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  • 151.
    Bessarab, Pavel F.
    et al.
    Univ Iceland, Sci Inst, IS-107 Reykjavik, Iceland.;ITMO Univ, St Petersburg 197101, Russia..
    Müller, Gideon P.
    Univ Iceland, Sci Inst, IS-107 Reykjavik, Iceland.;Forschungszentrum Julich, Peter Grunberg Inst, D-52425 Julich, Germany.;Forschungszentrum Julich, Inst Adv Simulat, D-52425 Julich, Germany.;JARA, D-52425 Julich, Germany..
    Lobanov, Igor S.
    ITMO Univ, St Petersburg 197101, Russia..
    Rybakov, Filipp N.
    KTH Royal Inst Technol, Dept Phys, SE-10691 Stockholm, Sweden..
    Kiselev, Nikolai S.
    Forschungszentrum Julich, Peter Grunberg Inst, D-52425 Julich, Germany.;Forschungszentrum Julich, Inst Adv Simulat, D-52425 Julich, Germany.;JARA, D-52425 Julich, Germany..
    Jonsson, Hannes
    Univ Iceland, Sci Inst, IS-107 Reykjavik, Iceland.;Aalto Univ, FI-00076 Espoo, Finland..
    Uzdin, Valery M.
    ITMO Univ, St Petersburg 197101, Russia.;St Petersburg State Univ, Dept Phys, St Petersburg 198504, Russia.;ITMO Univ, St Petersburg 197101, Russia..
    Blügel, Stefan
    Forschungszentrum Julich, Peter Grunberg Inst, D-52425 Julich, Germany.;Forschungszentrum Julich, Inst Adv Simulat, D-52425 Julich, Germany.;JARA, D-52425 Julich, Germany..
    Bergqvist, Lars
    KTH Royal Inst Technol, Sch Engn Sci, Dept Appl Phys, Electrum 229, SE-16440 Kista, Sweden.;KTH Royal Inst Technol, SeRC Swedish E Sci Res Ctr, SE-10044 Stockholm, Sweden..
    Delin, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Sch Engn Sci, Dept Appl Phys, Electrum 229, SE-16440 Kista, Sweden.;KTH Royal Inst Technol, SeRC Swedish E Sci Res Ctr, SE-10044 Stockholm, Sweden..
    Lifetime of racetrack skyrmions2018In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 3433Article in journal (Refereed)
    Abstract [en]

    The skyrmion racetrack is a promising concept for future information technology. There, binary bits are carried by nanoscale spin swirls-skyrmions-driven along magnetic strips. Stability of the skyrmions is a critical issue for realising this technology. Here we demonstrate that the racetrack skyrmion lifetime can be calculated from first principles as a function of temperature, magnetic field and track width. Our method combines harmonic transition state theory extended to include Goldstone modes, with an atomistic spin Hamiltonian parametrized from density functional theory calculations. We demonstrate that two annihilation mechanisms contribute to the skyrmion stability: At low external magnetic field, escape through the track boundary prevails, but a crossover field exists, above which the collapse in the interior becomes dominant. Considering a Pd/Fe bilayer on an Ir(111) substrate as a well-established model system, the calculated skyrmion lifetime is found to be consistent with reported experimental measurements. Our simulations also show that the Arrhenius pre-exponential factor of escape depends only weakly on the external magnetic field, whereas the pre-exponential factor for collapse is strongly field dependent. Our results open the door for predictive simulations, free from empirical parameters, to aid the design of skyrmion-based information technology.

  • 152.
    Beutier, G.
    et al.
    Univ Grenoble Alpes, CNRS, Grenoble INP, SIMaP, F-38000 Grenoble, France..
    Collins, S. P.
    Diamond Light Source Ltd, Diamond House,Harwell Sci & Innovat Campus, Didcot OX11 0DE, Oxon, England..
    Dimitrova, O. V.
    Moscow MV Lomonosov State Univ, Moscow 119991, Russia..
    Dmitrienko, V. E.
    RAS, FSRC Crystallog & Photon, AV Shubnikov Inst Crystallog, Moscow 119333, Russia..
    Katsnelson, M. I.
    Radboud Univ Nijmegen, Inst Mol & Mat, Heyendaalseweg 135, NL-6525 AJ Nijmegen, Netherlands.;Ural Fed Univ, Dept Theoret Phys & Appl Math, Mira 19, Ekaterinburg 620002, Russia..
    Kvashnin, Yaroslav
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Lichtenstein, A. I.
    Ural Fed Univ, Dept Theoret Phys & Appl Math, Mira 19, Ekaterinburg 620002, Russia.;Univ Hamburg, Inst Theoret Phys 1, Jungiusstr 9, D-20355 Hamburg, Germany..
    Mazurenko, V. V.
    Ural Fed Univ, Dept Theoret Phys & Appl Math, Mira 19, Ekaterinburg 620002, Russia..
    Nisbet, A. G. A.
    Diamond Light Source Ltd, Diamond House,Harwell Sci & Innovat Campus, Didcot OX11 0DE, Oxon, England..
    Ovchinnikova, E. N.
    Moscow MV Lomonosov State Univ, Moscow 119991, Russia..
    Pincini, D.
    Diamond Light Source Ltd, Diamond House,Harwell Sci & Innovat Campus, Didcot OX11 0DE, Oxon, England.;UCL, London Ctr Nanotechnol, London WC1E 6BT, England.;UCL, Dept Phys & Astron, London WC1E 6BT, England..
    Band Filling Control of the Dzyaloshinskii-Moriya Interaction in Weakly Ferromagnetic Insulators2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, no 16, article id 167201Article in journal (Refereed)
    Abstract [en]

    We observe and explain theoretically a dramatic evolution of the Dzyaloshinskii-Moriya interaction (DMI) in the series of isostructural weak ferromagnets, MnCO3, FeBO3, CoCO3, and NiCO3. The sign of the interaction is encoded in the phase of the x-ray magnetic diffraction amplitude, observed through interference with resonant quadrupole scattering. We find very good quantitative agreement with first-principles electronic structure calculations, reproducing both sign and magnitude through the series, and propose a simplified "toy model" to explain the change in sign with 3d shell filling. The model gives insight into the evolution of the DMI in Mott and charge transfer insulators.

  • 153. Bezerra-Neto, Manoel M.
    et al.
    Ribeiro, Marcelo S.
    Sanyal, Biplab
    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.
    Muniz, Roberto B.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Klautau, Angela B.
    Complex magnetic structure of clusters and chains of Ni and Fe on Pt(111)2013In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, p. 3054-Article in journal (Refereed)
    Abstract [en]

    We present an approach to control the magnetic structure of adatoms adsorbed on a substrate having a high magnetic susceptibility. Using finite Ni-Pt and Fe-Pt nanowires and nanostructures on Pt(111) surfaces, our ab initio results show that it is possible to tune the exchange interaction and magnetic configuration of magnetic adatoms (Fe or Ni) by introducing different numbers of Pt atoms to link them, or by including edge effects. The exchange interaction between Ni (or Fe) adatoms on Pt(111) can be considerably increased by introducing Pt chains to link them. The magnetic ordering can be regulated allowing for ferromagnetic or antiferromagnetic configurations. Noncollinear magnetic alignments can also be stabilized by changing the number of Pt-mediated atoms. An Fe-Pt triangularly-shaped nanostructure adsorbed on Pt(111) shows the most complex magnetic structure of the systems considered here: a spin-spiral type of magnetic order that changes its propagation direction at the triangle vertices.

  • 154.
    Bhandary, Sumanta
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    First Principles Studies of Functional Materials Based on Graphene and Organometallics2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Graphene is foreseen to be the basis of future electronics owing to its ultra thin structure, extremely high charge carrier mobility,  high thermal conductivity etc., which are expected to overcome the size limitation and heat dissipation problem in silicon based transistors. But these great prospects are hindered by the metallic nature of pristine graphene even at charge neutrality point, which allows to flow current even when a transistor is switched off. A part  of the thesis is dedicated to invoke electronic band gaps in graphene to overcome this problem. The concept of quantum confinement has been employed to tune the band gaps in graphene by  dimensional confinement along with the functionalization of the edges of these confined nanostructures. Thermodynamic stability of the functionalized zigzag edges with hydrogen, fluorine and reconstructed edges has been presented in the thesis. Keeping an eye towards the same goal of band gap opening,  a different route has been considered by admixing insulating hexagonal boron nitride (h-BN) with semimetal graphene. The idea has been implemented in two  dimensional h-BN-graphene composites and three dimensional stacked heterostructures. The study reveals the possibility of tuning band gaps by controlling the admixture. Occurrence of defects in graphene has significant effect on its electronic properties. By random insertion of defects, amorphous graphene is studied, revealing a semi-metal to a metal transition.

    The field of molecular electronics and spintronics aims towards device realization at the molecular scale. In this thesis, different aspects of magnetic bistability in organometallic molecules have been explored in order to design  practical spintronics devices. Manipulation of spin states in organometallic molecules, specifically metal porphyrin molecules, is achieved by controlling surface–molecule interaction. It has been shown that by strain engineering in defected graphene, the magnetic state of adsorbed molecules can be changed. The spin crossover between different spin states can also be achieved by chemisorption on magnetic surfaces. A significant part of the thesis demonstrates that the surface-molecule interaction not only changes the spin state of the molecule, but allows to manipulate magnetic anisotropies and spin dipole moments via modified ligand fields. Finally, in collaboration with experimentalists, a practical realization of switching surface–molecule magnetic interactions by external magnetic fields is demonstrated.

    List of papers
    1. Complex edge effects in zigzag graphene nanoribbons due to hydrogen loading
    Open this publication in new window or tab >>Complex edge effects in zigzag graphene nanoribbons due to hydrogen loading
    2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 16, p. 165405-Article in journal (Refereed) Published
    Abstract [en]

    We have performed density-functional calculations as well as employed a tight-binding theory, to study the effect of passivation of zigzag graphene nanoribbons (ZGNR) by hydrogen. We show that each edge C atom bonded with 2 H atoms open up a gap and destroys magnetism for small widths of the nanoribbon. However, a re-entrant magnetism accompanied by a metallic electronic structure is observed from eight rows and thicker nanoribbons. The electronic structure and magnetic state are quite complex for this type of termination, with sp(3) bonded edge atoms being nonmagnetic whereas the nearest neighboring atoms are metallic and magnetic. We have also evaluated the phase stability of several thicknesses of ZGNR and demonstrate that sp(3) bonded edge atoms with 2 H atoms at the edge can be stabilized over 1 H atom terminated edge at high temperatures and pressures.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-134336 (URN)10.1103/PhysRevB.82.165405 (DOI)000282422700005 ()
    Available from: 2010-11-25 Created: 2010-11-24 Last updated: 2017-12-12Bibliographically approved
    2. Controlling electronic structure and transport properties of zigzag graphene nanoribbons by mono- and difluorinated edge functionalization
    Open this publication in new window or tab >>Controlling electronic structure and transport properties of zigzag graphene nanoribbons by mono- and difluorinated edge functionalization
    Show others...
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    In this work, we report a detailed study of the electronic structure and transport properties of mono- and di-fluorinated edges of zigzag graphene nanoribbons (ZGNR) using density functional theory (DFT). The calculated formation energies at 0K indicate that the stability of the nanoribbons increases with the increase in the concentration of di-fluorinated edge C atoms along with an interesting variation of the energy gaps between 0.0 to 0.66 eV depending on the concentration. This gives a possibility of tuning the band gaps by controlling the concentration of F for terminating the edges of the nanoribbons. The DFT results have been reproduced by single band tight binding as well as density functional tight binding methods. Using non-equilibrium Green functional method, we have calculated the transmission coecients of several mono and di-fluorinated ZGNR as a function of unit cell size and degree of homogeneous disorder caused by the random placement of mono and di-fuorinated C atoms at the edges.

    Keywords
    Graphene nanoribbons, DFT, Band gap, Transport
    National Category
    Condensed Matter Physics
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-217155 (URN)
    Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2018-06-04
    3. Functionalization of edge reconstructed graphene nanoribbons by H and Fe: A density functional study
    Open this publication in new window or tab >>Functionalization of edge reconstructed graphene nanoribbons by H and Fe: A density functional study
    Show others...
    2012 (English)In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 152, no 18, p. 1719-1724Article in journal (Refereed) Published
    Abstract [en]

    In this paper, we have studied functionalization of 5-7 edge-reconstructed graphene nanoribbons by ab initio density functional calculations. Our studies show that hydrogenation at the reconstructed edges is favorable in contrast to the case of unreconstructed 6-6 zigzag edges, in agreement with previous theoretical results. Thermodynamical calculations reveal the relative stability of single and dihydro-genated edges under different temperatures and chemical potential of hydrogen gas. From phonon calculations, we find that the lowest optical phonon modes are hardened due to 5-7 edge reconstruction compared to the 6-6 unreconstructed hydrogenated edges. Finally, edge functionalization by Fe atoms reveals a dimerized Fe chain structure along the edges. The magnetic exchange coupling across the edges varies between ferromagnetic and antiferromagnetic ones with the variation of the width of the nanoribbons.

    Keywords
    Graphene, Functionalization, Magnetism, Phonons, Density functional theory
    National Category
    Atom and Molecular Physics and Optics
    Identifiers
    urn:nbn:se:uu:diva-177362 (URN)10.1016/j.ssc.2012.06.028 (DOI)000308776600003 ()
    Available from: 2012-07-11 Created: 2012-07-11 Last updated: 2017-12-07Bibliographically approved
    4. Interpolation of Atomically Thin Hexagonal Boron Nitride and Graphene: Electronic Structure and Thermodynamic Stability in Terms of All-Carbon Conjugated Paths and Aromatic Hexagons
    Open this publication in new window or tab >>Interpolation of Atomically Thin Hexagonal Boron Nitride and Graphene: Electronic Structure and Thermodynamic Stability in Terms of All-Carbon Conjugated Paths and Aromatic Hexagons
    2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 20, p. 10264-10271Article in journal (Refereed) Published
    Abstract [en]

    Two-dimensional hexagonal composite materials (BN)(n)(C-2)(m) (n, m = 1, 2,...), which all are isoelectronic with graphene and hexagonal boron nitride (h-BN), have been studied by density functional theory (DFT) with a focus on the relative energies of different material isomers and their band gaps. The well-established chemical concepts of conjugation and aromaticity were exploited to deduce a rationale for identifying the thermodynamically most stable isomer of the specific composites studied. We find that (BN)(n)(C-2)(m) materials will not adopt structures in which the B, C, and N atoms are finely dispersed in the 2D sheet. Instead, the C atoms and C-C bonds, which provide for improved conjugation when compared to B-N bonds, gather and form all-carbon hexagons and paths; that is, the (BN)(n)(C-2)(m) materials prefer nanostructured distributions. Importantly, there are several isomers of similarly low relative energy for each (BN)(n)(C-2)(m) composite type, but the band gaps for these nearly isoenergetic isomers differ by up to 1.0 eV. This feature in the band gap variation of the most stable few isomers is found for each of the four composites studied and at two different DFT levels. Consequently, the formation of a distinct (BN)(n)(C-2)(m) material isomer with a precise (small) band gap will likely be nontrivial. Therefore, one likely has to invoke nonstandard preparation techniques that exploit nanopatterned h-BN or graphene with voids that can be filled with the complementary all-carbon or boron nitride segments.

    National Category
    Physical Sciences Chemical Sciences
    Identifiers
    urn:nbn:se:uu:diva-154600 (URN)10.1021/jp2016616 (DOI)000290652200055 ()
    Available from: 2011-06-09 Created: 2011-06-08 Last updated: 2017-12-11Bibliographically approved
    5. Quasiperiodic van der Waals heterostructures of graphene and h-BN
    Open this publication in new window or tab >>Quasiperiodic van der Waals heterostructures of graphene and h-BN
    (English)Manuscript (preprint) (Other academic)
    Keywords
    Graphene, Boron nitride, Heterostructure, Fibonacci sequence, Band gap
    National Category
    Condensed Matter Physics
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-217159 (URN)
    Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2017-01-25
    6. Disorder-induced metallicity in amorphous graphene
    Open this publication in new window or tab >>Disorder-induced metallicity in amorphous graphene
    Show others...
    2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 20, p. 205414-Article in journal (Refereed) Published
    Abstract [en]

    We predict a transition to metallicity when a sufficient amount of disorder is induced in graphene. Calculations were performed by means of a first-principles stochastic quench method. The resulting amorphous graphene can be seen as nanopatches of graphene that are connected by a network of disordered small and large carbon rings. The buckling of the lattice is minimal and is a result of averaging of counteracting random in-plane stress forces. The linear response conductance is obtained by a model theory as function of lattice distortions, and results in a similar behavior as the first-principles calculation.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-163665 (URN)10.1103/PhysRevB.84.205414 (DOI)000296911900009 ()
    Available from: 2011-12-14 Created: 2011-12-13 Last updated: 2017-12-08Bibliographically approved
    7. Correlated electron behavior of metalorganic molecules: insights from density functional theory and exact diagonalization studies.
    Open this publication in new window or tab >>Correlated electron behavior of metalorganic molecules: insights from density functional theory and exact diagonalization studies.
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    The proper description of electronic structure of correlated orbitals in the metal centers of functional metalorganics is a challenging problem. In this letter, we apply density functional theory and exact diagonalization method in a many body approach to study the ground state electronic conguration of iron porphyrin (FeP) molecule. Our study reveals that FeP is a potential candidate for realizing a spin crossover due to a subtle balance of crystal elds and hybridization of the Fe d-orbitals and ligand N p-states. Moreover, the mechanism of switching between two close lying electronic congurations of Fe-d orbitals is revealed. This hybrid method can generally be applied to properly describe the electronic and related low energy physics of the whole class of correlated metal centered organometallic molecules.

    Keywords
    Molecular magnet, Porphyrin, DFT, DFT++, Spin crossover
    National Category
    Condensed Matter Physics
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-217168 (URN)
    Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2018-06-04
    8. Graphene as a Reversible Spin Manipulator of Molecular Magnets
    Open this publication in new window or tab >>Graphene as a Reversible Spin Manipulator of Molecular Magnets
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    2011 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 25, p. 257202-Article in journal (Refereed) Published
    Abstract [en]

    One of the primary objectives in molecular nanospintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. In this Letter, we demonstrate by first principles density functional calculations, as well as second order perturbation theory, that a strain induced change of the spin state, from S = 1 -> S = 2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in the sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-167201 (URN)10.1103/PhysRevLett.107.257202 (DOI)000298133000003 ()
    Available from: 2012-01-23 Created: 2012-01-23 Last updated: 2017-12-08Bibliographically approved
    9. Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates
    Open this publication in new window or tab >>Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates
    Show others...
    2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 2, p. 024401-Article in journal (Refereed) Published
    Abstract [en]

    One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this paper we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N K x-ray absorption (XA) and Fe L-2,L-3 x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state (S = 2) from an intermediate spin state (S = 1) is achieved. Due to chemisorption, ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy.

    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-204271 (URN)10.1103/PhysRevB.88.024401 (DOI)000321123200001 ()
    Available from: 2013-07-30 Created: 2013-07-29 Last updated: 2017-12-06Bibliographically approved
    10. Defect controlled magnetism in FeP/graphene/Ni(111)
    Open this publication in new window or tab >>Defect controlled magnetism in FeP/graphene/Ni(111)
    2013 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, p. 3405-Article in journal (Refereed) Published
    Abstract [en]

    Spin switching of organometallic complexes by ferromagnetic surfaces is an important topic in the area of molecular nanospintronics. Moreover, graphene has been shown as a 2D surface for physisorption of molecular magnets and strain engineering on graphene can tune the spin state of an iron porphyrin (FeP) molecule from S = 1 to S = 2. Our ab initio density functional calculations suggest that a pristine graphene layer placed between a Ni(111) surface and FeP yields an extremely weak exchange interaction between FeP and Ni whereas the introduction of defects in graphene shows a variety of ferromagnetic and antiferromagnetic exchange interactions. Moreover, these defects control the easy axes of magnetization, strengths of magnetic anisotropy energies and spin-dipolar contributions. Our study suggests a new way of manipulating molecular magnetism by defects in graphene and hence has the potential to be explored in designing spin qubits to realize logic operations in molecular nanospintronics.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-213899 (URN)10.1038/srep03405 (DOI)000327784400002 ()
    Available from: 2014-01-06 Created: 2014-01-05 Last updated: 2017-12-06Bibliographically approved
    11. Field-regulated switching of the magnetization of Co-porphyrin on graphene
    Open this publication in new window or tab >>Field-regulated switching of the magnetization of Co-porphyrin on graphene
    Show others...
    2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 14, p. 144411-Article in journal (Refereed) Published
    Abstract [en]

    Different magnetic coupling mechanisms have been identied for a few monolayers of Co-porphyrin molecules deposited on a graphene covered Ni(111) single crystal. A relatively strong antiferromagnetic coupling of the first molecular layer via graphene to the Ni crystal in comparison to a weaker inter-molecular coupling gives rise to a complex field-dependent response of this hybrid system. By continuously increasing the magnetic eld strength the net magnetization of the molecular system switches from antiparallel to parallel to the field direction at 2.5 T. Utilizing x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element specic magnetization and field dependence was probed. The nature of the magnetic couplings is identied by means of density functional theory and orbital dependent susceptibilities.

    Keywords
    Molecular magnet, Porphyrin, Magnetic field, XMCD
    National Category
    Condensed Matter Physics
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-217173 (URN)10.1103/PhysRevB.89.144411 (DOI)000337348300004 ()
    Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2017-12-06Bibliographically approved
  • 155.
    Bhandary, Sumanta
    et al.
    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.
    Panchmatia, Pooja M.
    Brumboiu, Iulia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bernien, Matthias
    Weis, Claudia
    Krumme, Bernhard
    Etz, Corina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kuch, Wolfgang
    Wende, Heiko
    Eriksson, Olle
    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.
    Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 2, p. 024401-Article in journal (Refereed)
    Abstract [en]

    One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this paper we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N K x-ray absorption (XA) and Fe L-2,L-3 x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state (S = 2) from an intermediate spin state (S = 1) is achieved. Due to chemisorption, ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy.

  • 156.
    Bhandary, Sumanta
    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.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Defect controlled magnetism in FeP/graphene/Ni(111)2013In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, p. 3405-Article in journal (Refereed)
    Abstract [en]

    Spin switching of organometallic complexes by ferromagnetic surfaces is an important topic in the area of molecular nanospintronics. Moreover, graphene has been shown as a 2D surface for physisorption of molecular magnets and strain engineering on graphene can tune the spin state of an iron porphyrin (FeP) molecule from S = 1 to S = 2. Our ab initio density functional calculations suggest that a pristine graphene layer placed between a Ni(111) surface and FeP yields an extremely weak exchange interaction between FeP and Ni whereas the introduction of defects in graphene shows a variety of ferromagnetic and antiferromagnetic exchange interactions. Moreover, these defects control the easy axes of magnetization, strengths of magnetic anisotropy energies and spin-dipolar contributions. Our study suggests a new way of manipulating molecular magnetism by defects in graphene and hence has the potential to be explored in designing spin qubits to realize logic operations in molecular nanospintronics.

  • 157.
    Bhandary, Sumanta
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ghosh, Saurabh
    Herper, Heike
    Wende, Heiko
    Eriksson, Olle
    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.
    Graphene as a Reversible Spin Manipulator of Molecular Magnets2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 25, p. 257202-Article in journal (Refereed)
    Abstract [en]

    One of the primary objectives in molecular nanospintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. In this Letter, we demonstrate by first principles density functional calculations, as well as second order perturbation theory, that a strain induced change of the spin state, from S = 1 -> S = 2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in the sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP.

  • 158.
    Bhandary, Sumanta
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Szunyogh, Laszlo
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Route towards finding large magnetic anisotropy in nanocomposites: Application to a W(1-x)Re(x)/Fe multilayer2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 9, p. 092401-Article in journal (Refereed)
    Abstract [en]

    We suggest here a nanolaminate, 5[Fe]/2[W(x)Re(1-x)] (x = 0.6-0.8), with enhanced magnetic hardness in combination with a large saturation moment. The calculated magnetic anisotropy of this material reaches values of 5.3-7.0 MJ/m(3), depending on alloying conditions. We also propose a recipe in how to identify other novel magnetic materials, such as nanolaminates and multilayers, with large magnetic anisotropy in combination with a high saturation moment.

  • 159.
    Bhandary, Sumanta
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Haldar, Soumyajyoti
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Quasiperiodic van der Waals heterostructures of graphene and h-BNManuscript (preprint) (Other academic)
  • 160.
    Bhandary, Sumanta
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Penazzi, Gabriele
    Univ Bremen, BCCMS, D-28359 Bremen, Germany..
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Frauenheim, Thomas
    Univ Bremen, BCCMS, D-28359 Bremen, Germany..
    Eriksson, Olle
    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.
    Controlling Electronic Structure and Transport Properties of Zigzag Graphene Nanoribbons by Edge Functionalization with Fluorine2015In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 36, p. 21227-21233Article in journal (Refereed)
    Abstract [en]

    In this work, we report a detailed study of the electronic structure and transport properties of mono- and difluorinated edges of zigzag graphene nanoribbons (ZGNR) using density functional theory (DFT). The calculated formation energies at 0 K indicate that the stability of the nanoribbons increases with the increase in the concentration of difluorinated edge C atoms along with an interesting variation of the energy gaps between 0.0 to 0.66 eV depending on the concentration. This gives a possibility of tuning the band gaps by controlling the concentration of F for terminating the edges of the nanoribbons. The DFT results have been reproduced by density functional tight binding method. Using the nonequilibrium Green functional method, we have calculated the transmission coefficients of several mono- and difluorinated ZGNR as a function of unit cell size and degree of homogeneous disorder caused by the random placement of mono and difluorinated C atoms at the edges.

  • 161.
    Bhandary, Sumanta
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Penazzi, Gabriele
    BCCMS, Universitat Bremen.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Frauenheim, Thomas
    BCCMS, Universitat Bremen.
    Eriksson, Olle
    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.
    Controlling electronic structure and transport properties of zigzag graphene nanoribbons by mono- and difluorinated edge functionalizationManuscript (preprint) (Other academic)
    Abstract [en]

    In this work, we report a detailed study of the electronic structure and transport properties of mono- and di-fluorinated edges of zigzag graphene nanoribbons (ZGNR) using density functional theory (DFT). The calculated formation energies at 0K indicate that the stability of the nanoribbons increases with the increase in the concentration of di-fluorinated edge C atoms along with an interesting variation of the energy gaps between 0.0 to 0.66 eV depending on the concentration. This gives a possibility of tuning the band gaps by controlling the concentration of F for terminating the edges of the nanoribbons. The DFT results have been reproduced by single band tight binding as well as density functional tight binding methods. Using non-equilibrium Green functional method, we have calculated the transmission coecients of several mono and di-fluorinated ZGNR as a function of unit cell size and degree of homogeneous disorder caused by the random placement of mono and di-fuorinated C atoms at the edges.

  • 162.
    Bhandary, Sumanta
    et al.
    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.
    Graphene-Boron Nitride Composite: A Material with Advanced Functionalities2012In: Composites and their properties / [ed] Ning Hu, INTECH, 2012Chapter in book (Refereed)
  • 163.
    Bhandary, Sumanta
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Wehling, Tim
    Institute for Theoretical Physics, University of Bremen.
    Eriksson, Olle
    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.
    Correlated electron behavior of metalorganic molecules: insights from density functional theory and exact diagonalization studies.Manuscript (preprint) (Other academic)
    Abstract [en]

    The proper description of electronic structure of correlated orbitals in the metal centers of functional metalorganics is a challenging problem. In this letter, we apply density functional theory and exact diagonalization method in a many body approach to study the ground state electronic conguration of iron porphyrin (FeP) molecule. Our study reveals that FeP is a potential candidate for realizing a spin crossover due to a subtle balance of crystal elds and hybridization of the Fe d-orbitals and ligand N p-states. Moreover, the mechanism of switching between two close lying electronic congurations of Fe-d orbitals is revealed. This hybrid method can generally be applied to properly describe the electronic and related low energy physics of the whole class of correlated metal centered organometallic molecules.

  • 164.
    Bhattacharjee, Satadeep
    et al.
    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.
    Taroni, Andrea
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Hellsvik, Johan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    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.
    Theoretical Analysis of Inertia-like Switching in Magnets: Applications to a Synthetic Antiferromagnet2012In: PHYS REV X, ISSN 2160-3308, Vol. 2, no 1, p. 011013-Article in journal (Refereed)
    Abstract [en]

    The magnetization dynamics of a synthetic antiferromagnet subjected to a short-magnetic-field pulse has been studied by using a combination of first principles calculations and atomistic spin-dynamics simulations. We observe switching phenomena on the time scale of tens of picoseconds, and inertia-like behavior in the magnetization dynamics. We explain the latter in terms of a dynamic redistribution of magnetic energy from the applied-field pulse to other possible energy terms, such as the exchange interaction and the magnetic anisotropy, without invoking concepts such as the inertia of an antiferro-magnetic vector. We also demonstrate that such dynamics can also be observed in a ferromagnetic material where the incident-field pulse pumps energy to the magnetic anisotropy.

  • 165.
    Bhattacharjee, Satadeep
    et al.
    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.
    Banerjee, Rudra
    Wende, Heiko
    Eriksson, Olle
    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.
    Electronic structure of Co-phthalocyanine calculated by GGA plus U and hybrid functional methods2010In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 377, no 1-3, p. 96-99Article in journal (Refereed)
    Abstract [en]

    Electronic structure calculations have been performed for the Co-phthalocyanine molecule using density functional theory (DFT) within the framework of Generalized Gradient Approximation (GGA). The electronic correlation in Co 3d orbitals is treated in terms of the GGA+U method in the framework of the Hubbard model. We find that for U = 6 eV, the calculated structural parameters as well as the spectral features are in good agreement with the experimental findings. From our calculation both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are dominated by the pyrrole carbon, with a HOMO-LUMO gap of about 1.4 eV. The GGA+U results obtained with U = 6 eV compare reasonably well with the calculations performed using Gaussian basis set and hybrid functionals in terms of ground state geometry, spin state and spectral features. The calculated valence band photoemission spectrum is in quite good agreement with the recently published experimental results.

  • 166.
    Bhattacharjee, Satadeep
    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.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    First principles calculations of magnetism, dielectric properties and spin-phonon coupling in double perovskite Bi2CoMnO62012In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 29, p. 295901-Article in journal (Refereed)
    Abstract [en]

    First principles electronic structure calculations have been performed for the double perovskite Bi2CoMnO6 in its non-centrosymmetric polar state using the generalized gradient approximation plus the Hubbard U approach. We find that the ferromagnetic state is more favored compared to the ferrimagnetic state with both Co and Mn in high spin states. The calculated dynamical charge tensors are anisotropic reflecting a low-symmetry structure of the compound. The magnetic structure dependent phonon frequencies indicate the presence of a weak spin-phonon coupling. Using the Berry phase method, we obtain a spontaneous ferroelectric polarization of 5.88 mu C cm(-2), which is close to the experimental value observed for a similar compound, Bi2NiMnO6.

  • 167.
    Bhattacharjee, Satadeep
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Atomistic Spin Dynamic Method with both Damping and Moment of Inertia Effects Included from First Principles2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, no 5, p. 057204-Article in journal (Refereed)
    Abstract [en]

    We consider spin dynamics for implementation in an atomistic framework and we address the feasibility of capturing processes in the femtosecond regime by inclusion of moment of inertia. In the spirit of an s-d-like interaction between the magnetization and electron spin, we derive a generalized equation of motion for the magnetization dynamics in the semiclassical limit, which is nonlocal in both space and time. Using this result we retain a generalized Landau-Lifshitz-Gilbert equation, also including the moment of inertia, and demonstrate how the exchange interaction, damping, and moment of inertia, all can be calculated from first principles.

  • 168.
    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.

  • 169.
    Bidermane, Ieva
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Brumboiu, Iulia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Totani, Roberta
    University of L'Aquila.
    Grazioli, Cesare
    University of Trieste.
    Shariati Nilsson, Masumeh Nina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Herper, Heike
    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.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ressel, B
    Univ Nova Gorica, Ajdovscina 5270, Slovenia.
    de Simone, Monica
    Lozzi, Luca
    University of L'Aquila.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Puglia, Carla
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Atomic Contributions to the Valence Band Photoelectron Spectra of Metal-free, Iron and Manganese Phthalocyanines2015In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 205, p. 92-97Article, review/survey (Other academic)
    Abstract [en]

    The present work reports a photoelectron spectroscopy study of the low-energy region of the valence band of metal-free phthalocyanine (H2Pc) compared with those of iron phthalocyanine (FePc) and manganese phthalocyanine (MnPc). We have analysed in detail the atomic orbital composition of the valence band both experimentally, by making use of the variation in photoionization cross-sections with photon energy, and theoretically, by means of density functional theory. The atomic character of the Highest Occupied Molecular Orbital (HOMO), reflected on the outermost valence band binding energy region, is different for MnPc as compared to the other two molecules. The peaks related to the C 2p contributions, result in the HOMO for H2Pc and FePc and in the HOMO-1 for MnPc as described by the theoretical predictions, in very good agreement with the experimental results. The DFT simulations, discerning the atomic contribution to the density of states, indicate how the central metal atom interacts with the C and N atoms of the molecule, giving rise to different partial and total density of states for these three Pc molecules.

  • 170.
    Bidermane, Ieva
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Lüder, Johann
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahmadi, S.
    Materialfysik, KTH-Electrum.
    Grazioli, C.
    CNR-IOM, Laboratorio TASC.
    Bouvet, M.
    Institut de Chimie Moléculaire de l’Université de Bourgogne.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Mårtensson, Niklas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Puglia, Carla
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Witkowski, N.
    Institut des Nanosciences de Paris, UPMC.
    When the Grafting of Double Decker Phthalocyanines on Si(100)-2 × 1 Partly Affects the Molecular Electronic Structure2016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 26, p. 14270-14276Article in journal (Refereed)
    Abstract [en]

    A combined X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density functional theory (DFT) study has been performed to characterize the adsorbate interaction of lutetium biphthalocyanine (LuPc2) molecules on the Si(100)-2 × 1 surface. Large molecule–substrate adsorption energies are computed and are found to compete with the molecule–molecule interactions of the double decker molecules. A particularly good matching between STM images and computed ones confirms the deformation of the molecule upon the absorption process. The comparison between DFT calculations and XP spectra reveals that the electronic distribution in the two plateaus of the biphthalocyanine are not affected in the same manner upon the adsorption onto the silicon surface. This finding can be of particular importance in the implementation of organic molecules in hybrid devices.

  • 171.
    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. 

  • 172.
    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.
    Totani, Roberta
    University of L'Aquila.
    Grazioli, Cesare
    University of Trieste.
    de Simone, Monica
    Coreno, Marcello
    Kivimäki, Antti
    Åhlund, John
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Lozzi, Luca
    University of L'Aquila.
    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.
    Characterization of Gas Phase of Iron Phthalocyanine with X-ray Photoelectron and Absorption Spectroscopies2015In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 252, no 6, p. 1259-1265Article in journal (Refereed)
    Abstract [en]

    Despite the numerous studies dedicated to phthalocyanine molecules adsorbed on surfaces, in monolayer or thin film, very few works have been focused on the characterization of vapors of these molecules. In this article we present the C 1s, N 1s and Fe 2p photoemission results as well as N K-edge X-ray absorption data of iron phthalocyanine (FePc) in gas phase. Presented comparison of X-ray photoelectron spectroscopy and X-ray absorption spectroscopy spectra of FePc films show a great similarity with the gas phase results, confirming the molecular character of thick films. The Fe2p photoemission spectrum of the gas phase FePc, shown for the first time, can be considered as a fingerprint of the Fe(II) ionic state of the central metal of the iron phthalocyanine. The performed multiplet calculations for describing the Fe 2p XP spectrum indicate 3Eg (a1g2eg32g1) state as the most probable ground state for thick film of iron phthalocyanine.

  • 173.
    Bijelovic, Stojanka
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Råsander, Mikael
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Wilhelmsson, Ola
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Lewin, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Eriksson, Olle
    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.
    Wear-resistant magnetic thin film material based on a Ti1−xFexC1−y nanocomposite alloy2010In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 81, no 1, p. 014405-Article in journal (Refereed)
    Abstract [en]

    In this study we report on the film growth and characterization of thin films deposited on amorphous quartz. The experimental studies have been complemented by first-principles density-functional theory metastable Ti-Fe-C film changes. With increasing annealing time, there is a depletion of iron close to the surface of the film, while regions enriched in iron are simultaneously formed deeper into the film. Both the magnetic ordering temperature and the saturation magnetization changes significantly upon annealing. The DFT calculations show that the critical temperature and the magnetic moment both increase with increasing Fe and C-vacancy concentration. The formation of the metastable iron-rich Ti-Fe-C compound is reflected in the strong increase in the magnetic ordering temperature. Eventually, after enough annealing time nanocrystalline -Fe starts to precipitate, the amount and size of which can be controlled by the annealing procedure; after 20 min of annealing, the experimental results indicate a nanocrystalline iron-film embedded in a wear-resistant TiC compound. This conclusion is further supported by transmission electron microscopy studies on epitaxial Ti-Fe-C films deposited on single-crystalline MgO substrates where, upon annealing, an iron film embedded in TiC is formed. Our results suggest that annealing of metastable Ti-Fe-C films can be used as an efficient way of creating a wear-resistant magnetic thin film material. approximately 50-nm-thick Ti-Fe-CDFT calculations. Upon annealing of as-prepared films, the composition of the10 min, nanocrystalline -Fe starts to precipitate, the amount and size of which can be controlled by the annealing procedure; after 20 min of annealing, the experimental results indicate a nanocrystalline iron-film embedded in a wear-resistant TiC compound. This conclusion is further supported by transmission electron microscopy studies on epitaxial Ti-Fe-C films deposited on single-crystalline MgO substrates where, upon annealing, an iron film embedded in TiC is formed. Our results suggest that annealing of metastable Ti-Fe-C films can be used as an efficient way of creating a wear-resistant magnetic thin film material.

  • 174.
    Björkman, Torbjörn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grånäs, Oscar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Adaptive Smearing for Brillouin Zone Integration2011In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 111, no 5, p. 1025-1030Article in journal (Refereed)
    Abstract [en]

    We suggest a simple scheme for automatically determining the width parameter of smearing methods of the Brillouin zone integration in electronic structure calculation. The scheme retains one free parameter that at any time can be eliminated by choosing a denser k-space mesh until the desired accuracy is obtained. The tests are carried out in the context of Methfessel-Paxton smearing. This adaptive Gaussian smearing (AGS) is easily implemented, variational with respect to partial occupancies and free from spurious occupancies that are negative or larger than one. Its convergence properties are similar to those obtained with the modified tetrahedron method for energy resolution of >= 0.1 meV.

  • 175.
    Björnson, Kristofer
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Topological band theory and Majorana fermions: With focus on self-consistent lattice models2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    One of the most central concepts in condensed matter physics is the electronic band structure. Although band theory was established more than 80 years ago, recent developments have led to new insights that are formulated in the framework of topological band theory. In this thesis a subset of topological band theory is presented, with particular focus on topological supercon- ductors and accompanying Majorana fermions. While simple models are used to introduce basic concepts, a physically more realistic model is also studied intensely in the papers. Through self- consistent tight-binding calculations it is confirmed that Majorana fermions appear in vortex cores and at wire end points when the superconductor is in the topologically non-trivial phase. Many other properties such as the topological invariant, experimental signatures in the local density of states and spectral function, unconventional and odd-frequency pairing, the precense of spin-polarized currents and spin-polarization of the Majorana fermions, and a local π-phase shift in the order parameter at magnetic impurities are also investigated. 

    List of papers
    1. Vortex states and Majorana fermions in spin-orbit coupled semiconductor-superconductor hybrid structures
    Open this publication in new window or tab >>Vortex states and Majorana fermions in spin-orbit coupled semiconductor-superconductor hybrid structures
    2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 2, p. 024501-Article in journal (Refereed) Published
    Abstract [en]

    We study the energy spectrum of a vortex core in a two-dimensional semiconductor with Rashba spin-orbit interaction and proximity coupled to a conventional superconductor and a ferromagnetic insulator. We perform self-consistent calculations using the microscopic tight-binding Bogoliubov-de Gennes method on a lattice and confirm the existence of Majorana fermions in the nontrivial topological phase. We also find two different topologically trivial bulk superconducting phases, only differing in the type of vortex core structure they support and separated by a zero-energy excitation. Furthermore, we find an asymmetry in the energy spectrum with respect to both Zeeman splitting and vortex rotation direction and explain its physical origin.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-204775 (URN)10.1103/PhysRevB.88.024501 (DOI)000321270100003 ()
    Available from: 2013-08-15 Created: 2013-08-12 Last updated: 2017-12-06Bibliographically approved
    2. Skyrmion spin texture in ferromagnetic semiconductor-superconductor heterostructures
    Open this publication in new window or tab >>Skyrmion spin texture in ferromagnetic semiconductor-superconductor heterostructures
    2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 13, p. 134518-Article in journal (Refereed) Published
    Abstract [en]

    We provide a derivation of a spin Skyrmion number classification for two-dimensional topological superconductors constructed from ferromagnetic and Rashba spin-orbit coupled semiconductor-superconductor heterostructures. We show that in the nontrivial topological phase, characterized by a nonzero Chern number, there is always a topological spin texture in the occupied bands represented by a Skyrmion number. The Skyrmion number has the advantage of being both physically intuitive and directly measurable using spin-sensitive band structure imaging techniques. In addition, we show that the Skyrmion classification can be extended to the equivalent one-dimensional topological superconductors.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-227201 (URN)10.1103/PhysRevB.89.134518 (DOI)000335498000004 ()
    Available from: 2014-06-26 Created: 2014-06-24 Last updated: 2017-12-05Bibliographically approved
    3. Probing vortex Majorana fermions and topology in semiconductor/superconductor heterostructures
    Open this publication in new window or tab >>Probing vortex Majorana fermions and topology in semiconductor/superconductor heterostructures
    2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 21, article id 214514Article in journal (Refereed) Published
    Abstract [en]

    We investigate the local density of states, spectral function, and superconducting pair amplitudes for signatures of Majorana fermions in vortex cores in ferromagnetic and spin-orbit coupled semiconductor/superconductor heterostructures. We show that the Majorana fermion quasiparticle momentum distribution is always symmetrically distributed at a finite radius around a high-symmetry point, thereby providing a necessary condition for a low-energy state to be a Majorana fermion. In real-space profiles of the local density of states through the vortex core the Majorana fermion, together with other finite-energy vortex states, forms a characteristic X-shape structure only present at nontrivial topology. Moreover, we find that the Mexican hat band structure property of the topologically nontrivial phase translates into multiple high-intensity band edges and also vortex core states located above the superconducting gap in the local density of states. Finally, we find no strong correlation between odd-frequency pairing and the appearance of Majorana fermions, but odd-frequency pairing exists as soon as ferromagnetism is present. In fact, we find that the only vortex superconducting pair amplitude directly related to any phase transition is the appearance of certain spin-triplet p-wave pairing components in the vortex core at a pre-topological vortex core widening transition.

    National Category
    Physical Sciences
    Ident