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  • 1.
    Abdullaev, Fatkhulla
    et al.
    Physical-Technical Institute, Uzbek Academy of Sciences, Tashkent, Uzbekistan; Instituto de Fisica Teorica, Universidade Estadual Paulista Júlio de Mesquita Filho, São Paulo, Brazil.
    Ögren, Magnus
    Department of Applied Mathematics and Computer Science, Technical University of Denmark, Kongens Lyngby, Denmark.
    Sørensen, M. P.
    Department of Applied Mathematics and Computer Science, Technical University of Denmark, Kongens Lyngby, Denmark.
    Faraday waves in quasi-one-dimensional superfluid Fermi-Bose mixtures2013In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 87, no 2, article id 023616Article in journal (Refereed)
    Abstract [en]

    The generation of Faraday waves in superfluid Fermi-Bose mixtures in elongated traps is investigated. The generation of waves is achieved by periodically changing a parameter of the system in time. Two types of modulations of parameters are considered: a variation of the fermion-boson scattering length and the boson-boson scattering length. We predict the properties of the generated Faraday patterns and study the parameter regions where they can be excited.

  • 2.
    Bondarenko, N.
    et al.
    Division of Materials theory, Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Division of Materials theory, Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Skorodumova, N. , V
    Division of Materials theory, Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden; Multiscale Materials Modelling, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden.
    Pereiro, M.
    Division of Materials theory, Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Multi-polaron solutions, nonlocal effects and internal modes in a nonlinear chain2019In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, no 41, article id 415401Article in journal (Refereed)
    Abstract [en]

    Multipolaron solutions were studied in the framework of the Holstein one-dimensional molecular crystal model. The study was performed in the continuous limit where the crystal model maps into the nonlinear Schrodinger equation for which a new periodic dnoidal solution was found for the multipolaron system. In addition, the stability of the multi-polaron solutions was examined, and it was found that dnoidal and dnoidal solutions stabilize in different ranges of the parameter space. Moreover, the model was studied under the influence of nonlocal effects and the polaronic dynamics was described in terms of internal solitonic modes.

  • 3.
    Corboz, Philippe
    et al.
    School of Mathematics and Physics, The University of Queensland, Brisbane Qld, Australia.
    Ögren, Magnus
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, The University of Queensland, Brisbane Qld, Australia.
    Kheruntsyan, Karén
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, The University of Queensland, Brisbane Qld, Australia.
    Corney, Joel F.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, The University of Queensland, Brisbane Qld, Australia.
    Phase-space methods for fermions2013In: Quantum Gases: Finite Temperature and Non-Equilibrium Dynamics / [ed] Davis, M.; Gardiner, S.; Proukakis, N.; Szymańska, M., London: Imperial College Press, 2013, p. 407-416Chapter in book (Other academic)
    Abstract [en]

    We review phase-space simulation techniques for fermions, showing how a Gaussian operator basis leads to exact calculations of the evolution of a many-body quantum system in both real and imaginary time. We apply such techniques to the Hubbard model and to the problem of molecular dissociation of bosonic molecules into pairs of fermionic atoms.

  • 4.
    Hasan, Mehedi
    et al.
    ITMO University, Saint Petersburg, Russia; Division of Physics and Applied Physics, Nanyang Technological University, Singapore, Singapore.
    Yudin, Dmitry
    ITMO University, Saint Petersburg, Russia.
    Iorsh, Ivan
    ITMO University, Saint Petersburg, Russia; Division of Physics and Applied Physics, Nanyang Technological University, Singapore, Singapore.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Shelykh, Ivan
    ITMO University, Saint Petersburg, Russia; Science Institute, University of Iceland, Reykjavik, Iceland.
    Topological edge-state engineering with high-frequency electromagnetic radiation2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 20, article id 205127Article in journal (Refereed)
    Abstract [en]

    We outline here how strong light-matter interaction can be used to induce quantum phase transition between normal and topological phases in two-dimensional topological insulators. We consider the case of a HgTe quantum well, in which band inversion occurs above a critical value of the well thickness, and demonstrate that coupling between electron states and the E field from an off-resonant linearly polarized laser provides a powerful tool to control topological transitions, even for a thickness of the quantum well that is below the critical value. We also show that topological phase properties of the edge states, including their group velocity, can be tuned in a controllable way by changing the intensity of the laser field. These findings open up the possibility for new experimental means with which to investigate topological insulators and shed new light on topological-insulator-based technologies that are under active discussion.

  • 5.
    Hellsvik, Johan
    et al.
    Nordita, Stockholm, Sweden; Department of Physics, KTH Royal Institute of Technology, Sweden.
    Thonig, Danny
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Modin, Klas
    Department of Mathematics, Chalmers University of Technology, Gothenburg, Sweden; Department of Mathematics, University of Gothenburg, Gothenburg, Sweden.
    Iusan, Diana
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Bergman, Anders
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Bergqvist, Lars
    Department of Applied Physics, School of Engineering Sciences, KTH Royal Institute of Technology, Electrum 229, Kista, Sweden; SeRC (Swedish e-Science Research Center), KTH Royal Institute of Technology, Stockholm, Sweden.
    Delin, Anna
    Department of Applied Physics, School of Engineering Sciences, KTH Royal Institute of Technology, Electrum 229, Kista, Sweden; SeRC (Swedish e-Science Research Center), KTH Royal Institute of Technology, Stockholm, Sweden; Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    General method for atomistic spin-lattice dynamics with first-principles accuracy2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 10, article id 104302Article in journal (Refereed)
    Abstract [en]

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

  • 6.
    Huttmann, Felix
    et al.
    II. Physikalisches Institut, Universität zu Köln, Köln, Germany.
    Rothenbach, Nico
    Fakultät für Physik, Universität Duisburg-Essen and Center for Nanointegration Duisburg-Essen (CENIDE), German.
    Kraus, Stefan
    II. Physikalisches Institut, Universität zu Köln, Köln, Germany.
    Ollefs, Katharina
    Fakultät für Physik, Universität Duisburg-Essen and Center for Nanointegration Duisburg-Essen (CENIDE), German.
    Arruda, Lucas M.
    Institut für Experimentalphysik, Freie Universität Berlin, Berlin, Germany.
    Bernien, Matthias
    Institut für Experimentalphysik, Freie Universität Berlin, Berlin, Germany.
    Thonig, Danny
    Department of Physics and Astronomy, Materials Theory, Uppsala University, Uppsala, Sweden.
    Delin, Anna
    Department of Physics and Astronomy, Materials Theory, Uppsala University, Uppsala, Sweden; Department of Applied Physics, School of Engineering Sciences, KTH Royal Institute of Technology, Kista, Sweden; SeRC (Swedish e-Science Research Center), KTH Royal Institute of Technology, Stockholm, Sweden.
    Fransson, Jonas
    Department of Physics and Astronomy, Materials Theory, Uppsala University, Uppsala, Sweden.
    Kummer, Kurt
    European Synchrotron Radiation Facility, Grenoble, France.
    Brookes, Nicholas B
    European Synchrotron Radiation Facility, Grenoble, France.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Materials Theory, Uppsala University, Uppsala, Sweden.
    Kuch, Wolfgang
    Institut für Experimentalphysik, Freie Universität Berlin, Berlin, Germany.
    Michely, Thomas
    II. Physikalisches Institut, Universität zu Köln, Köln, Germany.
    Wende, Heiko
    Fakultät für Physik, Universität Duisburg-Essen and Center for Nanointegration Duisburg-Essen (CENIDE), German.
    Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator2019In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 10, no 5, p. 911-917Article in journal (Refereed)
    Abstract [en]

    We investigate the magnetic and electronic properties of europium cyclooctatetraene (EuCot) nanowires by means of low-temperature X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS). The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as an insulator with a minority band gap of 2.3 eV. By means of Eu M5,4 edge XMCD, orbital and spin magnetic moments of (-0.1 ± 0.3)μB and (+7.0 ± 0.6)μB, respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M5 edge show hysteresis for grazing X-ray incidence at 5 K, thus confirming EuCot as a ferromagnetic material. Our density functional theory calculations reproduce the experimentally observed minority band gap. Modeling the experimental results theoretically, we find that the effective interatomic exchange interaction between Eu atoms is on the order of millielectronvolts, that magnetocrystalline anisotropy energy is roughly half as big, and that dipolar energy is approximately ten times lower.

  • 7.
    Kavoulakis, G. M.
    et al.
    Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Yu, Y.
    Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Ögren, Magnus
    Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Reimann, S. M.
    Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Superfluidity in a gas of strongly interacting bosons2006In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 76, no 2, p. 215-221Article in journal (Refereed)
    Abstract [en]

    We consider small systems of bosonic atoms rotating in a toroidal trap. Using the method of exact numerical diagonalization of the many-body Hamiltonian, we examine the transition from the Bose-Einstein condensed state to the Tonks-Girardeau state. The system supports persistent currents in a wide range between the two limits, even in the absence of Bose-Einstein condensation.

  • 8.
    Keshavarz, Samara
    et al.
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Kontos, Sofia
    Department of Engineering Sciences, Division of Solid State Physics, Uppsala University, Uppsala, Sweden.
    Wardecki, Dariusz
    Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, Sweden; Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Warsaw, Poland; Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Gothenburg, Sweden.
    Kvashnin, Yaroslav O.
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Pereiro, Manuel
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Panda, Swarup K.
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Sanyal, Biplab
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Grins, Jekabs
    Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, Sweden.
    Svensson, Gunnar
    Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, Sweden.
    Gunnarsson, Klas
    Department of Engineering Sciences, Division of Solid State Physics, Uppsala University, Uppsala, Sweden.
    Svedlindh, Peter
    Department of Engineering Sciences, Division of Solid State Physics, Uppsala University, Uppsala, Sweden.
    Magnetic properties of Ruddlesden-Popper phases Sr3−x Yx (Fe1.25 Ni0.75) O7−δ: A combined experimental and theoretical investigation2018In: Physical Review Materials, E-ISSN 2475-9953, Vol. 2, no 4, article id 044005Article in journal (Refereed)
    Abstract [en]

    We present a comprehensive study of the magnetic properties of Sr3-xYx(Fe1.25Ni0.75)O-7(-delta )(0 <= x <= 0.75). Experimentally, the magnetic properties are investigated using superconducting quantum interference device (SQUID) magnetometry and neutron powder diffraction (NPD). This is complemented by a theoretical study based on density functional theory as well as the Heisenberg exchange parameters. Experimental results show an increase in the Ned temperature (T-N) with an increase of Y concentrations and O occupancy. The NPD data reveal that all samples are antiferromagnetically ordered at low temperatures, which has been confirmed by our theoretical simulations for the selected samples. Our first-principles calculations suggest that the three-dimensional magnetic order is stabilized due to finite interlayer exchange couplings. The latter give rise to finite interlayer spin-spin correlations, which disappear above T-N.

  • 9.
    Koumpouras, Konstantinos
    et al.
    Applied Physics, Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå, Sweden.
    Yudin, Dmitry
    ITMO University, Saint Petersburg, Russia.
    Adelmann, Christoph
    Imec, Leuven, Belgium.
    Bergman, Anders
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden; Maison de la Simulation, CEA, CNRS, Univ. Paris-Sud, UVSQ, Université Paris-Saclay, Gif-sur-Yvette, France; INAC-MEM, CEA, Grenoble, France.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Pereiro, Manuel
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    A majority gate with chiral magnetic solitons2018In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, no 37, article id 375801Article in journal (Refereed)
    Abstract [en]

    In magnetic materials, nontrivial spin textures may emerge due to the competition among different types of magnetic interactions. Among such spin textures, chiral magnetic solitons represent topologically protected spin configurations with particle-like properties. Based on atomistic spin dynamics simulations, we demonstrate that these chiral magnetic solitons are ideal to use for logical operations, and we demonstrate the functionality of a three- input majority gate, in which the input states can be controlled by applying an external electromagnetic field or spin-polarized currents. One of the main advantages of the proposed device is that the input and output signals are encoded in the chirality of solitons, that may be moved, allowing to perform logical operations using only minute electric currents. As an example we illustrate how the three input majority gate can be used to perform logical relations, such as Boolean AND and OR.

  • 10.
    Lüder, Johann
    et al.
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden; Department of Mechanical Engineering, National University, Singapore, Singapore.
    Schött, Johan
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Brena, Barbara
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Haverkort, Maurits W.
    Institute for Theoretical Physics, Heidelberg University, Heidelberg, Germany.
    Thunström, Patrik
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Sanyal, Biplab
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Di Marco, Igor
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Kvashnin, Yaroslav O.
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Theory of L-edge spectroscopy of strongly correlated systems2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 24, article id 245131Article in journal (Refereed)
    Abstract [en]

    X-ray absorption spectroscopy measured at the L edge of transition metals (TMs) is a powerful element selective tool providing direct information about the correlation effects in the 3d states. The theoretical modeling of the 2p -> 3d excitation processes remains to be challenging for contemporary ab initio electronic structure techniques, due to strong core-hole and multiplet effects influencing the spectra. In this work, we present a realization of the method combining the density-functional theory with multiplet ligand field theory, proposed in Haverkort et al. [Phys. Rev. B 85, 165113 (2012)]. In this approach, a single-impurity Anderson model (SIAM) is constructed, with almost all parameters obtained from first principles, and then solved to obtain the spectra. In our implementation, we adopt the language of the dynamical mean-field theory and utilize the local density of states and the hybridization function, projected onto TM 3d states, in order to construct the SIAM. The developed computational scheme is applied to calculate the L-edge spectra for several TM monoxides. A very good agreement between the theory and experiment is found for all studied systems. The effect of core-hole relaxation, hybridization discretization, possible extensions of the method as well as its limitations are discussed.

  • 11. Oresic, Matej
    et al.
    Pirc, Rasa
    Model of a quasi-one-dimensional spin glass1993In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 47, no 5, p. 2655-2660Article in journal (Refereed)
    Abstract [en]

    We consider a system of linear chains of Ising spins with short-range nearest-neighbor ferromagnetic interactions of strength K along the chains, and with infinite-range random interchain interactions of a Sherrington-Kirkpatrick type. Also included in the model is a local Gaussian random field with variance Δ. In the replica symmetric case, the system is mapped exactly on the one-dimensional Ising model in a random field and a recursion method for calculating the spin-correlation functions is developed. For Δ=0, an analytic relation for freezing temperature Tf (K) is obtained. The replica-symmetric spin-glass phase is shown to be stable above the freezing temperature Tf (K,Δ), which is determined numerically.

  • 12.
    Pal, Somnath
    et al.
    Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru, India.
    Jana, Somnath
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Govinda, Sharada
    Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru, India.
    Pal, Banabir
    Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru, India.
    Mukherjee, Sumanta
    Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru, India.
    Keshavarz, Samara
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Thonig, Danny
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Kvashnin, Yaroslav
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Pereiro, Manuel
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Mathieu, Roland
    Department of Engineering Sciences, Uppsala University, Uppsala, Sweden.
    Nordblad, Per
    Department of Engineering Sciences, Uppsala University, Uppsala, Sweden.
    Freeland, John W.
    Argonne National Laboratory, Argonne Illinois, USA.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Karis, Olof
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Sarma, D. D.
    Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru, India.
    Peculiar magnetic states in the double perovskite Nd2NiMnO62019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 4, article id 045122Article in journal (Refereed)
    Abstract [en]

    We present magnetic measurements on Nd2NiMnO6 which exhibits a well-known insulating paramagnetic state to an insulating ferromagnetic state transition when cooled below 200 K. Beyond this basic fact, there is a great deal of diversity in the reported magnetic properties and interpretation of specific anomalies observed in the magnetic data of this compound below the Curie temperature. We address specifically two anomalies discussed in the past, namely, a spin-glass like behavior observed in some samples near 100 K and a downturn in the magnetization with a lowering of the temperature below approximately 50 K. We show for the first time that the application of an increasing magnetic field can systematically change the low-temperature behavior to make the down-turn in the magnetization into an upturn. With the help of first principle calculations and extensive simulations along with our experimental observations, we provide a microscopic understanding of all magnetic properties observed in this interesting system to point out that the glassiness around 100 K is absent in well-ordered samples and that the low-temperature magnetic anomaly below 50 K is a consequence of a ferromagnetic coupling of the Nd spin moments with the spin of the Ni-Mn ordered sublattice without giving rise to any ordering of the Nd sublattice that remains paramagnetic, contrary to earlier claims. We explain this counter-intuitive interpretation of a ferromagnetic coupling of Nd spins with Ni-Mn spin giving rise to a decrease in the total magnetic moment by noting the less than half-filled 4f occupation of Nd that ensures orbital and spin moments of Nd to be opposite to each other due to the spin-orbit coupling. Since the ground state total magnetic moment of Nd has a contribution from the orbital moment, that is larger than the spin moment, the total moment of Nd is indeed pointing in a direction opposite to the direction of spin moments of the Ni-Mn sublattice as a consequence of the ferromagnetic exchange coupling between Nd and Ni-Mn spins.

  • 13.
    Roussou, Alexandra
    et al.
    Department of Applied Mathematics, University of Crete, Heraklion, Greece.
    Smyrnakis, Ioannis
    Technological Education Institute of Crete, Heraklion, Greece.
    Magiropoulos, Manolis
    Technological Education Institute of Crete, Heraklion, Greece.
    Efremidis, Nikolaos
    Department of Applied Mathematics, University of Crete, Heraklion, Greece.
    Kavoulakis, Georgios
    Technological Education Institute of Crete, Heraklion, Greece.
    Sandin, Patrik
    Örebro University, School of Science and Technology.
    Ögren, Magnus
    Örebro University, School of Science and Technology.
    Gulliksson, Mårten
    Örebro University, School of Science and Technology.
    Excitation spectrum of a mixture of two Bose gases confined in a ring potential with interaction asymmetry2018In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 20, article id 045006Article in journal (Refereed)
    Abstract [en]

    We study the rotational properties of a two-component Bose-Einstein condensed gas of distinguishable atoms which are confined in a ring potential using both the mean-field approximation, as well as the method of diagonalization of the many-body Hamiltonian. We demonstrate that the angular momentum may be given to the system either via single-particle, or "collective" excitation. Furthermore, despite the complexity of this problem, under rather typical conditions the dispersion relation takes a remarkably simple and regular form. Finally, we argue that under certain conditions the dispersion relation is determined via collective excitation. The corresponding many-body state, which, in addition to the interaction energy minimizes also the kinetic energy, is dictated by elementary number theory.

  • 14.
    Sandin, Patrik
    et al.
    Örebro University, School of Science and Technology.
    Ögren, Magnus
    Örebro University, School of Science and Technology.
    Gulliksson, Mårten
    Örebro University, School of Science and Technology.
    Smyrnakis, J.
    Technological Education Institute of Crete, Heraklion, Greece.
    Magiropoulos, M.
    Technological Education Institute of Crete, Heraklion, Greece.
    Kavoulakis, G. M.
    Technological Education Institute of Crete, Heraklion, Greece.
    Dimensional reduction in Bose-Einstein condensed clouds of atoms confined in tight potentials of any geometry and any interaction strength2017In: Physical Review E, ISSN 2470-0045, Vol. 95, no 1, article id 012142Article in journal (Refereed)
    Abstract [en]

    Motivated by numerous experiments on Bose-Einstein condensed atoms which have been performed in tight trapping potentials of various geometries (elongated and/or toroidal/annular), we develop a general method which allows us to reduce the corresponding three-dimensional Gross-Pitaevskii equation for the order parameter into an effectively one-dimensional equation, taking into account the interactions (i.e., treating the width of the transverse profile variationally) and the curvature of the trapping potential. As an application of our model we consider atoms which rotate in a toroidal trapping potential. We evaluate the state of lowest energy for a fixed value of the angular momentum within various approximations of the effectively one-dimensional model and compare our results with the full solution of the three-dimensional problem, thus getting evidence for the accuracy of our model.

  • 15.
    Schmitz-Antoniak, Carolin
    et al.
    Peter-Grünberg-Institut (PGI-6), Forschungszentrum Jülich, Jülich, Germany.
    Schmitz, Detlef
    Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin, Germany.
    Warland, Anne
    Fakultät für Physik and Center for Nanointegration Duisburg-Essen (CENIDE), Universität Duisburg-Essen, Duisburg, Germany.
    Darbandi, Masih
    Fakultät für Physik and Center for Nanointegration Duisburg-Essen (CENIDE), Universität Duisburg-Essen, Duisburg, Germany; Chemistry faculty, University of Tabriz, Tabriz, Iran.
    Haldar, Soumyajyoti
    Institute of Theoretical Physics & Astrophysics, University of Kiel, Kiel, Germany.
    Bhandary, Sumanta
    Centre de Physique Théorique (CPHT), Ecole Polytechnique, Palaiseau cedex, France.
    Sanyal, Biplab
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Eriksson, Olle
    Örebro University, School of Science and Technology.
    Wende, Heiko
    Fak Phys, Univ Duisburg Essen, Duisburg, Germany; Ctr Nanointegrat Duisburg Essen CENIDE, Univ Duisburg Essen, Duisburg, Germany.
    Suppression of the Verwey Transition by Charge Trapping2018In: Annalen der Physik, ISSN 0003-3804, E-ISSN 1521-3889, Vol. 530, no 3, article id 1700363Article in journal (Refereed)
    Abstract [en]

    The Verwey transition in Fe3O4 nanoparticles with a mean diameter of 6.3 nm is suppressed after capping the particles with a 3.5 nm thick shell of SiO2. By X-ray absorption spectroscopy and its associated X-ray magnetic circular dichroism this suppression can be correlated to localized Fe2+ states and a reduced double exchange visible in different site-specific magnetization behavior in high magnetic fields. The results are discussed in terms of charge trapping at defects in the Fe3O4/ SiO2 interface and the consequent difficulties in the formation of the common phases of Fe3O4. By comparison to X-ray absorption spectra of bare Fe3O4 nanoparticles in course of the Verwey transition, particular changes in the spectral shape could be correlated to changes in the number of unoccupied d states for Fe ions at different lattice sites. These findings are supported by density functional theory calculations.

  • 16.
    Schneider, Natalia L.
    et al.
    Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, Kiel, Germany.
    Johansson, Peter
    Örebro University, School of Science and Technology.
    Berndt, Richard
    Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, Kiel, Germany.
    Hot electron cascades in the scanning tunneling microscope2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 045409Article in journal (Refereed)
    Abstract [en]

    The nonequilibrium distribution of electrons at the junction of a scanning tunneling microscope is investigated by detecting photons with energies hv > eV, where V is the bias voltage. Electrons are found at energies exceeding the Fermi level by almost eV. While their distribution deviates from a Fermi-Dirac function it is consistent with a model of hot electrons and holes that diffuse in energy and real space. DOI: 10.1103/PhysRevB.87.045409

  • 17.
    Schwind, Markus
    et al.
    Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden.
    Miljkovic, Vladimir D.
    Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden.
    Zäch, Michael
    Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden.
    Gusak, Viktoria
    Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden.
    Käll, Mikael
    Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden.
    Zoric, Igor
    Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden.
    Johansson, Peter
    Örebro University, School of Science and Technology. Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden.
    Diffraction from Arrays of Plasmonic Nanoparticles with Short-Range Lateral Order2012In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 6, no 11, p. 9455-9465Article in journal (Refereed)
    Abstract [en]

    We have measured the angular distribution of light scattered off 2D plasmonic Al nanoparticle ensembles. We created.. these samples with disk-like nanoparticles, 175 and 500 nm in diameter, respectively, using hole-mask colloidal lithography and electron beam lithography. The nanoparticle arrangements In the samples display the Short-range order (but no long-range order) characteristic for an ensemble formed by random sequential adsorption. As a consequence of this, the ensemble scattering patterns can be quantitatively well described by combining the single-particle scattering pattern with a static structure factor that carries information about the diffraction effects caused by the short-range order of the ensemble. We also performed sensing experiments in which we monitored changes in the angle-resolved scattering intensity for a fixed wavelength as a function of the thickness of an ultrathin SiO2 coating covering the Al nanoparticles. The data show that the angle and strength of the main diffraction peak vary linearly, with SiO2 coating thickness In the range 1.5-4.5 nm and suggest that measurements of the scattering profile could be a competitive alternative to traditional transmission measurements in terms of sensitivity.

  • 18.
    Shao, Lei
    et al.
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Andrén, Daniel
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Jones, Steven
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Johansson, Peter
    Örebro University, School of Science and Technology.
    Käll, Mikael
    Department of Physics, Chalmers University of Technology, Göteborg, Sweden.
    Optically controlled stochastic jumps of individual gold nanorod rotary motors2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 8, article id 085404Article in journal (Refereed)
    Abstract [en]

    Brownian microparticles diffusing in optical potential-energy landscapes constitute a generic test bed for nonequilibrium statistical thermodynamics and have been used to emulate a wide variety of physical systems, ranging from Josephson junctions to Carnot engines. Here we demonstrate that it is possible to scale down this approach to nanometric length scales by constructing a tilted washboard potential for the rotation of plasmonic gold nanorods. The potential depth and tilt can be precisely adjusted by modulating the light polarization. This allo`ws for a gradual transition from continuous rotation to discrete stochastic jumps, which are found to follow Kramers dynamics in excellent agreement with stochastic simulations. The results widen the possibilities for fundamental experiments in statistical physics and provide insights into how to construct light-driven nanomachines and multifunctional sensing elements.

  • 19.
    Shaw, Justin M.
    et al.
    Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder CO, United States.
    Delczeg-Czirjak, Erna K.
    Department of Physics and Astronomy, University Uppsala, Uppsala, Sweden.
    Edwards, Eric R. J.
    Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder CO, United States.
    Kvashnin, Yaroslav
    Department of Physics and Astronomy, University Uppsala, Uppsala, Sweden.
    Thonig, Danny
    Department of Physics and Astronomy, University Uppsala, Uppsala, Sweden.
    Schoen, Martin A. W.
    Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder CO, United States; Department of Physics, University of Regensburg, Regensburg, Germany.
    Pufall, Matt
    Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder CO, United States.
    Schneider, Michael L.
    Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder CO, United States.
    Silva, Thomas J.
    Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder CO, United States.
    Karis, Olof
    Department of Physics and Astronomy, University Uppsala, Uppsala, Sweden.
    Rice, Katherine P.
    CAMECA Instruments, Madison WI, United States.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, University Uppsala, Uppsala, Sweden.
    Nembach, Hans T.
    Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder CO, United States.
    Magnetic damping in sputter-deposited Co2MnGe Heusler compounds with A2, B2, and L2(1) orders: Experiment and theory2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 9, article id 094420Article in journal (Refereed)
    Abstract [en]

    We show that very low values of the magnetic damping parameter can be achieved in sputter deposited polycrystalline films of Co2MnGe annealed at relatively low temperatures ranging from 240 degrees C to 400 degrees C. Damping values as low as 0.0014 are obtained with an intrinsic value of 0.0010 after spin-pumping contributions are considered. Of importance to most applications is the low value of inhomogeneous linewidth that yields measured linewidths of 1.8 and 5.1 mT at 10 and 40 GHz, respectively. The damping parameter monotonically decreases as the B2 order of the films increases. This trend is reproduced and explained by ab initio calculations of the electronic structure and damping parameter. Here, the damping parameter is calculated as the structure evolves from A2 to B2 to L2(1) orders. The largest decrease in the damping parameter occurs during the A2 to B2 transition as the half-metallic phase becomes established.

  • 20.
    Szilva, A.
    et al.
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Thonig, D.
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Bessarab, P. F.
    Science Institute, University of Iceland, Reykjavik, Iceland; Department of Nanophotonics and Metamaterials, ITMO University, St. Petersburg, Russia.
    Kvashnin, Y. O.
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Rodrigues, D. C. M.
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden; Faculdade de Física, Universidade Federal do Pará, Belém, Brazil.
    Cardias, R.
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden; Faculdade de Física, Universidade Federal do Pará, Belém, Brazil.
    Pereiro, M.
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Nordström, L.
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Bergman, A.
    Maison de la Simulation, USR 3441, CEA-CNRS-INRIA-Université Paris-Sud-Université de Versailles, Gif-sur-Yvette, France; Institut Nanosciences et Cryogénie (Inac) and Modeling and Exploration of Materials (MEM), Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), Grenoble, France.
    Klautau, A. B.
    Faculdade de Física, Universidade Federal do Pará, Belém, Brazil.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Theory of noncollinear interactions beyond Heisenberg exchange: Applications to bcc Fe2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 14, article id 144413Article in journal (Refereed)
    Abstract [en]

    We show for a simple noncollinear configuration of the atomistic spins (in particular, where one spin is rotated by a finite angle in a ferromagnetic background) that the pairwise energy variation computed in terms of multiple-scattering formalism cannot be fully mapped onto a bilinear Heisenberg spin model even in the absence of spin-orbit coupling. The non-Heisenberg terms induced by the spin-polarized host appear in leading orders in the expansion of the infinitesimal angle variations. However, an E-g - T-2g symmetry analysis based on the orbital decomposition of the exchange parameters in bcc Fe leads to the conclusion that the nearest-neighbor exchange parameters related to the T-2g orbitals are essentially Heisenberg-like: they do not depend on the spin configuration, and can, in this case, be mapped onto a Heisenberg spin model even in extreme noncollinear cases.

  • 21.
    Sørensen, Mads Peter
    et al.
    Department of Applied Mathematics and Computer Science, Technical University of Denmark, Kongens Lyngby, Denmark.
    Falsig Pedersen, Niels
    Department of Applied Mathematics and Computer Science, Technical University of Denmark, Kongens Lyngby, Denmark.
    Ögren, Magnus
    Örebro University, School of Science and Technology.
    The dynamics of magnetic vortices in type II superconductors with pinning sites studied by the time dependent Ginzburg–Landau model2017In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 533, p. 40-43Article in journal (Refereed)
    Abstract [en]

    We investigate the dynamics of magnetic vortices in type II superconductors with normal state pinning sites using the Ginzburg–Landau equations. Simulation results demonstrate hopping of vortices between pinning sites, influenced by external magnetic fields and external currents. The system is highly nonlinear and the vortices show complex nonlinear dynamical behaviour.

  • 22.
    Vekilova, Olga Yu
    et al.
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Fayyazi, Bahar
    Materials Science, TU Darmstadt, Darmstadt, Germany.
    Skokov, Konstantin P.
    Materials Science, TU Darmstadt, Darmstadt, Germany.
    Gutfleisch, Oliver
    Materials Science, TU Darmstadt, Darmstadt, Germany.
    Echevarria-Bonet, Cristina
    BCMaterials, UPV/EHU Science Park, Leioa, Spain.
    Barandiarán, Jose Manuel
    BCMaterials, UPV/EHU Science Park, Leioa, Spain.
    Kovacs, Alexander
    Department for Integrated Sensor Systems, Danube University Krems, Wiener Neustadt, Austria.
    Fischbacher, Johann
    Department for Integrated Sensor Systems, Danube University Krems, Wiener Neustadt, Austria.
    Schrefl, Thomas
    Department for Integrated Sensor Systems, Danube University Krems, Wiener Neustadt, Austria.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Herper, Heike C.
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Tuning the magnetocrystalline anisotropy of Fe3Sn by alloying2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 2, article id 024421Article in journal (Refereed)
    Abstract [en]

    The electronic structure, magnetic properties, and phase formation of hexagonal ferromagnetic Fe3Sn-based alloys have been studied from first principles and by experiment. The pristine Fe3Sn compound is known to fulfill all the requirements for a good permanent magnet, except for the magnetocrystalline anisotropy energy (MAE). The latter is large, but planar, i.e., the easy magnetization axis is not along the hexagonal c direction, whereas a good permanent magnet requires the MAE to be uniaxial. Here we consider Fe3Sn0.75M0.25, where M = Si, P, Ga, Ge, As, Se, In, Sb, Te, Pb, and Bi, and show how different dopants affect the MAE and can alter it from planar to uniaxial. The stability of the doped Fe3Sn phases is elucidated theoretically via the calculations of their formation enthalpies. A micromagnetic model is developed to estimate the energy density product (BH)(max) and coercive field mu H-0(c) of a potential magnet made of Fe3Sn0.75M0.25, the most promising candidate from theoretical studies. The phase stability and magnetic properties of the Fe3Sn compound doped with Sb and Mn have been checked experimentally on the samples synthesised using the reactive crucible melting technique as well as by solid state reaction. The Fe3Sn-Sb compound is found to be stable when alloyed with Mn. It is shown that even small structural changes, such as a change of the c/a ratio or volume, that can be induced by, e.g., alloying with Mn, can influence anisotropy and reverse it from planar to uniaxial and back.

  • 23.
    Yu, Y.
    et al.
    Division of Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Ögren, Magnus
    Division of Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Åberg, S.
    Division of Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Reimann, S. M.
    Division of Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Brack, M.
    Institut für Theoretische Physik, Universität Regensburg, Regensburg, Germany.
    Supershell structure in trapped dilute Fermi gases2005In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, no 5, article id 051602Article in journal (Refereed)
    Abstract [en]

    We show that a dilute harmonically trapped two-component gas of fermionic atoms with a weak repulsive interaction has a pronounced super-shell structure: The shell fillings due to the spherical harmonic trapping potential are modulated by a beat mode. This changes the "magic numbers" occurring between the beat nodes by half a period. The length and amplitude of this beating mode depend on the strength of the interaction. We give a simple interpretation of the beat structure in terms of a semiclassical trace formula for the symmetry breaking U(3)→SO(3).

  • 24.
    Ögren, Magnus
    Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    Klassisk mekanik vs kvantmekanik2010In: Forskning, ISSN 1654-8876, no 3-4, p. 50-54Article, review/survey (Other (popular science, discussion, etc.))
  • 25.
    Ögren, Magnus
    et al.
    Department of Mathematics, Technical University of Denmark, Kgs. Lyngby, Denmark.
    Carlsson, M.
    Center for Mathematical Sciences, Lund University, Lund, Sweden.
    On the dynamics of the Fermi-Bose model2013In: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 46, no 1, article id 015005Article in journal (Refereed)
    Abstract [en]

    We consider the exponential matrix representing the dynamics of the Fermi-Bose model in an undepleted bosonic field approximation. A recent application of this model is molecular dimers dissociating into its atomic compounds. The problem is solved in D spatial dimensions by dividing the system matrix into blocks with generalizations of Hankel matrices, here referred to as D-block-Hankel matrices. The method is practically useful for treating large systems, i.e. dense computational grids or higher spatial dimensions, either on a single standard computer or a cluster. In particular the results can be used for studies of three-dimensional physical systems of arbitrary geometry. We illustrate the generality of our approach by giving numerical results for the dynamics of Glauber type atomic pair correlation functions for a non-isotropic three-dimensional harmonically trapped molecular Bose-Einstein condensate.

  • 26.
    Ögren, Magnus
    et al.
    Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    Heiselberg, H.
    University of Southern Denmark, Odense, Denmark.
    Supershell structures and pairing in ultracold trapped Fermi gases2007In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, no 2, article id 021601Article in journal (Refereed)
    Abstract [en]

    We calculate level densities and pairing gaps for an ultracold dilute gas of fermionic atoms in harmonic traps under the influence of mean field and anharmonic quartic trap potentials. Supershell nodes, which were found in Hartree-Fock calculations, are calculated analytically within periodic orbit theory as well as from WKB calculations. For attractive interactions, the underlying level densities are crucial for pairing and supershell structures in gaps are predicted.

  • 27.
    Ögren, Magnus
    et al.
    Department of Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    Kavoulakis, G. M.
    Department of Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    Persistent currents in a Bose-Einstein condensate in the presence of disorder2007In: Journal of Low Temperature Physics, ISSN 0022-2291, E-ISSN 1573-7357, Vol. 149, no 3-4, p. 176-184Article in journal (Refereed)
    Abstract [en]

    We examine bosonic atoms that are confined in a toroidal, quasi-one-dimensional trap, subjected to a random potential. The resulting inhomogeneous atomic density is smoothened for sufficiently strong, repulsive interatomic interactions. Statistical analysis of our simulations show that the gas supports persistent currents, which become more fragile due to the disorder.

  • 28.
    Ögren, Magnus
    et al.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Physical Sciences, University of Queensland, Brisbane, Australia; Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    Kavoulakis, G. M.
    Technological Education Institute of Crete, Heraklion, Greece.
    Stability of persistent currents in a Bose-Einstein condensate confined in a toroidal trap2009In: Journal of Low Temperature Physics, ISSN 0022-2291, E-ISSN 1573-7357, Vol. 154, no 1-2, p. 30-40Article in journal (Refereed)
    Abstract [en]

    Motivated by recent experiments in Bose-Einstein condensed atoms that have been confined in toroidal traps, we examine the stability of persistent currents in such systems. We investigate the extent that the stability of these currents may be tunable, and the possible difficulties in their creation and detection.

  • 29.
    Ögren, Magnus
    et al.
    Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    Kavoulakis, G. M.
    Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    Jackson, A. D.
    Niels Bohr Institute, Copenhagen, Denmark.
    Solitary waves in elongated clouds of strongly interacting bosons2005In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, no 2, article id 021603Article in journal (Refereed)
    Abstract [en]

    We examine the propagation of solitary waves in elongated clouds of trapped bosonic atoms as the confinement, the strength of the interatomic interaction, and the atom density are varied. We identify three different physical regimes and develop a general formalism that allows us to interpolate between them. Finally we pay special attention to the transition to the Tonks-Girardeau limit of strongly interacting bosons.

  • 30.
    Ögren, Magnus
    et al.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Physical Sciences, University of Queensland, Brisbane, Australia.
    Kheruntsyan, K. V.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Physical Sciences, University of Queensland, Brisbane, Australia.
    Atom-atom correlations and relative number squeezing in dissociation of spatially inhomogeneous molecular condensates2008In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 78, no 1, article id 011602Article in journal (Refereed)
    Abstract [en]

    We study atom-atom correlations and relative number squeezing in the dissociation of a Bose-Einstein condensate (BEC) of molecular dimers made of either bosonic or fermionic atom pairs. Our treatment addresses the role of the spatial inhomogeneity of the molecular BEC on the strength of correlations in the short time limit. We obtain explicit analytic results for the density-density correlation functions in momentum space, and show that the correlation widths and the degree of relative number squeezing are determined merely by the shape of the molecular condensate.

  • 31.
    Ögren, Magnus
    et al.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Physical Sciences, University of Queensland, Brisbane, Australia.
    Kheruntsyan, K. V.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Physical Sciences, University of Queensland, Brisbane, Australia.
    Atom-atom correlations in colliding Bose-Einstein condensates2009In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 79, no 2, article id 021606Article in journal (Refereed)
    Abstract [en]

    We analyze atom-atom correlations in the s -wave scattering halo of two colliding condensates. By developing a simple perturbative approach, we obtain explicit analytic results for the collinear (CL) and back-to-back (BB) correlations corresponding to realistic density profiles of the colliding condensates with interactions. The results in the short-time limit are in agreement with the first-principles simulations using the positive- P representation and provide analytic insights into the experimental observations of Perrin [Phys. Rev. Lett. 99, 150405 (2007)]. For long collision durations, we predict that the BB correlation becomes broader than the CL correlation.

  • 32.
    Ögren, Magnus
    et al.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, University of Queensland, Brisbane, Australia.
    Kheruntsyan, K. V.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, University of Queensland, Brisbane, Australia.
    Role of spatial inhomogeneity in dissociation of trapped molecular condensates2010In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 82, no 1, article id 013641Article in journal (Refereed)
    Abstract [en]

    We theoretically analyze dissociation of a harmonically trapped Bose-Einstein condensate of molecular dimers and examine how the spatial inhomogeneity of the molecular condensate affects the conversion dynamics and the atom-atom pair correlations in the short-time limit. Both fermionic and bosonic statistics of the constituent atoms are considered. Using the undepleted molecular-field approximation, we obtain explicit analytic results for the asymptotic behavior of the second-order correlation functions and for the relative number squeezing between the dissociated atoms in one, two, and three spatial dimensions. Comparison with the numerical results shows that the analytic approach employed here captures the main underlying physics and provides useful insights into the dynamics of dissociation for conversion efficiencies up to 10%. The results show explicitly how the strength of atom-atom correlations and relative number squeezing degrade with the reduction of the size of the molecular condensate.

  • 33.
    Ögren, Magnus
    et al.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, University of Queensland, Brisbane, Australia.
    Kheruntsyan, K. V.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, University of Queensland, Brisbane, Australia.
    Corney, J. F.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, University of Queensland, Brisbane, Australia.
    First-principles quantum dynamics for fermions: Application to molecular dissociation2010In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 92, no 3, article id 36003Article in journal (Refereed)
    Abstract [en]

    We demonstrate that the quantum dynamics of a many-body Fermi-Bose system can be simulated using a Gaussian phase-space representation method. In particular, we consider the application of the mixed fermion-boson model to ultracold quantum gases and simulate the dynamics of dissociation of a Bose-Einstein condensate of bosonic dimers into pairs of fermionic atoms. We quantify deviations of atom-atom pair correlations from Wick's factorization scheme, and show that atom-molecule and molecule-molecule correlations grow with time, in clear departures from pairing mean-field theories. As a first-principles approach, the method provides benchmarking of approximate approaches and can be used to validate dynamical probes for characterizing strongly correlated phases of fermionic systems.

  • 34.
    Ögren, Magnus
    et al.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, University of Queensland, Brisbane, Australia.
    Kheruntsyan, K. V.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, University of Queensland, Brisbane, Australia.
    Corney, J. F.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Mathematics and Physics, University of Queensland, Brisbane, Australia.
    Stochastic simulations of fermionic dynamics with phase-space representations2011In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 182, no 9, p. 1999-2003Article in journal (Refereed)
    Abstract [en]

    A Gaussian operator basis provides a means to formulate phase-space simulations of the real- and imaginary-time evolution of quantum systems. Such simulations are guaranteed to be exact while the underlying distribution remains well-bounded, which defines a useful simulation time. We analyse the application of the Gaussian phase-space representation to the dynamics of the dissociation of an ultra-cold molecular gas. We show how the choice of mapping to stochastic differential equations can be used to tailor the stochastic behaviour, and thus the useful simulation time. In the phase-space approach, it is only averages of stochastic trajectories that have a direct physical meaning. Whether particular constants of the motion are satisfied by individual trajectories depends on the choice of mapping, as we show in examples.

  • 35.
    Ögren, Magnus
    et al.
    Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    Kärkkäinen, K.
    Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    Yu, Y.
    Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    Reimann, S. M.
    Mathematical Physics, Lund Institute of Technology, Lund, Sweden.
    On the spin asymmetry of ground states in trapped two-component Fermi gases with repulsive interactions2007In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 40, no 13, p. 2653-2660Article in journal (Refereed)
    Abstract [en]

    We examine the spin asymmetry of ground states for two-dimensional, harmonically trapped two-component gases of fermionic atoms at zero temperature with weakly repulsive short-range interactions. Our main result is that, in contrast to the three-dimensional case, in two dimensions a non-trivial spin-asymmetric phase can only be caused by the shell structure. A simple, qualitative description is given in terms of an approximate single-particle model, comparing well to the standard results of Hartree-Fock or direct diagonalization methods.

  • 36.
    Ögren, Magnus
    et al.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Physical Sciences, University of Queensland, Brisbane, Australia.
    Savage, C. M.
    Department of Quantum Science, ARC Centre of Excellence for Quantum-Atom Optics, Australian National University, Canberra, Australia.
    Kheruntsyan, K. V.
    ARC Centre of Excellence for Quantum-Atom Optics, School of Physical Sciences, University of Queensland, Brisbane, Australia.
    Directional effects due to quantum statistics in dissociation of elongated molecular condensates2009In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 79, no 4, article id 043624Article in journal (Refereed)
    Abstract [en]

    Ultracold clouds of dimeric molecules can dissociate into quantum mechanically correlated constituent atoms that are either both bosons or both fermions. We theoretically model the dissociation of two-dimensional anisotropic molecular condensates for which this difference manifests as complementary geometric structures of the dissociated atoms. Atomic bosons are preferentially emitted along the long axis of the molecular condensate, while atomic fermions are preferentially emitted along the short axis. This anisotropy potentially simplifies the measurement of correlations between the atoms through relative number squeezing.

  • 37.
    Ögren, Magnus
    et al.
    Dept. of Mathematics, Technical University of Denmark, Kongens Lyngby, Denmark.
    Sørensen, M. P.
    Dept. of Mathematics, Technical University of Denmark, Kongens Lyngby, Denmark.
    Pedersen, N. F.
    Dept. of Mathematics, Technical University of Denmark, Kongens Lyngby, Denmark.
    Self-consistent Ginzburg-Landau theory for transport currents in superconductors2012In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 479, p. 157-159Article in journal (Refereed)
    Abstract [en]

    We elaborate on boundary conditions for Ginzburg-Landau (GL) theory in the case of external currents. We implement a self-consistent theory within the finite element method (FEM) and present numerical results for a two-dimensional rectangular geometry. We emphasize that our approach can in principle also be used for general geometries in three-dimensional superconductors. © 2012 Elsevier B.V. All rights reserved.

  • 38.
    Ögren, Magnus
    et al.
    Division of Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Yu, Y.
    Division of Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Åberg, S.
    Division of Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Reimann, S. M.
    Division of Mathematical Physics, LTH, Lund University, Lund, Sweden.
    Brack, M.
    Institut für Theoretische Physik, Universität Regensburg, Regensburg, Germany.
    Super-shell structure in harmonically trapped fermionic gases and its semi-classical interpretation2006In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, no T125, p. 37-40Article in journal (Refereed)
    Abstract [en]

    It was recently shown in self-consistent Hartree–Fock calculations that a harmonically trapped dilute gas of fermionic atoms with a repulsive two-body interaction exhibits a pronounced super-shell structure: the shell fillings due to the spherical harmonic trapping potential are modulated by a beat mode. This changes the 'magic numbers' occurring between the beat nodes by half a period. The length and amplitude of the beating mode depends on the strength of the interaction. We give a qualitative interpretation of the beat structure in terms of a semi-classical trace formula that uniformly describes the symmetry breaking U(3) → SO(3) in a three-dimensional harmonic oscillator potential perturbed by an anharmonic term ∝ r 4 with arbitrary strength. We show that at low Fermi energies (or particle numbers), the beating gross-shell structure of this system is dominated solely by the twofold degenerate circular and (diametrically) pendulating orbits.

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