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  • 301. Bentell, J.
    et al.
    Wennekes, F.
    Salomonsson, F.
    Hammar, M.
    Streubel, K.
    Characterisation of n-InP/n-GaAs Wafer Fused Heterojunctions1999In: Physica scripta. T, ISSN 0281-1847, Vol. 79, 206-208 p.Article in journal (Refereed)
    Abstract [en]

    We have investigated the properties of wafer-fused Si-doped isotype hetero-junctions between GaAs and InP. Current/voltage measurements were conducted to study the influence of the doping concentrations on each side of the interface on the electrical conductivity. An almost ohmic behavior with a very low series resistance was obtained for the highest examined doping level on the GaAs side, whereas the doping concentration on the InP side was found to be of little significance. Fusion at different temperatures showed that the conductivity degrades significantly below 500°C, although mechanically stable junctions were obtained also at temperatures as low as 305°C. Secondary ion mass spectroscopy measurements showed no redistribution of Si, but indicated the presence of small amounts of C and Fe impurities at the interface.

  • 302.
    Beran, P.
    et al.
    Nucl Phys Inst ACSR, Rez, Czech Republic..
    Ivanov, Sergey A.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics. Karpov Inst Phys Chem, Ctr Mat Sci, Moscow 105064, Russia..
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Middey, S.
    Indian Assoc Cultivat Sci, Ctr Adv Mat, Kolkata 700032, India..
    Nag, A.
    Indian Assoc Cultivat Sci, Dept Mat Sci, Kolkata 700032, India..
    Sarma, D. D.
    Indian Assoc Cultivat Sci, Ctr Adv Mat, Kolkata 700032, India.;Indian Inst Sci, Solid State & Struct Chem Unit, Bangalore 560012, Karnataka, India.;CSIR, NISE, New Delhi 110001, India..
    Ray, S.
    Indian Assoc Cultivat Sci, Ctr Adv Mat, Kolkata 700032, India.;Indian Assoc Cultivat Sci, Dept Mat Sci, Kolkata 700032, India..
    Mathieu, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Neutron powder diffraction study of Ba3ZnRu2-xIrxO9 (x=0, 1, 2) with 6H-type perovskite structure2015In: Solid State Sciences, ISSN 1293-2558, E-ISSN 1873-3085, Vol. 50, 58-64 p.Article in journal (Refereed)
    Abstract [en]

    The triple perovskites Ba3ZnRu2-xIrxO9 with x = 0, 1, and 2 are insulating compounds in which Ru(Ir) cations form a dimer state. Polycrystalline samples of these materials were studied using neutron powder diffraction (NPD) at 10 and 295 K. No structural transition nor evidence of long range magnetic order was observed within the investigated temperature range. The results from structural refinements of the NPD data and its polyhedral analysis are presented, and discussed as a function of Ru/Ir content.

  • 303.
    Berg, Alexander
    et al.
    Lund University, Lund, Sweden.
    Yazdi, Sadegh
    Technical University of Denmark, Lyngby, Denmark.
    Nowzari, Ali
    Lund University, Lund, Sweden.
    Storm, Kristian
    Lund University, Lund, Sweden.
    Jain, Vishal
    Halmstad University, School of Information Technology, Halmstad Embedded and Intelligent Systems Research (EIS), MPE-lab. Lund University, Lund, Sweden.
    Vainorius, Neimantas
    Lund University, Lund, Sweden.
    Samuelson, Lars
    Lund University, Lund, Sweden.
    Wagner, Jakob B.
    Technical University of Denmark, Lyngby, Denmark.
    Borgström, Magnus T.
    Lund University, Lund, Sweden.
    Radial Nanowire Light-Emitting Diodes in the (AlxGa1-x)yIn1-yP Material System2016In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 16, no 1, 656-662 p.Article in journal (Refereed)
    Abstract [en]

    Nanowires have the potential to play an important role for next-generation light-emitting diodes. In this work, we present a growth scheme for radial nanowire quantum-well structures in the AlGaInP material system using a GaInP nanowire core as a template for radial growth with GaInP as the active layer for emission and AlGaInP as charge carrier barriers. The different layers were analyzed by X-ray diffraction to ensure lattice-matched radial structures. Furthermore, we evaluated the material composition and heterojunction interface sharpness by scanning transmission electron microscopy energy dispersive X-ray spectroscopy. The electro-optical properties were investigated by injection luminescence measurements. The presented results can be a valuable track toward radial nanowire light-emitting diodes in the AlGaInP material system in the red/orange/yellow color spectrum. © 2015 American Chemical Society.

  • 304.
    Berger, Rolf
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Fritzsche, M
    Broddefalk, A
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science. Fasta tillst. fysik. Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Nordblad, P.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science. Fasta tillst. fysik. Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Malaman, B.
    A study of the structural and magnetic properties of TlCo2-xCuxSe22002In: J Alloys Comp, Vol. 343, 186-191 p.Article in journal (Refereed)
    Abstract [en]

    The structural and magnetic properties of TlCo(2-x)Cu(x)Se2 have been investigated experimentally. The antiferromagnetic coupling already found in TlCo2Se2 prevails on copper substitution but is weakened. For high copper contents, x larger tahn 1.3, Pauli paramagnetism occurs. -- The magnetic structure of teh unsubstituted TlCo2Se2 has been reinvestigated by neutron powder diffraction. Below 80K it is characterised by a helimagnetic arrangement of spins confined to the Co sheets, the helix running along the c-axis.

  • 305.
    Berggren, Lars
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Jonsson, Jacob C.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Niklasson, Gunnar A.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Optical absorption in lithiated tungsten oxide thin films: Experiment and theory2007In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 102, no 8, 083538- p.Article in journal (Refereed)
    Abstract [en]

    Amorphous tungsten oxide exhibits electrochromism when intercalated with protons, lithium, sodium, and other ions. Thin films of the material were prepared by dc magnetron sputtering and then electrochemically intercalated with lithium. The optical absorption in the wavelength range of 300-2500 nm was measured for a number of lithium concentrations. The optical absorption shows a maximum for lithium/tungsten ratios of 0.3-0.5. The optical spectra can be fitted by a superposition of three Gaussian peaks, representing the three possible electronic transitions between W6+, W5+, and W4+ sites. The variation of the peak strength with lithium concentration is consistent with an extended site-saturation theory.

  • 306.
    Berggren, Lars
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Niklasson, Gunnar A
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics. Fasta tillståndets fysik.
    Optical Charge Transfer Absorption in Lithium-intercalated Tungsten Oxide Thin Films2006In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, no 88, 081906- p.Article in journal (Refereed)
    Abstract [en]

    Amorphous tungsten oxide exhibits electrochromism when intercalated with protons or lithium ions.Thin films of the material were prepared by dc magnetron sputtering and subsequentlyelectrochemically intercalated with lithium. The optical absorption in the wavelength range300 to 2500 nm was measured for a number of lithium concentrations. All the spectra can be fittedby a superposition of three Gaussian peaks, representing the three possible electronic transitionsbetween W6+, W5+, and W4+ sites. The variation of the peak strength with lithium concentration isconsistent with the predictions of a statistical theory.

  • 307.
    Berggren, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Elastic and inelastic scattering effects in conductance measurements at the nanoscale: A theoretical treatise2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Elastic and inelastic interactions are studied in tunnel junctions of a superconducting nanoelectromechanical setup and in response to resent experimental superconducting scanning tunneling microscope findings on a paramagnetic molecule. In addition, the electron density of molecular graphene is modeled by a scattering theory approach in very good agreement with experiment. All studies where conducted through the use of model Hamiltonians and a Green function formalism. The nanoelectromechanical system comprise two fixed superconducting leads in-between which a cantilever suspended superconducting island oscillates in an asymmetric fashion with respect to both fixed leads. The Josephson current is found to modulate the island motion which in turn affects the current, such that parameter regions of periodic, quasi periodic and chaotic behavior arise. Our modeled STM setup reproduces the experimentally obtained spin excitations of the paramagnetic molecule and we show a probable cause for the increased uniaxial anisotropy observed when closing the gap distance of tip and substrate. A wider parameter space is also investigated including effects of external magnetic fields, temperature and transverse anisotropy. Molecular graphene turns out to be well described by our adopted scattering theory, producing results that are in good agreement with experiment. Several point like scattering centers are therefore well suited to describe a continuously decaying potential and effects of impurities are easily calculated.

  • 308.
    Bergholtz, E.J.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Karlhede, A.
    Stockholm University, Faculty of Science, Department of Physics.
    A simple view on the quantum Hall system2007In: Bulletin of the American Physical Society, 2007Conference paper (Other academic)
    Abstract [en]

    The physics of the quantum Hall system becomes very simple on a thin torus. Remarkably, however, the rich structure of the system still exists in this limit. On the very thin torus the many body problem reduces to a one-dimensional classical electrostatic problem and both the abelian and the non-abelian quantum Hall states are manifested as gapped one-dimensional crystals, Tao-Thouless states, with fractionally charged excitations that appear as domain walls between degenerate ground states. These states represent, but are extreme forms of, the observed states in the bulk and their qualitative properties (such as quasiparticle degeneracies, quantum numbers, relative size of the gaps etc.) remain the same. For the gapless states, there is a phase transition at finite thickness to phases different from the gapped crystals. At half-filling in the lowest Landau level, this new phase is a Luttinger liquid of neutral dipoles which is adiabatically connected to the gapless state observed in the bulk. The existence of the gapless phase on the thin (but finite) torus provides an explicit microscopic example of how weakly interacting quasiparticles moving in a reduced (zero) magnetic field emerge as the low energy sector of strongly interacting electrons in a strong magnetic field.

  • 309.
    Bergholtz, E.J.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Karlhede, A.
    Stockholm University, Faculty of Science, Department of Physics.
    An Exact Solution for the Half-filled Lowest Landau Level2006In: Bulletin of the American Physical Society, 2006Conference paper (Other academic)
    Abstract [en]

    We present an exact solution for the interacting electron gas in the half-filled lowest Landau level on a thin torus. The low energy sector consists of non-interacting, one-dimensional, neutral fermions (dipoles). The ground state, which is homogeneous, is the Fermi sea obtained by filling the negative energy states and the excited states are the gapless neutral excitations out of this one-dimensional sea. We identify this ground state as a version of the Rezayi-Read state, and find that it develops continuously, as the circumference grows, into the Rezayi-Read state that is believed to describe the observed metallic phase in the two-dimensional system. This suggests a Luttinger liquid description of the half-filled Landau level.

  • 310.
    Bergholtz, Emil J
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Karlhede, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Half-filled lowest Landau level on a thin torus.2005In: Phys Rev Lett, ISSN 0031-9007, Vol. 94, no 2, 026802- p.Article in journal (Other academic)
    Abstract [en]

    We solve a model that describes an interacting electron gas in the half-filled lowest Landau level on a thin torus, with radius of the order of the magnetic length. The low-energy sector consists of noninteracting, one-dimensional, neutral fermions. The ground state, which is homogeneous, is the Fermi sea obtained by filling the negative energy states, and the excited states are gapless neutral excitations out of this one-dimensional sea. Although the limit considered is extreme, the solution has a striking resemblance to the composite fermion description of the bulk \nu = 1/2 state—the ground state is homogeneous and the excitations are neutral and gapless. This suggests a one-dimensional Luttinger liquid description, with possible observable effects in transport experiments, of the bulk state where it develops continuously from the state on a thin torus as the radius increases.

  • 311.
    Bergholtz, Emil J
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Karlhede, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    'One-dimensional' theory of the quantum Hall system2006In: Journal of Statistical Mechanics: Theory and Experiment, no L04001Article in journal (Refereed)
    Abstract [en]

    We consider the lowest Landau level on a torus as a function of its circumference L_1. When L_1 → 0, the ground state at general rational filling fraction is a crystal with a gap—a Tao–Thouless state. For filling fractions ν = p/(2pm + 1), these states are the limits of Laughlin's or Jain's wavefunctions describing the gapped quantum Hall states when L_1 → ∞. For the half-filled Landau level, there is a transition to a Fermi sea of non-interacting neutral fermions (dipoles), or rather to a Luttinger liquid modification thereof, at L_1 ~ 5 magnetic lengths. Using exact diagonalization we identify this state as a version of the Rezayi–Read state, and find that it develops continuously into the state that is believed to describe the observed metallic phase as L_1 → ∞. Furthermore, the effective Landau level structure that emerges within the lowest Landau level is found to be a consequence of the magnetic symmetries.

  • 312.
    Bergkvist, Sara
    et al.
    KTH, Superseded Departments, Physics.
    Henelius, Patrik
    KTH, Superseded Departments, Physics.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Ground state of the random-bond spin-1 Heisenberg chain2002In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 66, 134407-1-134407-9 p.Article in journal (Refereed)
    Abstract [en]

    Stochastic series expansion quantum Monte Carlo is used to study the ground state of the antiferromagnetic spin-1 Heisenberg chain with bond disorder. Typical spin- and string-correlation functions behave in accordance with real-space renormalization group predictions for the random-singlet phase. The average string-correlation function decays algebraically with an exponent of -0.378(6), in very good agreement with the prediction of -(3-root5)/2similar or equal to-0.382, while the average spin-correlation function is found to decay with an exponent of about -1, quite different from the expected value of -2. By implementing the concept of directed loops for the spin-1 chain we show that autocorrelation times can be reduced by up to two orders of magnitude.

  • 313.
    Bergkvist, Sara
    et al.
    KTH, Superseded Departments, Physics.
    Henelius, Patrik
    KTH, Superseded Departments, Physics.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Local-density approximation for confined bosons in an optical lattice2004In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 70, 053601-1-053601-7 p.Article in journal (Refereed)
    Abstract [en]

    We investigate local and global properties of the one-dimensional Bose-Hubbard model with an external confining potential, describing an atomic condensate in an optical lattice. Using quantum Monte Carlo techniques we demonstrate that a local-density approximation, which relates the unconfined and the confined model, yields quantitatively correct results in most of the interesting parameter range. We also examine claims of universal behavior in the confined system, and demonstrate the origin of a previously calculated fine structure in the experimentally accessible momentum distribution.

  • 314.
    Bergkvist, Sara
    et al.
    KTH, Superseded Departments, Physics.
    Henelius, Patrik
    KTH, Superseded Departments, Physics.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Reduction of the sign problem using the meron-cluster approach2003In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 68, 016122-1-016122-8 p.Article in journal (Refereed)
    Abstract [en]

    The sign problem in quantum Monte Carlo calculations is analyzed using the meron-cluster solution. A meron is a loop that alters the sign of the configuration, and the concept of merons can be used to solve the sign problem for a limited class of models. Here we show that the method can be used to reduce the sign problem in a wider class of models. We investigate how the meron solution evolves between a point in parameter space where it eliminates the sign problem and a point where it does not affect the sign problem at all. In this intermediate regime, the merons can be used to reduce the sign problem. The average sign still decreases exponentially with system size and inverse temperature, but with a different prefactor. The sign exhibits the slowest decrease in the vicinity of points where the meron-cluster solution eliminates the sign problem. We have used stochastic series expansion quantum Monte Carlo combined with the concept of directed loops.

  • 315.
    Bergkvist, Sara
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    McCulloch, Ian
    KTH.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Spinful bosons in an optical lattice2006In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, no 5, 053419-1-053419-8 p.Article in journal (Refereed)
    Abstract [en]

    We analyze the behavior of cold spin-1 particles with antiferromagnetic interactions in a one-dimensional optical lattice using density matrix renormalization group calculations. Correlation functions and the dimerization are shown and we also present results for the energy gap between ground state and the spin excited states. We confirm the anticipated phase diagram, with Mott-insulating regions of alternating dimerized S=1 chains for odd particle density versus on-site singlets for even density. We find no evidence for any additional ordered phases in the physically accessible region, however for sufficiently large spin interaction, on-site singlet pairs dominate leading, for odd density, to a breakdown of the Mott insulator or, for even density, a real-space singlet superfluid.

  • 316.
    Bergkvist, Sara
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Saers, Robert
    Umeå Univ, Dept Phys.
    Lundh, Emil
    Umeå Univ, Dept Phys.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Rehn, Magnus
    Umeå Univ, Dept Phys.
    Kastberg, Anders
    Umeå Univ, Dept Phys.
    Transition from a two-dimensional superfluid to a one-dimensional mott insulator2007In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 99, no 11, 110401-1-110401-5 p.Article in journal (Refereed)
    Abstract [en]

    A two-dimensional system of atoms in an anisotropic optical lattice is studied theoretically. If the system is finite in one direction, it is shown to exhibit a transition between a two-dimensional superfluid and a one-dimensional Mott insulating chain of superfluid tubes. Monte Carlo simulations are consistent with the expectation that the phase transition is of Kosterlitz-Thouless type. The effect of the transition on experimental time-of-flight images is discussed.

  • 317.
    Bergkvist Sylvan, Sara
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Numerical studies of spin chains and cold atoms in optical lattices2007Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    An important, but also difficult, research field in condensed matter physics is that of strongly correlated systems. This thesis considers two topics in this field.

    The first topic is disorder and frustration in spin models. The introduction of disorder into quantum spin chains creates a complex problem. The ground state of the random-bond spin-1 Heisenberg chain is studied by means of stochastic series expansion quantum Monte Carlo simulation, applying the concept of directed loops. It is found that this system undergoes a phase transition to the random-singlet phase if the bond disorder is strong enough. Further a frustrated spin system is investigated. The frustration is introduced by having spins positioned on a triangular lattice. Performing a quantum Monte Carlo simulation for such a frustrated lattice leads to the occurrence of the infamous sign problem. This problem is investigated and it is shown that it is possible to use a meron cluster approach to reduce its effect for some specific models.

    The second topic concerns atomic condensates in optical lattices. A system of trapped bosonic atoms in such a lattice is described by a Bose-Hubbard model with an external confining potential. Using quantum Monte Carlo simulations it is demonstrated that the local density approximation that relates the observables of the unconfined and the confined models yields quantitatively correct results in most of the interesting parameter range of the model. Further, the same model with the addition that the atoms carry spin-1 is analyzed using density matrix renormalization group calculations. The anticipated phase diagram, with Mott insulating regions of dimerized spin-1 chains for odd particle density, and on-site singlets for even density is confirmed. Also an ultracold gas of bosonic atoms in an anisotropic two dimensional optical lattice is studied. It is found that if the system is finite in one direction it exhibits a quantum phase transition. The Monte Carlo simulations performed show that the transition is of Kosterlitz-Thouless type.

  • 318.
    Bergman, Anders
    et al.
    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, Materials Theory.
    Delin, Anna
    KTH.
    Atomistic spin dynamics of atomic Pt wiresManuscript (preprint) (Other academic)
  • 319.
    Bergman, Anders
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Skubic, B
    Hellsvik, J
    Nordstrom, L
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Eriksson, O
    Ultrafast switching in a synthetic antiferromagnetic magnetic random-access memory device2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 22Article in journal (Refereed)
    Abstract [en]

    The dynamics of a synthetic antiferromagnet (a metallic trilayer) have been explored and are shown to exhibit ultrafast switching on a time scale of tens of ps. This conclusion is based on first-principles, atomistic spin dynamics simulations. The simulations are performed at finite temperature, as well as at T = 0 K (the macrospin limit), and we observe a marked temperature dependence of the switching phenomenon. It is shown that, to reach very high switching speeds, it is important that the two ferromagnetic components of the synthetic antiferromagnet have oppositely directed external fields to one another. Then a complex collaboration between precession switching of an internal exchange field and the damping switching of the external field occurs, which considerably accelerates the magnetization dynamics. We discuss a possible application of this fast switching as a magnetic random access memory device, which has as a key component intrinsic antiferromagnetic couplings and an applied Oersted field.

  • 320.
    Bergman, Anders
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Skubic, Björn
    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, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Delin, Anna
    KTH.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ultrafast switching in a synthetic antiferromagnetic random-access memory device.Manuscript (preprint) (Other academic)
  • 321.
    Bergman, Anders
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Taroni, Andrea
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Bergqvist, Lars
    Uppsala University, Sweden.
    Hellsvik, Johan
    Hjörvarsson, Björgvin
    Eriksson, Olle
    Magnon softening in a ferromagnetic monolayer: A first-principles spin dynamics study2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 14, 144416- p.Article in journal (Refereed)
    Abstract [en]

    We study the Fe/W(110) monolayer system through a combination of first-principles calculations and atomistic spin dynamics simulations. We focus on the dispersion of the spin-waves parallel to the [001] direction. Our results compare favorably with the experimental data of Prokop et al. [Phys. Rev. Lett. 102, 177206 (2009)] and correctly capture a drastic softening of the magnon spectrum, with respect to bulk bcc Fe. The suggested shortcoming of the itinerant electron model, in particular that given by density functional theory, is refuted. We also demonstrate that finite-temperature effects are significant, and that atomistic spin dynamics simulations represent a powerful tool with which to include these.

  • 322.
    Bergman, Anders
    et al.
    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.
    Bergqvist, Lars
    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, Materials Theory.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Magnon softening in a ferromagnetic monolayer: A first-principles spin dynamics study2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 14, 144416- p.Article in journal (Refereed)
    Abstract [en]

    We study the Fe/W(110) monolayer system through a combination of first-principles calculations and atomistic spin dynamics simulations. We focus on the dispersion of the spin-waves parallel to the [001] direction. Our results compare favorably with the experimental data of Prokop et al. [Phys. Rev. Lett. 102, 177206 (2009)] and correctly capture a drastic softening of the magnon spectrum, with respect to bulk bcc Fe. The suggested shortcoming of the itinerant electron model, in particular that given by density functional theory, is refuted. We also demonstrate that finite-temperature effects are significant, and that atomistic spin dynamics simulations represent a powerful tool with which to include these.

  • 323.
    Bergqvist, Lars
    et al.
    Institut für Festkörperforschung, Forschungszentrum Jülich.
    Belhadji, Brahim
    Picozzi, Silvia
    Dederichs, Peter H.
    Volume dependence of the Curie temperatures in diluted magnetic semiconductors2008In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 77, no 1, 014418- p.Article in journal (Refereed)
    Abstract [en]

    Using electronic structure methods and statistical methods, we have studied theoretically the volume dependence of the exchange interactions and Curie temperatures in the diluted magnetic semiconductors. In both Mn-doped GaAs and Mn-doped InAs, the calculated Curie temperatures from numerical exact Monte Carlo simulations are more or less constant for a large volume interval. We have compared the exchange mechanisms in Mn-doped GaAs using both the local density approximation (LDA) and the LDA+U method. It is demonstrated that the magnetic properties are understood within Zener’s p-d exchange model for the LDA+U, while in LDA they reflect a mixture between double and p-d exchange mechanisms.

  • 324.
    Bergqvist, Lars
    et al.
    Institut für Festkörperforschung, Forschungszentrum Jülich.
    Dederichs, Peter H.
    A theoretical study of half-metallic antiferromagnetic diluted magnetic semiconductors2007In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 21, 216220- p.Article in journal (Refereed)
    Abstract [en]

    Based on electronic structure calculations and statistical methods, we investigate a new class of materials for spintronic applications: half-metallic antiferromagnetic diluted magnetic semiconductors (HMAF-DMSs). As shown recently by Akai and Ogura, these DMS systems contain equal amounts of low-valent and high-valent transition metal impurities, such that their local moments exactly compensate each other. We present ab initio calculations using the KKR-CPA and the PAW-supercell methods, and show that quite a few half-metallic antiferromagnets should exist. Our calculations demonstrate that the exchange coupling parameters in these systems are dominated by a strong antiferromagnetic interaction between the two impurities. The Néel temperatures are calculated by Monte Carlo simulations and in mean-field approximation. It is shown that the latter method strongly overestimates the critical temperatures and that the more realistic values obtained by Monte Carlo techniques are rather low.

  • 325.
    Bergqvist, Lars
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Eriksson, O
    Kudrnovsky, J
    Drchal, V
    Korzhavyi, Pavel
    KTH, Superseded Departments, Materials Science and Engineering.
    Turek, I
    Magnetic percolation in diluted magnetic semiconductors2004In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 93, no 13, 137202- p.Article in journal (Refereed)
    Abstract [en]

    We demonstrate that the magnetic properties of diluted magnetic semiconductors are dominated by short ranged interatomic exchange interactions that have a strong directional dependence. By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we reproduce the observed critical temperatures of a broad range of diluted magnetic semiconductors. We also show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned. This suggests that the ordering of diluted magnetic semiconductors is heavily influenced by magnetic percolation, and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations.

  • 326.
    Bergqvist, Lars
    et al.
    Uppsala University.
    Eriksson, Olle
    Theory of weakly coupled two-dimensional magnets2006In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 18, no 20, 4853-4860 p.Article in journal (Refereed)
    Abstract [en]

    We demonstrate that the experimental findings of the magnetic properties of the weakly coupled trilayer system Ni4/CuN/Co2are reproduced by a theory that combines first principles calculations of the exchange interactions in a classical Heisenberg model with Monte Carlo simulations. Through an analysis of the spin–spin correlation function we show that two distinct temperatures can be identified; a higher temperature where long range magnetic order disappears and a lower temperature where the spin–spin correlation of the Ni atoms undergoes a drastic change. We argue that our findings hold in general for 'weak exchange link' systems.

  • 327.
    Bergqvist, Lars
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Eriksson, Olle
    Bergman, Anders
    Hellsvik, Johan
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Atomistic Spin Dynamics: Foundations and Applications2017Book (Other academic)
  • 328.
    Bergqvist, Lars
    et al.
    Uppsala University.
    Eriksson, Olle
    Kudrnosky, Josef
    Drchal, V.
    Bergman, Anders
    Nordström, Lars
    Turek, Ilja
    Magnetic properties and disorder effects in diluted magnetic semiconductors2005In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 72, no 19, 195210- p.Article in journal (Refereed)
    Abstract [en]

    We present calculations of the exchange interactions and critical temperatures for several diluted magnetic semiconductor systems. It is shown that the exchange interactions are dominated by short-ranged interactions that have a strong directional dependence. Using a combination of first-principles calculations of the exchange interactions together with Monte Carlo simulations of the classical Heisenberg model, in which the positional disorder and spin fluctuations are properly included, the calculated critical temperatures are in good agreement with experimantal observations. It is shown that agreement between theory and experiment, as regards ordering temperatures, is obtained only when the magnetic atoms are randomly positioned in a simulation cell which proves that disorder effects play a very important role. The effect of strong electron-electron interaction has been studied by means of the LSDA+U scheme. We investigate in detail the nature of the anisotropic exchange interactions by means of a Fermi surface analysis.

  • 329.
    Bergqvist, Lars
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Korzhavyi, Pavel A.
    KTH, Superseded Departments, Materials Science and Engineering.
    Sanyal, B.
    Mirbt, S.
    Abrikosov, I. A.
    Nordstrom, L.
    Smirnova, E. A.
    Mohn, P.
    Svedlindh, P.
    Eriksson, O.
    Magnetic and electronic structure of (Ga1-xMnx)As2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, no 20, 205201- p.Article in journal (Refereed)
    Abstract [en]

    We present theoretical calculations of the magnetic and electronic structure of Mn-doped GaAs (in the zinc-blende structure). The magnetic properties are shown to be very sensitive to structural defects, in particular, As antisite defects and Mn at interstitial positions. Only when considering such defects can the experimental magnetic moments be reproduced by first-principles theory. We present a simple model for understanding the connection between the magnetic ordering and the As antisites, and the way in which the defects help to stabilize a partial disordered local-moment state. The connection between the energetics of the Mn substitution and the As antisite concentration is also analyzed. In addition, we compare the calculated magnetic properties and electronic structures of Mn situated on substitutional sites (Mn replacing a Ga atom) and on interstitial sites, where in agreement with observations the interstitial site is found to be less favorable. Finally, combining our first-principles calculations of the spin-wave excitation energies with a classical Heisenberg Hamiltonian we have calculated interatomic exchange interactions, and using Monte Carlo simulations we present theoretical values of the critical temperature as a function of Mn concentration.

  • 330.
    Bergqvist, Lars
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Sato, K.
    Katayama-Yoshida, H.
    Dederichs, P. H.
    Computational materials design for high-T(c) (Ga, Mn)As with Li codoping2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 16, 165201Article in journal (Refereed)
    Abstract [en]

    Based on first-principles calculations and kinetic Monte Carlo simulations, we design a realistic and practical codoping technique for increasing the concentration of Mn atoms in GaAs and realizing high Curie temperatures in (Ga, Mn) As. We found that using codoping of Li interstitial atoms during the crystal growth has two great advantages. First, due to lower formation energy of Li interstitials compared to Mn interstitials, Li prevents formation of unwanted Mn interstitials. Second, Li interstitials can be removed by using post-growth annealing at low temperatures. This codoping method offers a general strategy to go far beyond the solubility limit and it should be applicable also to other diluted magnetic semiconductor systems.

  • 331. Bergqvist, Lars
    et al.
    Taroni, Andrea
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Etz, Corina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Olle, Eriksson
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Atomistic spin dynamics of low-dimensional magnets2013In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 87, no 144401Article in journal (Refereed)
    Abstract [en]

    We investigate the magnetic properties of a range of low-dimensional   ferromagnets using a combination of first-principles calculations and   atomistic spin dynamics simulations. This approach allows us to evaluate   the ground state and finite temperature properties of experimentally   well characterized systems such as Co/Cu(111), Co/Cu(001), Fe/Cu(001)   and Fe/W(110), for different thicknesses of the magnetic layer. We   compare our calculated spin wave spectra with experimental data   available in the literature, and find a good quantitative agreement. We   also predict magnon spectra for systems for which no experimental data   exist at the moment, and estimate the role of temperature effects.

  • 332.
    Bergvall, Anders
    et al.
    Chalmers University of Technology.
    Fogelström, Mikael
    Chalmers University of Technology.
    Holmqvist, Cecilia
    Universität Konstanz, Germany.
    Löfwander, Tomas
    Chalmers University of Technology.
    Basic theory of electron transport through molecular contacts2015In: Handbook of Single Molecule Electronics / [ed] K. Moth-Poulsen, Pan Stanford Publishing, 2015, 31-78 p.Chapter in book (Refereed)
  • 333. Bernabé, S.
    et al.
    Stevens, R.
    Volpert, M.
    Hamelin, R.
    Rossat, C.
    Berger, F.
    Lombard, L.
    Kopp, C.
    Berggren, Jesper
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Sundgren, Petrus
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Hammar, Mattias
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Highly integrated VCSEL-based 10Gb/s miniature optical sub-assembly2005In: Proceedings - Electronic Components and Technology Conference, Lake Buena Vista, FL: IEEE , 2005, Vol. 2, 1333-1338 p.Conference paper (Refereed)
    Abstract [en]

    In order to fit with the present and future needs of the Datacom transceiver market, newly designed high rate transmitter optical subassemblies (TOSAs) have to be compact, low cost and compatible with mass production. We propose here an innovative design strategy that reaches all these targets by integrating a 10Gbps 850nmVCSEL laser diode and its laser driver with other functionalities (e.g. power monitoring and thermal monitoring) in a small form factor package. Taking advantages of the optical properties of the VCSEL and using flip-chip techniques, the transmitter exhibits excellent hyperfrequency performances and compatibility with mass production due to the use of collective manufacturing technologies and passive optical alignment. This versatile approach is also applicable to high rate receivers, parallel optics emitters, and singlemode low cost transmitter integrating long wavelength VCSELs.

  • 334.
    Bernad, Joszef Zsolt
    et al.
    Massey University.
    Jääskeläinen, Markku
    Dalarna University. Institute of Fundamental Sciences and MacDiarmid Institute for Advanced Materials and Nanotechnology, Massey University, Palmerston North, New Zealand.
    Zulicke, Ulrich
    Massey University.
    Effects of a quantum measurement on the electric conductivity: Application to graphene2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 7, 073403Article in journal (Refereed)
    Abstract [en]

    We generalize the standard linear-response (Kubo) theory to obtain the conductivity of a system that is subject to a quantum measurement of the current. Our approach can be used to specifically elucidate how back-action inherent to quantum measurements affects electronic transport. To illustrate the utility of our general formalism, we calculate the frequency-dependent conductivity of graphene and discuss the effect of measurement-induced decoherence on its value in the dc limit. We are able to resolve an ambiguity related to the parametric dependence of the minimal conductivity.

  • 335.
    Bernard, L.
    et al.
    Empa, Switzerland; Eindhoven University of Technology, Netherlands.
    Khikhlovskyi, V.
    TNO Dutch Org Appl Science Research, Netherlands.
    van Breemen, A.
    TNO Dutch Org Appl Science Research, Netherlands.
    Michels, J. J.
    Max Planck Institute Polymer Research, Germany.
    Janssen, R.
    Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering. Eindhoven University of Technology, Netherlands.
    Gelinck, G.
    Eindhoven University of Technology, Netherlands; TNO Dutch Org Appl Science Research, Netherlands.
    Pilet, N.
    Paul Scherrer Institute, Switzerland.
    Study of the morphology of organic ferroelectric diodes with combined scanning force and scanning transmission X-ray microscopy2018In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 53, 242-248 p.Article in journal (Refereed)
    Abstract [en]

    Organic ferroelectric diodes attract increasing interest as they combine non-destructive data read-out and low cost fabrication, two requirements in the development of novel non-volatile memory elements. The macroscopic electrical characteristics and performances of such devices strongly depend on their structural properties. Various studies of their global microscopic morphology have already been reported. Here, a multi-technique approach including different scanning force and X-ray microscopies permitted to reveal and locally study nanometer-scale unexpected sub-structures within a P(VDF-TrFE):F8BT ferroelectric diode. The strong impact of these structures on the local polarizability of the ferroelectric is shown. Two alternative fabrication methods are proposed that prevent the formation of these structures and demonstrate improved macroscopic device performances such as endurance and ON/OFF ratio.

  • 336. Berner, G.
    et al.
    Glawion, S.
    Walde, J.
    Pfaff, F.
    Hollmark, H.M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Duda, Laurent-Claudius
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Paeterl, S.
    Richter, C.
    Mannhart, J.
    Sing, M.
    Claessen, R.
    LaAlO3/SrTiO3 oxide heterostructures studied by resonant inelastic x-ray scattering2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 24, 241405Article in journal (Refereed)
  • 337.
    Bernhardsson, Sebastian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Minnhagen, Petter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Selective pressure on metabolic network structures as measured from the random blind-watchmaker network2010In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 12, no 103047Article in journal (Refereed)
    Abstract [en]

    A random null model termed the Blind Watchmaker network (BW) has been shown to reproduce the degree distribution found in metabolic networks. This might suggest that natural selection has had little influence on this particular network property. We here investigate to what extent other structural network properties have evolved under selective pressure from the corresponding ones of the random null model: The clustering coefficient and the assortativity measures are chosen and it is found that these measures for the metabolic network structure are close enough to the BW-network so as to fit inside its reachable random phase space. It is furthermore shown that the use of this null model indicates an evolutionary pressure towards low assortativity and that this pressure is stronger for larger networks. It is also shown that selecting for BW networks with low assortativity causes the BW degree distribution to slightly deviate from its power-law shape in the same way as the metabolic networks. This implies that an equilibrium model with fluctuating degree distribution is more suitable as a null model, when identifying selective pressures, than a randomized counterpart with fixed degree sequence, since the overall degree sequence itself can change under selective pressure on other global network properties.

  • 338.
    Berntsen, Magnus H.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Consequences of a non-trivial band-structure topology in solids: Investigations of topological surface and interface states2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The development and characterization of experimental setups for angle-resolved photoelectron spectroscopy (ARPES) and spin- and angle-resolved photoelectron spectroscopy (SARPES) is described. Subsequently, the two techniques are applied to studies of the electronic band structure in topologically non-trivial materials.

    The laser-based ARPES setup works at a photon energy of 10.5 eV and a typical repetition rate in the range 200 kHz to 800 kHz. By using a time-of-flight electron energy analyzer electrons emitted from the sample within a solid angle of up to ±15 degrees can be collected and analyzed simultaneously. The SARPES setup is equipped with a traditional hemispherical electron energy analyzer in combination with a mini-Mott electron polarimeter. The system enables software-controlled switching between angle-resolved spin-integrated and spin-resolved measurements, thus providing the possibility to orient the sample by mapping out the electronic band structure using ARPES before performing spin-resolved measurements at selected points in the Brillouin zone.

    Thin films of the topological insulators (TIs) Bi2Se3, Bi2Te3 and Sb2Te3 are grown using e-beam evaporation and their surface states are observed by means of ARPES. By using a combination of low photon energies and cryogenic sample temperatures the topological states originating from both the vacuum interface (surface) and the substrate interface are observed in Bi2Se3 films and Bi2Se3/Bi2Te3 heterostructures, with total thicknesses in the ultra-thin limit (six to eight quintuple layers), grown on Bi-terminated Si(111) substrates. Band alignment between Si and Bi2Se3 at the interface creates a band bending through the films. The band bending is found to be independent of the Fermi level (EF) position in the bulk of the substrate, suggesting that the surface pinning of EF in the Si(111) substrate remains unaltered after deposition of the TI films. Therefore, the type and level of doping of the substrate does not show any large influence on the size of the band bending.

    Further, we provide experimental evidence for the realization of a topological crystalline insulator (TCI) phase in the narrow-band semiconductor Pb1−xSnxSe. The TCI phase exists for temperatures below the transition temperature Tc and is characterized by an inverted bulk band gap accompanied by the existence of non-gapped surface states crossing the band gap. Above Tc the material is in a topologically trivial phase where the surface states are gapped. Thus, when lowering the sample temperature across Tc a topological phase transition from a trivial insulator to a TCI is observed. SARPES studies indicate a helical spin structure of the surface states both in the topologically trivial and the TCI phase.

  • 339.
    Berntsen, Magnus H.
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Götberg, Olof
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Wojek, Bastian M.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Tjernberg, Oscar
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Direct observation of decoupled Dirac states at the interface between topological and normal insulators2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 19, 195132Article in journal (Refereed)
    Abstract [en]

    Several proposed applications and exotic effects in topological insulators rely on the presence of helical Dirac states at the interface between a topological insulator and a normal insulator. In the present work, we have used low-energy angle-resolved photoelectron spectroscopy to uncover and characterize the interface states of Bi2Se3 thin films and Bi2Te3/Bi2Se3 heterostructures grown on Si(111). The results establish that Dirac fermions are indeed present at the topological-normal-insulator boundary and absent at the topological-topological-insulator interface. Moreover, it is demonstrated that band bending present within the topological-insulator films leads to a substantial separation of the interface and surface states in energy. These results pave the way for further studies and the realization of interface-related phenomena in topological-insulator thin-film heterostructures.

  • 340.
    Berritta, Marco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Mondal, Ritwik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Carva, Karel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Charles Univ Prague, Fac Math & Phys, Dept Condensed Matter Phys, Ke Karlovu 5, CZ-12116 Prague 2, Czech Republic..
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab Initio Theory of Coherent Laser-Induced Magnetization in Metals2016In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 117, no 13Article in journal (Refereed)
    Abstract [en]

    We present the first materials specific ab initio theory of the magnetization induced by circularly polarized laser light in metals. Our calculations are based on nonlinear density matrix theory and include the effect of absorption. We show that the induced magnetization, commonly referred to as inverse Faraday effect, is strongly materials and frequency dependent, and demonstrate the existence of both spin and orbital induced magnetizations which exhibit a surprisingly different behavior. We show that for nonmagnetic metals (such as Cu, Au, Pd, Pt) and antiferromagnetic metals the induced magnetization is antisymmetric in the light's helicity, whereas for ferromagnetic metals (Fe, Co, Ni, FePt) the imparted magnetization is only asymmetric in the helicity. We compute effective optomagnetic fields that correspond to the induced magnetizations and provide guidelines for achieving all-optical helicity-dependent switching.

  • 341.
    Bertone, Daniele
    et al.
    CSELT Centro Studi e Laboratori, Telecomunicazioni S.p.A, Torino.
    Campi, Roberta
    CSELT Centro Studi e Laboratori, Telecomunicazioni S.p.A, Torino.
    Fang, Ruiyi yi
    CSELT Centro Studi e Laboratori, Telecomunicazioni S.p.A, Torino.
    Meliga, Marina
    CSELT Centro Studi e Laboratori, Telecomunicazioni S.p.A, Torino.
    Morello, G.
    CSELT Centro Studi e Laboratori, Telecomunicazioni S.p.A, Torino.
    Murgia, S.
    CSELT Centro Studi e Laboratori, Telecomunicazioni S.p.A, Torino.
    Paoletti, Roberto
    CSELT Centro Studi e Laboratori, Telecomunicazioni S.p.A, Torino.
    High reliability, high yield, high modulation bandwidth, low threshold current 1.55 μm MQW laser by new in-situ etching technique1998In: European Conference on Optical Communication, ECOC, 1998, 75-76 p.Conference paper (Refereed)
    Abstract [en]

    For the first time, to our knowledge, an InP based Fabry-Perot MQW semiconductor laser is obtained by using an in-situ etching technique. Good static and dynamic laser characteristics together with high process yield and reliability confirm the validity of this new technological process.

  • 342.
    Bertone, Daniele
    et al.
    Centro Studi e Laboratori Telecomunicazioni.
    Campi, Roberta
    Morello, G.
    Centro Studi e Laboratori Telecomunicazioni.
    Etching of InP-based MQW laser structure in a MOCVD reactor by chlorinated compounds1998In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 195, no 1-4, 624-629 p.Article in journal (Refereed)
    Abstract [en]

    Four different chlorinated compounds: 2-chloropropane, dichloromethane, chloroform and carbon tetrachloride have been used to etch InGaAsP/InP MQW laser structures partially masked. Etching experiments were performed in a home-made LP-MOCVD reactor with argon or argon + hydrogen as carrier gas, using phosphine (PH3) or tertiarybutylphosphine (TBP) to prevent thermal decomposition. The etching temperature as well as the chlorinated compound flow were varied to obtain the best trade-off between etch rate and surface morphology. The optimized experimental conditions were applied to etch mesa stripes in a SCH-MQW laser structure, for the first time to our knowledge, followed by lateral InP : Fe regrowth in the same step. Threshold current as low as 4 mA (best value)-6 mA (typical value) and differential quantum efficiency higher than 20% for SI-BM MQW laser have been achieved.

  • 343.
    Bertoni, M. I.
    et al.
    Massachusetts Institute of Technology, Cambridge, USA .
    Fenning, D. P.
    Massachusetts Institute of Technology, Cambridge, USA .
    Rinio, Markus
    Fraunhofer ISE, Laboratory and Servicecenter, Auf der Reihe 2, Gelsenkirchen, Germany .
    Rose, V.
    USA.
    Holt, M.
    USA.
    Maser, J.
    USA.
    Buonassisi, Tonio
    USA.
    Nanoprobe X-ray fluorescence characterization of defects in large-area solar cells2011In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706, Vol. 4, 4252-4257 p.Article in journal (Refereed)
    Abstract [en]

    The performance of centimeter-sized energy devices is regulated by inhomogeneously distributednanoscale defects. To improve device efficiency and reduce cost, accurate characterization of thesenanoscale defects is necessary. However, the multiscale nature of this problem presentsa characterization challenge, as non-destructive techniques often specialize in a single decade of lengthscales, and have difficulty probing non-destructively beneath the surface of materials with sub-micronspatial resolution. Herein, we push the resolution limits of synchrotron-based nanoprobe X-rayfluorescence mapping to 80 nm, to investigate a recombination-active intragranular defect in industrialsolar cells. Our nano-XRF measurements distinguish fundamental differences between benign anddeleterious dislocations in solar cell devices: we observe recombination-active dislocations to containa high degree of nanoscale iron and copper decoration, while recombination-inactive dislocationsappear clean. Statistically meaningful high-resolution measurements establish a connection betweencommercially relevant materials and previous fundamental studies on intentionally contaminatedmodel defect structures, pointing the way towards optimization of the industrial solar cell process.Moreover, this study presents a hierarchical characterization approach that can be broadly extended toother nanodefect-limited energy systems with the advent of high-resolution X-ray imaging beamlines

  • 344.
    Beshkova, M.
    et al.
    Bulgarian Academy of Science, Sofia, Bulgaria.
    Zakhariev, Z.
    Bulgarian Academy of Science, Sofia, Bulgaria.
    Abrashev, M. V.
    University of Sofia, Bulgaria.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Postovit, A.
    Institute of Problem Microelectronics Technology and High Purity Materials, Moskow, Russia.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Properties of AlN epitaxial layers on 6H-SiC substrate grown by sublimation in argon, nitrogen, and their mixtures2006In: Materials Science & Engineering: B. Solid-state Materials for Advanced Technology, ISSN 0921-5107, E-ISSN 1873-4944, Vol. 129, no 1-3, 228-231 p.Article in journal (Refereed)
    Abstract [en]

    Epitaxial layers of aluminum nitride (AlN) have been grown at temperature 1900 °C on 10 mm × 10 mm 6H-SiC substrate via sublimation-recondensation in RF heated graphite furnace. The source material was polycrystalline sintered AlN. Growth of AlN layers in pure nitrogen, mixed nitrogen/argon and pure argon atmosphere of 50 mbar were compared. A maximum growth rate of about 30 µm/h was achieved in pure nitrogen atmosphere. The surface morphology reflects the hexagonal symmetry of the seed, which is characteristic of an epitaxial growth for samples grown in a pure nitrogen and mixed nitrogen/argon atmosphere. X-ray diffraction (XRD) measurements show very strong and well defined (0 0 0 2) reflection positioned at around 36° in symmetric ?-2? scans. Micro-Raman spectroscopy reveals that the films have a wurtzite structure. Secondary-ion mass spectroscopy (SIMS) results showed a low concentration of carbon incorporation in the AlN layers. This study demonstrates that nitrogen is necessary for the successful epitaxial growth of AlN on 6H-SiC by sublimation. © 2006 Elsevier B.V. All rights reserved.

  • 345.
    Beyer, Franziska
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Pedersen, Henrik
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Henry, Anne
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Janzén, Erik
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Defects in 4H-SiC Layers Grown by Chloride-based Epitaxy2009In: Materials Science Forum Vols. 615-617 / [ed] Amador Pérez-Tomás, Trans Tech Publications , 2009, 373- p.Conference paper (Refereed)
    Abstract [en]

    Chloride-based 4H-SiC epitaxial layers were investigated by DLTS, MCTS and PL. The DLTS spectra of the as grown samples showed dominance of the Z1/2 and the EH6/7 peaks. For growth rates exceeding 100 µm/h, an additional peak occurred in the DLTS spectra which can be assigned to the UT1 defect. The shallow and the deep boron complexes as well as the HS1 defect are observed in MCTS measurements. The PL spectra are completely dominated by the near band gap (NBG) emission. No luminescence from donor-acceptor pair occurred. The PL line related to the D1 centre was weakly observed. In the NBG region nitrogen bound exciton (N-BE) and free exciton (FE) related lines could be seen. The addition of chlorine in the growth process gives the advantage of high growth rates without the introduction of additional defects.

  • 346.
    Beyer, Jan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Buyanova, Irina A
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Suraprapapich, S.
    Department of Electrical and Computer Engineering, University of California, USA .
    Tu, C. W.
    Department of Electrical and Computer Engineering, University of California, USA .
    Chen, Weimin
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Strong room-temperature optical and spin polarization in InAs/GaAs quantum dot structures2011In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 20, 203110- p.Article in journal (Refereed)
    Abstract [en]

    Room-temperature optical and spin polarization up to 35% is reported in InAs/GaAs quantum dots in zero magnetic field under optical spin injection using continuous-wave optical orientation spectroscopy. The observed strong spin polarization is suggested to be facilitated by a shortened trion lifetime, which constrains electron spin relaxation. Our finding provides experimental demonstration of the highly anticipated capability of semiconductor quantum dots as highly polarized spin/light sources and efficient spin detectors, with efficiency greater than 35% in the studied quantum dots.

  • 347.
    Beyer, Jan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Buyanova, Irina A
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Suraprapapich, Suwaree
    UC San Diego, USA.
    Tu, Charles
    UC San Diego, USA.
    Chen, Weimin
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Spin injection in lateral InAs quantum dot structures by optical orientation spectroscopy2009In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 20, no 37, 375401- p.Article in journal (Refereed)
    Abstract [en]

    Optical spin injection is studied in novel laterally-arranged self-assembled InAs/GaAs quantum dot structures, by using optical orientation measurements in combination with tunable laser spectroscopy. It is shown that spins of uncorrelated free carriers are better conserved during the spin injection than the spins of correlated electrons and holes in an exciton. This is attributed to efficient spin relaxation promoted by the electron–hole exchange interaction of the excitons. Our finding suggests that separate carrier injection, such as that employed in electrical spin injection devices, can be advantageous for spin conserving injection. It is also found that the spin injection efficiency decreases for free carriers with high momentum, due to the acceleration of spin relaxation processes.

  • 348.
    Beyer, Jan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Buyanova, Irina A
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Suraprapapich, Suwaree
    Dept of Electrical and Computer Engineering, University of California at San Diego, La Jolla, USA.
    Tu, Charles W
    Dept of Electrical and Computer Engineering, University of California at San Diego, La Jolla, USA.
    Chen, Weimin
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Efficiency of spin injection in novel InAs quantum dotstructures: exciton vs. free carrier injection2010Conference paper (Refereed)
    Abstract [en]

    Unambiguous experimental evidence for a significant difference in efficiency of excitonic vs. free carrier spin injection is provided in novel laterally arranged self-assembled InAs/GaAs quantum dot structures, from optical orientation and tunable laser spectroscopy. A lower efficiency of exciton spin injection as compared to free carrier spin injection from wetting layers into QDs results in a distinct feature in luminescence polarization of the QDs as a function of excitation photon energy. It is shown that this difference is not related to carrier density and state-filling effects arising from the difference in optical absorption efficiency between the excitons and free carriers. Rather, it is a genuine property for exciton spin injection that suffers stronger spin relaxation due to Coulomb exchange interaction.

  • 349.
    Beyer, Jan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Buyanova, Irina
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Sernelius, Bo E.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Suraprapapich, S
    Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, USA .
    Tu, C. W.
    Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California, USA .
    Chen, Weimin
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Strong suppression of spin generation at a Fano resonance in a semiconductor nanostructure2012Manuscript (preprint) (Other academic)
    Abstract [en]

    We observe remarkable, complete suppression of spin generation under optical excitation in a thin InAs/GaAs wetting layer close to the light-hole excitonic resonance, leading to zero electron spin polarization as monitored by adjacent InAs quantum dots. The suppression is attributed to efficient spin relaxation/scattering at the Fano resonance between the light-hole exciton states and the heavy-hole continuum of the wetting layer. The complete suppression is found to remain effective up to temperatures exceeding 100 K.

  • 350.
    Beyer, Jan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Buyanova, Irina
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Suraprapapich, S
    Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, USA .
    Tu, C. W.
    Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California, USA .
    Chen, Weimin
    Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
    Hanle effect and electron spin polarization in InAs/GaAs quantum dots up to room temperature2012In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 23, no 13, 135705- p.Article in journal (Refereed)
    Abstract [en]

    Hanle effect in InAs/GaAs quantum dots (QDs) is studied under optical orientation as a function of temperature over the range of 150-300 K, with the aim to understand the physical mechanism responsible for the observed sharp increase of electron spin polarization with increasing temperature. The deduced spin lifetime Ts of positive trions in the QDs is found to be independent of temperature, and is also insensitive to excitation energy and density. It is argued that the measured Ts is mainly determined by the longitudinal spin flip time (T1) and the spin dephasing time (T2 *) of the studied QD ensemble, of which both are temperatureindependent over the studied temperature range and the latter makes a larger contribution. The observed sharply rising of the QD spin polarization degree with increasing temperature, on the other hand, is shown to be induced by an increase in spin injection efficiency from the barrier/wetting layer and also by a moderate increase in spin detection efficiency of the QD.

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