attributable to Si-O-Si bridged oxygen or Si-OH surface defects.

  • 22.
    Kislitsyn, Dmitry A.
    et al.
    Univ Oregon, Oregon Ctr Opt Mol & Quantum Sci, Dept Chem & Biochem, Inst Mat Sci, 1253 Univ Oregon, Eugene, OR 97403 USA..
    Mills, Jon M.
    Univ Oregon, Oregon Ctr Opt Mol & Quantum Sci, Dept Chem & Biochem, Inst Mat Sci, 1253 Univ Oregon, Eugene, OR 97403 USA..
    Kocevski, Vancho
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Northwestern Univ, Dept Mat Sci & Engn, Evanston, IL 60208 USA..
    Chiu, Sheng-Kuei
    Portland State Univ, Dept Chem, Portland, OR 97201 USA..
    DeBenedetti, William J. I.
    Portland State Univ, Dept Chem, Portland, OR 97201 USA.;Cornell Univ, Dept Chem & Chem Biol, Ithaca, NY 14853 USA..
    Gervasi, Christian F.
    Univ Oregon, Oregon Ctr Opt Mol & Quantum Sci, Dept Chem & Biochem, Inst Mat Sci, 1253 Univ Oregon, Eugene, OR 97403 USA..
    Taber, Benjamen N.
    Univ Oregon, Oregon Ctr Opt Mol & Quantum Sci, Dept Chem & Biochem, Inst Mat Sci, 1253 Univ Oregon, Eugene, OR 97403 USA..
    Rosenfield, Ariel E.
    Univ Oregon, Oregon Ctr Opt Mol & Quantum Sci, Dept Chem & Biochem, Inst Mat Sci, 1253 Univ Oregon, Eugene, OR 97403 USA..
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Goforth, Andrea M.
    Portland State Univ, Dept Chem, Portland, OR 97201 USA..
    Nazin, George V.
    Univ Oregon, Oregon Ctr Opt Mol & Quantum Sci, Dept Chem & Biochem, Inst Mat Sci, 1253 Univ Oregon, Eugene, OR 97403 USA..
    Communication: Visualization and spectroscopy of defects induced by dehydrogenation in individual silicon nanocrystals2016In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 144, no 24, article id 241102Article in journal (Refereed)
    Abstract [en]

    We present results of a scanning tunneling spectroscopy (STS) study of the impact of dehydrogenation on the electronic structures of hydrogen-passivated silicon nanocrystals (SiNCs) supported on the Au(111) surface. Gradual dehydrogenation is achieved by injecting high-energy electrons into individual SiNCs, which results, initially, in reduction of the electronic bandgap, and eventually produces midgap electronic states. We use theoretical calculations to show that the STS spectra of midgap states are consistent with the presence of silicon dangling bonds, which are found in different charge states. Our calculations also suggest that the observed initial reduction of the electronic bandgap is attributable to the SiNC surface reconstruction induced by conversion of surface dihydrides to monohydrides due to hydrogen desorption. Our results thus provide the first visualization of the SiNC electronic structure evolution induced by dehydrogenation and provide direct evidence for the existence of diverse dangling bond states on the SiNC surfaces. Published by AIP Publishing.

  • 23.
    Kocevski, Vancho
    et al.
    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.
    Gerard, Celine
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sarma, D. D.
    Indian Institute of Science, Bangalore-560 012, India.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Influence of dimensionality and interface type on optical and electronic properties of CdS/ZnS core-shell nanocrystals: a first-principles study2015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, no 16, article id 164701Article in journal (Refereed)
    Abstract [en]

    Semiconducting nanocrystals (NCs) have become one of the leading materials in a variety of applications, mainly due to their size tunable band gap and high intensity emission. Their photoluminescence properties can be notably improved by capping the nanocrystals with a shell of another semiconductor, making core-shell structures. We focus our study on the CdS/ZnS core-shell nanocrystals, that are closely related to extensively studied CdSe/CdS NCs, albeit exhibiting rather different photoluminescence properties. We employ density functional theory to investigate the changes in the electronic and optical properties of these nanocrystals with size, core/shell ratio and interface structure between the core and the shell. We show that the band gap depends on the size of the NCs and the core/shell ratio. We suggest that the differences in the density of states and absorption are mainly governed by the core/shell ratio. We present that both the LUMO and the HOMO wavefunctions are localised in the core of the NCs, with the distribution of the LUMO wavefunction being more sensitive to the size and the core/shell ratio. We also demonstrate that the Coulomb interaction energies closely follow the behaviour of the localisation of the HOMO and LUMO wavefunctions, and are decreasing with increasing NC size. Furthermore, we investigated the electronic and optical properties of the NCs with different interfaces between the core and the shell, and different core types. We find that the different interfaces and core types have rather small influence on the band gaps and the absorption indices, as well as on the confinement of the HOMO and LUMO wavefunctions. In addition, we compare these results with the previous results for CdSe/CdS NCs, reflecting the different PL properties of these two types of NCs. We argue that the difference in their Coulomb interaction energies is one of the main reasons for their distinct PL properties.

  • 24.
    Kocevski, Vancho
    et al.
    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.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Band alignment switching and the interaction between neighbouring silicon nanocrystals embedded in a SiC matrix2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, p. 165429-165435Article in journal (Refereed)
    Abstract [en]

    We present results from density functional theory of the electronic properties of silicon nanocrystals (Si NCs) embedded in a silicon carbide (SiC) matrix, considering different combinations of various NCs and host matrix sizes. We show that the NC and the host matrix form a type-II band alignment, with the states at the top of the valence band being in the Si NC and the states at the bottom of the conduction band in the host matrix. Moreover, this band alignment can be interchanged with introducing oxygen at the interface. We demonstrate that the charge densities of some valence band states can overlap with the charge densities of the neighbouring NCs. We also demonstrate that this leakage of states is significant when the distance between the neighbouring NCs is less than ~1.6 nm.

  • 25.
    Kocevski, Vancho
    et al.
    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.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Size dependence of the stability, electronic structure, and optical properties of silicon nanocrystals with various surface impurities2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 12, article id 125402Article in journal (Refereed)
    Abstract [en]

    We present a comprehensive, ground-state density functional theory study of the size dependence of the optical and electronic properties and the stability of spherical silicon nanocrystals (NCs) with different impurities on the surface. We vary the size of the NCs from 1.0 to 3.5 nm, considering single-bonded (CH3, F, Cl, OH) and double-bonded (O, S) impurities and bridged oxygen. We show that the density of states (DOS) and absorption indices of the NCs with single-bonded impurities are very similar to each other and the fully hydrogenated NCs, except for the 1.0-nm NCs, where a slight difference is present. In the case of the NCs with double-bonded impurities, the DOS and absorption indices exhibit a significant difference, compared to the fully hydrogenated NCs, for sizes up to 2.5 nm. We argue that this difference arises from the difference in the contribution from the impurity to the states around the gap, which can considerably change the character of the states. We demonstrate that the double-bonded impurities contribute significantly to the states around the gap, compared to the single-bonded impurities, causing changes in the symmetry of these states. This observation was further supported by analyzing the changes of the Fourier transform of the charge densities of the highest occupied and lowest unoccupied eigenstate. We also show that the formation energies of NCs with bridged oxygen and fluorine are the lowest, regardless of the size. Furthermore, we show that high hydrogen concentration can be used to suppress the addition of oxygen and fluorine on the surface of the Si NCs.

  • 26.
    Kocevski, Vancho
    et al.
    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.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Transition between direct and indirect band gap in silicon nanocrystals2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 24, p. 245401-Article in journal (Refereed)
    Abstract [en]

    Using ground-state density functional theory we study the transition from indirect to direct band gap in hydrogen-terminated silicon nanocrystals (NCs) as a function of decreasing diameter. The studied range, from 1.0 to 4.6 nm diameter of nanocrystals, with spherical and Wulff-shape NCs, covers the transition from nano-to bulk regime. A change in the symmetry of the lowest unoccupied state as a function of decreasing NC diameter is observed, gradually increasing the oscillator strength of transitions from the highest occupied to the lowest unoccupied state. Real space and Fourier space characteristics of highest occupied and lowest unoccupied states are explored in detail and linked to a smooth transition from nano-to bulk regime.

  • 27.
    Kocevski, Vancho
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Rusz, Jan
    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.
    Sarma, Dipankar Das
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics. Indian Institute of Science, Bangalore-560 012, India.
    First-principles study of the influence of different interfaces and core types on the properties of CdSe/CdS core-shell nanocrystals2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 10865Article in journal (Refereed)
    Abstract [en]

    With the expanding field of nanoengineering and the production of nanocrystals (NCs) with higher quality and tunable size, having reliable theoretical calculations to complement the experimental results is very important. Here we present such a study of CdSe/CdS core-shell NCs using density functional theory, where we focus on dependence of the properties of these NCs on core types and interfaces between the core and the shell, as well as on the core/shell ratio. We show that the density of states and the absorption indices depend rather weakly on the type of interface and core type. We demonstrate that the HOMO wavefunction is mainly localised in the core of the nanocrystal, depending primarily on the core/shell ratio. On the other hand the LUMO wavefunction spreads more into the shell of the nanocrystal, where its confinement in the core is almost the same in each of the studied structural models. Furthermore, we show that the radiative lifetimes decrease with increasing core sizes due to changes in the dipolar overlap integral of the HOMO and LUMO wavefunctions. In addition, the electron-hole Coulomb interaction energies follow a similar pattern as the localisation of the wavefunctions, with the smaller NCs having higher Coulomb interaction energies.

  • 28. Krivanek, Ondrej L.
    et al.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Idrobo, Juan-Carlos
    Lovejoy, Tracy J.
    Dellby, Niklas
    Toward Single Mode, Atomic Size Electron Vortex Beams2014In: Microscopy and Microanalysis, ISSN 1431-9276, E-ISSN 1435-8115, Vol. 20, no 3, p. 832-836Article in journal (Refereed)
    Abstract [en]

    We propose a practical method of producing a single mode electron vortex beam suitable for use in a scanning transmission electron microscope (STEM). The method involves using a holographic "fork" aperture to produce a row of beams of different orbital angular momenta, as is now well established, magnifying the row so that neighboring beams are separated by about 1 mu m, selecting the desired beam with a narrow slit, and demagnifying the selected beam down to 1-2 angstrom in size. We show that the method can be implemented by adding two condenser lenses plus a selection slit to a straight-column cold-field emission STEM. It can also be carried out in an existing instrument, the monochromated Nion high-energy-resolution monochromated electron energy-loss spectroscopy-STEM, by using its monochromator in a novel way. We estimate that atom-sized vortex beams with <= 20 pA of current should be attainable at 100-200 keV in either instrument.

  • 29.
    Kudo, Tomohiro
    et al.
    Nagoya University.
    Tatsumi, Kazuyoshi
    Nagoya University.
    Muto, Shunsuke
    Nagoya University.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Magnetocrystalline anisotropy of hexagonal Co by relative intensities of electron magnetic circular dichroic signals2015Conference paper (Other academic)
  • 30. Kuz'min, M. D.
    et al.
    Skokov, K. P.
    Radulov, I.
    Schwoebel, C. A.
    Foro, S.
    Donner, W.
    Werwinski, M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Delczeg-Czirjak, Erna
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Gutfleisch, O.
    Magnetic anisotropy of La2Co72015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 118, no 5, article id 053905Article in journal (Refereed)
    Abstract [en]

    A magnetization study of a La2Co7 single crystal has obtained the following anisotropy constants: K-1 = 1.4 MJ/m(3) and K-2 = 0.02 MJ/m(3) (at room temperature). The corresponding anisotropy field is 6.7 T; an earlier report of a much higher value (17 T) has not been confirmed. A significant (10%) magnetization anisotropy has been observed. Density-functional calculations are in qualitative agreement with the new data. (C) 2015 AIP Publishing LLC.

  • 31.
    Legut, D.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Magnetic exchange interactions and estimation of T-N in CsNiF3 from first principles2008In: Acta Physica Polonica. A, ISSN 0587-4246, E-ISSN 1898-794X, Vol. 113, no 1, p. 503-506Article in journal (Refereed)
    Abstract [en]

    The CsNiF3 has been for long time studied as a prototype of quasi-1D planar ferromagnetic system. At very low temperature the studied system is insulator and therefore the magnetic exchange interactions should decay very rapidly. We treated the magnetic exchange coupling within the Heisenberg model for the nearest neighbor interaction between the antiferromagnetically coupled Ni-chains. The influence of up to the second-nearest neighbors on the ferromagnetic exchange coupling along the Ni-chains was determined. The exchange interactions were calculated for the experimental volume by the density functional theory, within the all-electron approach using the local density approximation for the exchange and correlation. The Neel temperature was calculated by means of the mean-field theory and by the random-phase approximation method.

  • 32.
    Leifer, Klaus
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Rubino, Stefano
    Lidbaum, Hans
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter
    Eriksson, Olle
    Liebig, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hjörvarsson, Björgvin
    Mapping of the EMCD signal in real and reciprocal space forquantitative analysis of magnetic samples2010In: Scandem 2010 Proceedings; Stockholm, Sweden, 8-11 June 2010, 2010Conference paper (Refereed)
  • 33.
    Leifer, Klaus
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Rubino, Stefano
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Liebig, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hjörvarsson, Björgvin
    Oppener, Peter
    Eriksson, Olle
    Mapping of the EMCD signal in real and reciprocal space for quantitative analysis of magnetic samples2010In: IMC 17, Rio de Janeiro,19-24 September 2010; Proceedings, 2010Conference paper (Refereed)
  • 34.
    Lidbaum, Hans
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Rubino, Stefano
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Liebig, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Oppeneer, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Quantitative EMCD yields nano-information on magnetic moments2009In: EDGE 2009: International EELS-Workshop, May 17 – May 22, 2009, Banff, Alberta, Canada, 2009Conference paper (Other academic)
  • 35.
    Lidbaum, Hans
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Liebig, A
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Oppeneer, Peter
    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.
    Coronel, E
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    EMCD in the TEM - Optimization of signal acquisition and data evaluation2008Conference paper (Refereed)
    Abstract [en]

    The method of electron magnetic circular dichroism (EMCD) was recently proposed anddemonstrated by Peter Schattschneider et al. [I]. The EMCD signal consists in differences of L3 andL2 edge intensities of ferromagnetic materials at specific diffraction vectors. EMCD is thereforeelement specific. Furthermore. it was shown recently that sum rules apply to the EMCD methodwhich is an essential progress to obtain quantitative magnetic information from this method [2-3].Though. the theoretical derivations of sum rule suppose that the obtained spin- to orbital magneticmoments do not depend on the choice of scattering vector. This is a simplification which must beverified both by simulations and experiments to make EMCD a quantitative method for the study ofmagnetic moments.In the experiment. the dichroic signal. i.e. the change ofLJ and L2 edges at different diffractionvectors is small. i.e. of the order of 5-15% of the total signal intensity. Therefore. the acquisitionconditions must be optimized to both approach a di ffraction geometry where sum rules can beapplied and to maximize the signal to noise ratio.In this work. we optimize both. the signal and the signal/noise ratio. The experiments were carriedout on a FEI Tecnai F30 equipped with a Gatan GIF2002 energy filter. Instead of recording k-vectordependent single spectra as in the original work [I], we acquire energy filtered diffraction patternsin the energy interval around the transition metal L-edge. This allows for extraction of the EMCDsignal at k-vectors where the signal contains quantitative magnetic information. All measurementsof2 dimensional k-space maps of the EMCD signal are compared with simulations of the EM CDsignal. We find very good agreement between theoretical predictions and experimental values forboth. the k-space evolution and quantitative agreement the EMCD signal. The data evaluationincludes a careful normalization procedure. a statistical optimization of the signal to noise ratio aswell as the consideration of the entire edge intensity. Finally. we demonstrate the quantitativeprecision of the proposed method at the example ofa slightly textured iron layer (figures I and 2).References[I] P Schattschneider et aJ.. Nature 44 I (2006) 486.[2] J. Rusz, O. Eriksson, P. Novak, P. M. Oppeneer. Phys. Rev. B 76 (2007) 060408.[3] L. Calmels et aJ.. Phys. Rev. B 76 (2007) 060409.

  • 36.
    Lidbaum, Hans
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Liebig, A
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Oppeneer, Peter
    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.
    Coronel, E
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Experimental conditions and data evaluation forquantitative EMCD measurements in the TEM2008In: European Microscopy Conference, 2008Conference paper (Refereed)
    Abstract [en]

    The recently demonstrated technique electron energy-loss spectroscopy (EMCD) [I)opens new routes for characterization of magnetic materials using transmission electronmicroscopy. The technique enables quantitative measurements of orbital to spinmagnetic moments with element specificity, according to the recently derived sum rules[2). Electron energy-loss spectra is obtained at well defined scattering geometries, seefigure I.The principle of the technique having been demonstrated, further progress isrequired to obtain reliable quantitative information about the magnetic properties of thesample. By using energy filtered diffraction patterns, the distribution of the EM CDsignal in reciprocal space is obtained. The ava,lability of these data sets from a fullreciprocal plane allows for the optimisation of the data treatment. We study the theinfluence of experimental geometries on the EMCD signal and optimise data analysis ofthe probed reciprocal plane. This is essential to obtain correct and reliable magneticinformation. Especially normalization, signal to noise optimization and consideration ofthe entire edge intensities are important. The data cubes consisting of the reciprocalplane and energy-loss were acquired using a FEI Tecnai F30ST microscope equippedwith a Gatan GIF2002 spectrometer. In figure 2, two spectra that were extracted at theP+ and P- positions are shown. The experimental results are compared with calculationsof the EMCD signal for a thin Fe film, showing very good agreement.I. P. Schattschneider, S. Rubino, C. H~bert, J. Rusz, J. Kune~, P Novak, E. Carlino,M. Fabrizioli, G. Panaccione and G. Rossi, Nature 441 (2006), p. 486-488.2. J. Rusz, O. Eriksson, P. Novak and P.M. Oppeneer, Phys. Rev. B 76 (2007),060408(R).

  • 37.
    Lidbaum, Hans
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Liebig, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Björgvin, Hjörvarsson
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Physics.
    Oppeneer, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Coronel, Ernesto
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Quantitative magnetic information from reciprocal space maps in transmission electron microscopy2009In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 102, no 3, p. 037201-Article in journal (Refereed)
    Abstract [en]

    One of the most challenging issues in the characterization of magnetic materials is to obtain a quantitative analysis on the nanometer scale. Here we describe how electron magnetic circular dichroism (EMCD) measurements using the transmission electron microscope can be used for that purpose, utilizing reciprocal space maps. Applying the EMCD sum rules, an orbital to spin moment ratio of mL/mS=0.08±0.01 is obtained for Fe, which is consistent with the commonly accepted value. Hence, we establish EMCD as a quantitative element-specific technique for magnetic studies, using a widely available instrument with superior spatial resolution.

  • 38.
    Lidbaum, Hans
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter
    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.
    Coronel, Ernesto
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Conductivity engineering of graphene and carbon nanosheets by defect formation2008In: Conference of the American Vacuum Society, Boston, 2008, 2008Conference paper (Refereed)
  • 39.
    Lidbaum, Hans
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter
    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.
    Coronel, Ernesto
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Electron magnetic circular dichroism -Optimization of signal acquisition and data evaluation2008In: European Microscopy Conference, Aachen, 2008, 2008Conference paper (Refereed)
  • 40.
    Lidbaum, Hans
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Rusz, Ján
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Rubino, Stefano
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Liebig, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Reciprocal and real space maps for EMCD experiments2010In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 110, no 11, p. 1380-1389Article in journal (Refereed)
    Abstract [en]

    Electron magnetic chiral dichroism (EMCD) is an emerging tool for quantitative measurements of magnetic properties using the transmission electron microscope (TEM), with the possibility of nanometer resolution. The geometrical conditions, data treatment and electron gun settings are found to influence the EMCD signal. In this article, particular care is taken to obtain a reliable quantitative measurement of the ratio of orbital to spin magnetic moment using energy filtered diffraction patterns. For this purpose, we describe a method for data treatment, normalization and selection of mirror axis. The experimental results are supported by theoretical simulations based on dynamical diffraction and density functional theory. Special settings of the electron gun, so called telefocus mode, enable a higher intensity of the electron beam, as well as a reduction of the influence from artifacts on the signal. Using these settings, we demonstrate the principle of acquiring real space maps of the EMCD signal. This enables advanced characterization of magnetic materials with superior spatial resolution.

  • 41.
    Lin, J.
    et al.
    Tsinghua Univ, Natl Ctr Electron Microscopy Beijing, Sch Mat Sci & Engn, Key Lab Adv Mat MOE,State Key Lab New Ceram & Fin, Beijing 100084, Peoples R China.
    Zhong, X. Y.
    Tsinghua Univ, Natl Ctr Electron Microscopy Beijing, Sch Mat Sci & Engn, Key Lab Adv Mat MOE,State Key Lab New Ceram & Fin, Beijing 100084, Peoples R China.
    Song, C.
    Tsinghua Univ, Key Lab Adv Mat MOE, Sch Mat Sci & Engn, Beijing 100084, Peoples R China.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Uppsala Univ, Dept Phys & Astron, Box 516, S-75120 Uppsala, Sweden.
    Kocevski, Vancho
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Northwestern Univ, Dept Mat Sci & Engn, Evanston, IL 60208 USA.
    Xin, H. L.
    Brookhaven Natl Lab, Ctr Funct Nanomat, Upton, NY 11973 USA.
    Cui, B.
    Tsinghua Univ, Key Lab Adv Mat MOE, Sch Mat Sci & Engn, Beijing 100084, Peoples R China.
    Han, L. L.
    Brookhaven Natl Lab, Ctr Funct Nanomat, Upton, NY 11973 USA.
    Lin, R. Q.
    Brookhaven Natl Lab, Ctr Funct Nanomat, Upton, NY 11973 USA.
    Chen, X. F.
    Tsinghua Univ, Natl Ctr Electron Microscopy Beijing, Sch Mat Sci & Engn, Key Lab Adv Mat MOE,State Key Lab New Ceram & Fin, Beijing 100084, Peoples R China.
    Zhu, J.
    Tsinghua Univ, Natl Ctr Electron Microscopy Beijing, Sch Mat Sci & Engn, Key Lab Adv Mat MOE,State Key Lab New Ceram & Fin, Beijing 100084, Peoples R China.
    Detection of magnetic circular dichroism in amorphous materials utilizing a single-crystalline overlayer2017In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 1, no 7, article id UNSP 071404Article in journal (Refereed)
    Abstract [en]

    Electron energy-loss magnetic chiral dichroism (EMCD) is a novel technique that allows magnetic information determination down to the nanoscale. However, constrained by the predefined diffraction geometry in regular EMCD experiments, it has not yet been feasible to obtain EMCD signals from amorphous materials, due to the lack of long-range ordering. Here we propose a protocol for EMCD detection in amorphous materials utilizing a single-crystalline overlayer acting as a two-beam splitter. Phase locking of the EMCD signals is observed and explained by two conceivable scenarios. Both experimental results and theoretical calculations demonstrate significant EMCD signals of amorphous materials.

  • 42.
    Lubk, A.
    et al.
    Tech Univ Dresden, Inst Struct Phys, Triebenberg Lab, D-01062 Dresden, Germany..
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Jacob's ladder of approximations to paraxial dynamic electron scattering2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 23, article id 235114Article in journal (Refereed)
    Abstract [en]

    Dynamical scattering theory describes the dominant scattering process of beam electrons at targets in the transmission electron microscope (TEM). Hence, practically every quantitative TEM study has to consider its ramifications, typically by some approximate modeling. Here, we elaborate on a hierarchy within the various approximations focusing on the two principal approaches used in practice, Bloch wave and multislice. We reveal characteristic differences in the capability of these methods to reproduce the correct local propagation of the wave function, while convergent results are obtained over larger propagation distances. We investigate the dependency of local variations of the wave function on the atomic number of the atomic scatterers and discuss their significance for, e.g., inelastic scattering.

  • 43.
    Löfgren, André
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Zeiger, Paul
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kocevski, Vancho
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Rusz, Ján
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Influence of nuclear quantum effects on frozen phonon simulations of electron vortex beam HAADF-STEM images.2016In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 164, p. 62-69Article in journal (Refereed)
    Abstract [en]

    We have evaluated atomic resolution high-angle annular dark field images with ordinary beams and electron vortex beams for thin crystals of bcc iron, explicitly considering the atomic vibrations using molecular dynamics. The shape of the image representing an atomic column depends on the orbital angular momentum, sample thickness and temperature. For electron vortex beams we observe characteristic doughnut-shaped images of atomic columns. It is shown how the thermal diffuse scattering reduces the depth of their central minima, which get further smeared by finite source size effects. In addition, it is shown that in calculations of HAADF-STEM images at low temperatures one has to explicitly consider the nuclear quantum effects (zero point vibrations), otherwise the effect of atomic vibrations is strongly underestimated.

  • 44. Magnani, N.
    et al.
    Caciuffo, R.
    Wilhelm, F.
    Colineau, E.
    Eloirdi, R.
    Griveau, J. -C
    Rusz, Jan
    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.
    Rogalev, A.
    Lander, G. H.
    Magnetic Polarization of the Americium J=0 Ground State in AmFe22015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, no 9, article id 097203Article in journal (Refereed)
    Abstract [en]

    Trivalent americium has a nonmagnetic (J = 0) ground state arising from the cancellation of the orbital and spin moments. However, magnetism can be induced by a large molecular field if Am3+ is embedded in a ferromagnetic matrix. Using the technique of x-ray magnetic circular dichroism, we show that this is the case in AmFe2. Since < J(z)> = 0, the spin component is exactly twice as large as the orbital one, the total Am moment is opposite to that of Fe, and the magnetic dipole operator < T-z > can be determined directly; we discuss the progression of the latter across the actinide series.

  • 45. Mukherjee, Soham
    et al.
    Nag, Angshuman
    Kocevski, Vancho
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Santra, Pralay K.
    Balasubramanian, Mahalingam
    Chattopadhyay, Soma
    Shibata, Tomohiro
    Schaefers, Franz
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Gerard, Celine
    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.
    Segre, C. U.
    Sarma, Dipankar Das
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Microscopic description of the evolution of the local structure and an evaluation of the chemical pressure concept in a solid solution2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 22, p. 224105-Article in journal (Refereed)
    Abstract [en]

    Extended x-ray absorption fine-structure studies have been performed at the Zn K and Cd K edges for a series of solid solutions of wurtzite Zn1-xCdxS samples with x = 0.0, 0.1, 0.25, 0.5, 0.75, and 1.0, where the lattice parameter as a function of x evolves according to the well-known Vegard's law. In conjunction with extensive, large-scale first-principles electronic structure calculations with full geometry optimizations, these results establish that the percentage variation in the nearest-neighbor bond distances are lower by nearly an order of magnitude compared to what would be expected on the basis of lattice parameter variation, seriously undermining the chemical pressure concept. With experimental results that allow us to probe up to the third coordination shell distances, we provide a direct description of how the local structure, apparently inconsistent with the global structure, evolves very rapidly with interatomic distances to become consistent with it. We show that the basic features of this structural evolution with the composition can be visualized with nearly invariant Zn-S-4 and Cd-S-4 tetrahedral units retaining their structural integrity, while the tilts between these tetrahedral building blocks change with composition to conform to the changing lattice parameters according to the Vegard's law within a relatively short length scale. These results underline the limits of applicability of the chemical pressure concept that has been a favored tool of experimentalists to control physical properties of a large variety of condensed matter systems.

  • 46. Muto, S.
    et al.
    Tatsumi, K.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Parameter-free extraction of EMCD from an energy-filtered diffraction datacube using multivariate curve resolution2013In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 125, p. 89-96Article in journal (Refereed)
    Abstract [en]

    We present a parameter-free method of extraction of the electron magnetic circular dichroism spectra from energy-filtered diffraction patterns measured on a crystalline specimen. The method is based on a multivariate curve resolution technique. The main advantage of the proposed method is that it allows extraction of the magnetic signal regardless of the symmetry and orientation of the crystal, as long as there is a sufficiently strong magnetic component of the signal in the diffraction plane. This method essentially overcomes difficulties in extraction of the EMCD signal caused by complexity of dynamical diffraction effects.

  • 47. Muto, Shunsuke
    et al.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Tatsumi, Kazumasa
    Thersleff, Thomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    (Invited) Current progress in nanometric magnetic moment measurements based on electron magnetic circular dichroism2015Conference paper (Refereed)
  • 48. Muto, Shunsuke
    et al.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Tatsumi, Kazuyoshi
    Adam, Roman
    Arai, Shigeo
    Kocevski, Vancho
    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.
    Buergler, Daniel E.
    Schneider, Claus M.
    Quantitative characterization of nanoscale polycrystalline magnets with electron magnetic circular dichroism2014In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, p. 3138-Article in journal (Refereed)
    Abstract [en]

    Electron magnetic circular dichroism (EMCD) allows the quantitative, element-selective determination of spin and orbital magnetic moments, similar to its well-established X-ray counterpart, X-ray magnetic circular dichroism (XMCD). As an advantage over XMCD, EMCD measurements are made using transmission electron microscopes, which are routinely operated at sub-nanometre resolution, thereby potentially allowing nanometre magnetic characterization. However, because of the low intensity of the EMCD signal, it has not yet been possible to obtain quantitative information from EMCD signals at the nanoscale. Here we demonstrate a new approach to EMCD measurements that considerably enhances the outreach of the technique. The statistical analysis introduced here yields robust quantitative EMCD signals. Moreover, we demonstrate that quantitative magnetic information can be routinely obtained using electron beams of only a few nanometres in diameter without imposing any restriction regarding the crystalline order of the specimen.

  • 49.
    Negi, Devendra Singh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Datta, Ranjan
    Jawaharlal Nehru Ctr Adv Sci Res, Int Ctr Mat Sci, Bangalore 560064, Karnataka, India;Jawaharlal Nehru Ctr Adv Sci Res, Chem & Phys Mat Unit, Bangalore 560064, Karnataka, India.
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Defect driven spin state transition and the existence of half-metallicity in CoO2019In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, no 11, article id 115602Article in journal (Refereed)
    Abstract [en]

    We unveil the native defect induced high spin to low spin state transition in Co+3 and half- metallicity in CoO. First principles calculations unravel that, defect density holds a key role in dictating the spin-state transition in Co+3 ion in CoO, and introducing the half-metallicity. Charge transfer in the vicinity of vacancy plane favors the stabilization and coexistence of bivalent Co+2 and trivalent Co+3 ion in CoO. We propose that defect engineering could serve as a route to design the half metallicity in transition metal mono-oxides.

  • 50.
    Negi, Devendra Singh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Idrobo, Juan Carlos
    Ctr Nanophase Mat Sci, Oak Ridge Natl Lab, Oak Ridge, TN 37831 USA..
    Rusz, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Probing the localization of magnetic dichroism by atomic-size astigmatic and vortex electron beams2018In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 4019Article in journal (Refereed)
    Abstract [en]

    We report localization of a magnetic dichroic signal on atomic columns in electron magnetic circular dichroism ( EMCD), probed by beam distorted by four-fold astigmatism and electron vortex beam. With astigmatic probe, magnetic signal to noise ratio can be enhanced by blocking the intensity from the central part of probe. However, the simulations show that for atomic resolution magnetic measurements, vortex beam is a more effective probe, with much higher magnetic signal to noise ratio. For all considered beam shapes, the optimal SNR constrains the signal detection at low collection angles of approximately 6-8 mrad. Irrespective of the material thickness, the magnetic signal remains strongly localized within the probed atomic column with vortex beam, whereas for astigmatic probes, the magnetic signal originates mostly from the nearest neighbor atomic columns. Due to excellent signal localization at probing individual atomic columns, vortex beams are predicted to be a strong candidate for studying the crystal site specific magnetic properties, magnetic properties at interfaces, or magnetism arising from individual atomic impurities.

123 1 - 50 of 136
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