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  • 1.
    Andersson, Ove
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Relaxation time of water's high-density amorphous ice phase2005Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Physical Review Letters, Vol. 95, nr 20, s. 205503-Artikel i tidskrift (Refereegranskat)
  • 2.
    Bamford, R. A.
    et al.
    RAL Space, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom.
    Kellett, B.
    RAL Space, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom.
    Bradford, W. J.
    RAL Space, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom.
    Norberg, Carol
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Box 812, SE-981 28 Kiruna, Sweden.
    Thornton, A.
    York Plasma Institute, Department of Physics, University of York, Heslington, York, YO10 5DD, United Kingdom.
    Gibson, K. J.
    York Plasma Institute, Department of Physics, University of York, Heslington, York, YO10 5DD, United Kingdom.
    Crawford, I. A.
    Department of Earth and Planetary Sciences, Birkbeck College, London, United Kingdom.
    Silva, L.
    Instituto Superior Técnico, 1049-00, Lisboa, Portugal.
    Gargate, L.
    Instituto Superior Técnico, 1049-00, Lisboa, Portugal.
    Bingham, Ruth
    University of Strathclyde, Glasgow, Scotland, United Kingdom and Central Laser Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom.
    Minimagnetospheres above the lunar surface and the formation of lunar swirls2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, nr 8, s. 081101-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper we will present the in-situ satellite data, theory and laboratory validation that show how small scale collisionless shocks and mini-magnetospheres can form on the electron inertial scale length. The resulting retardation and deflection of the solar wind ions could be responsible for the unusual "lunar swirl" patterns seen on the surface of the Moon.

  • 3.
    Bergkvist, Sara
    et al.
    Department of Theoretical Physics, Royal Institute of Technology, AlbaNova, SE-106 91 Stockholm, Sweden.
    Rosengren, Anders
    Department of Theoretical Physics, Royal Institute of Technology, AlbaNova, SE-106 91 Stockholm, Sweden.
    Saers, Robert
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Lundh, Emil
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Rehn, Magnus
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Kastberg, Anders
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Transition from a Two-Dimensional Superfluid to a One-Dimensional Mott Insulator2007Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 99, nr 11, s. 110401-Artikel i tidskrift (Refereegranskat)
    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.

  • 4.
    Brodin, Gert
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Eliasson, Bengt
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Shukla, Padma Kant
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Quantum-electrodynamical photon splitting in magnetized Nonlinear pair plasmas2007Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 98, nr 12, s. 5001-4 sidorArtikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present for the first time the nonlinear dynamics of quantum electrodynamic (QED) photon splitting in a strongly magnetized electron-positron (pair) plasma. By using a QED corrected Maxwell equation, we derive a set of equations that exhibit nonlinear couplings between electromagnetic (EM) waves due to nonlinear plasma currents and QED polarization and magnetization effects. Numerical analyses of our coupled nonlinear EM wave equations reveal the possibility of a more efficient decay channel, as well as new features of energy exchange among the three EM modes that are nonlinearly interacting in magnetized pair plasmas. Possible applications of our investigation to astrophysical settings, such as magnetars, are pointed out.

  • 5.
    Brodin, Gert
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Manfredi, Giovanni
    Quantum plasma effects in the classical regime2008Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 100, nr 17, s. 5001-4 sidorArtikel i tidskrift (Refereegranskat)
    Abstract [en]

    For quantum effects to be significant in plasmas it is often assumed that the temperature over density ratio must be small. In this paper we challenge this assumption by considering the contribution to the dynamics from the electron spin properties. As a starting point we consider a multicomponent plasma model, where electrons with spin-up and spin-down are regarded as different fluids. By studying the propagation of Alfvén wave solitons we demonstrate that quantum effects can survive in a relatively high-temperature plasma. The consequences of our results are discussed.

  • 6.
    Brodin, Gert
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Zamanian, Jens
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Ericsson, Åsa
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Mana, Piero L
    Effects of the g factor in semiclassical kinetic plasma theory2008Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 101, nr 24, s. 245002-245006Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A kinetic theory for spin plasmas is put forward, generalizing those of previous authors. In the model, the ordinary phase space is extended to include the spin degrees of freedom. Together with Maxwell's equations, the system is shown to be energy conserving. Analyzing the linear properties, it is found that new types of wave-particle resonances are possible that depend directly on the anomalous magnetic moment of the electron. As a result, new wave modes, not present in the absence of spin, appear. The implications of our results are discussed.

  • 7.
    Brodin, Gert
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Misra, Amar P
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Spin contribution to the ponderomotive force in a plasma2010Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, nr 10, s. 105004-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The concept of a ponderomotive force due to the intrinsic spin of electrons is developed. An expression containing both the classical as well as the spin-induced ponderomotive force is derived. The results are used to demonstrate that an electromagnetic pulse can induce a spin-polarized plasma. Furthermore, it is shown that, for certain parameters, the nonlinear backreaction on the electromagnetic pulse from the spin magnetization current can be larger than that from the classical free current. Suitable parameter values for a direct test of this effect are presented.

  • 8. Buck, A.
    et al.
    Wenz, J.
    Xu, Jiancai
    Khrennikov, K.
    Schmid, K.
    Heigoldt, M.
    Mikhailova, J. M.
    Geissler, M.
    Shen, B.
    Krausz, F.
    Karsch, S.
    Veisz, László
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Shock-Front Injector for High-Quality Laser-Plasma Acceleration2013Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, nr 18, artikel-id 185006Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report the generation of stable and tunable electron bunches with very low absolute energy spread (ΔE≈5  MeV) accelerated in laser wakefields via injection and trapping at a sharp downward density jump produced by a shock front in a supersonic gas flow. The peak of the highly stable and reproducible electron energy spectrum was tuned over more than 1 order of magnitude, containing a charge of 1–100 pC and a charge per energy interval of more than 10  pC/MeV. Laser-plasma electron acceleration with Ti:sapphire lasers using this novel injection mechanism provides high-quality electron bunches tailored for applications.

  • 9.
    Bychkov, Vitaly
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Matyba, Piotr
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Akkerman, Vyacheslav
    Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544-5263, USA.
    Modestov, Mikhail
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Valiev, Damir
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Law, Chung K.
    Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544-5263, USA.
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Edman, Ludvig
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Speedup of doping fronts in organic semiconductors through plasma instability2011Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, nr 1, s. 016103-016107Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The dynamics of doping transformation fronts in organic semiconductor plasma is studied for application in light-emitting electrochemical cells. We show that new fundamental effects of the plasma dynamics can significantly improve the device performance. We obtain an electrodynamic instability, which distorts the doping fronts and increases the transformation rate considerably. We explain the physical mechanism of the instability, develop theory, provide experimental evidence, perform numerical simulations, and demonstrate how the instability strength may be amplified technologically. The electrodynamic plasma instability obtained also shows interesting similarity to the hydrodynamic Darrieus-Landau instability in combustion, laser ablation, and astrophysics.

  • 10.
    Bychkov, Vitaly
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Valiev, Damir
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Eriksson, Lars-Erik
    Chalmers, Dept Appl Mech, S-41296 Gothenburg, Sweden.
    Physical mechanism of ultrafast flame acceleration2008Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 101, nr 16, s. 164501-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We explain the physical mechanism of ultra-fast flame accelerationin obstructed channels used in modern experiments on detonationtriggering.  It is demonstrated that delayed burning between theobstacles creates a powerful jet-flow, driving the acceleration.This mechanism is much stronger than the classical Shelkinscenario of flame acceleration due to non-slip at the channelwalls.  The mechanism under study isindependent of the Reynolds number, with turbulence playing only asupplementary role. The flame front accelerates exponentially; theanalytical formula for the growth rate is obtained. The theory isvalidated by extensive direct numerical simulations and comparisonto previous experiments.

  • 11. Chou, Shao-wei
    et al.
    Xu, J.
    Khrennikov, K.
    Cardenas, Daniel E.
    Wenz, J.
    Heigoldt, M.
    Hofmann, L.
    Veisz, Laszlo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Max-Planck Institut für Quantenoptik, 85748 Garching, Germany.
    Karsch, S.
    Collective Deceleration of Laser-Driven Electron Bunches2016Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 117, nr 14, artikel-id 144801Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Few-fs electron bunches from laser wakefield acceleration (LWFA) can efficiently drive plasma wakefields (PWFs), as shown by their propagation through underdense plasma in two experiments. A strong and density-insensitive deceleration of the bunches has been observed in 2 mm of 1018 cm−3 density plasma with 5.1 GV=m average gradient, which is attributed to a self-driven PWF. This observation implies that the physics of PWFs, usually relying on large-scale rf accelerators as drivers, can be studied by tabletop LWFA electron sources.

  • 12. Eliasson , B.
    et al.
    Shukla, P.K.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Production of nonisothermal electrons and langmuir waves because of colliding ion holes and trapping of plasmons in an ion hole2004Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 92, s. 95006-Artikel i tidskrift (Refereegranskat)
  • 13.
    Eliasson, Bengt
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Shukla, Padma Kant
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Numerical investigation of the instability and nonlinear evolution of narrow-band directional ocean waves2010Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, nr 1, s. 014501-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The instability and nonlinear evolution of directional ocean waves is investigated numerically by means of simulations of the governing kinetic equation for narrow-band surface waves. Our simulation results reveal the onset of the modulational instability for long-crested wave trains, which agrees well with recent large-scale experiments in wave basins, where it was found that narrower directional spectra lead to self-focusing of ocean waves and an enhanced probability of extreme events. We find that the modulational instability is nonlinearly saturated by a broadening of the wave spectrum, which leads to the stabilization of the water-wave system. Applications of our results to other fields of physics, such as nonlinear optics and plasma physics, are discussed.

  • 14.
    Fallani, Leonardo
    et al.
    LENS, Dipartimento di Fisica, and INFM, Università di Firenze, via Nello Carrara 1, I-50019 Sesto Fiorentino (FI), Italy.
    De Sarlo, Luigi
    LENS, Dipartimento di Fisica, and INFM, Università di Firenze, via Nello Carrara 1, I-50019 Sesto Fiorentino (FI), Italy.
    Lye, Jessica E.
    LENS, Dipartimento di Fisica, and INFM, Università di Firenze, via Nello Carrara 1, I-50019 Sesto Fiorentino (FI), Italy.
    Modugno, Michele
    LENS, Dipartimento di Fisica, and INFM, Università di Firenze, via Nello Carrara 1, I-50019 Sesto Fiorentino (FI), Italy, BEC-INFM Center, Università di Trento, I-38050 Povo (TN), Italy .
    Saers, Robert
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Fort, Chiara
    LENS, Dipartimento di Fisica, and INFM, Università di Firenze, via Nello Carrara 1, I-50019 Sesto Fiorentino (FI), Italy.
    Inguscio, Massimo
    LENS, Dipartimento di Fisica, and INFM, Università di Firenze, via Nello Carrara 1, I-50019 Sesto Fiorentino (FI), Italy.
    Observation of Dynamical Instability for a Bose-Einstein Condensate in a Moving 1D Optical Lattice2004Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 93, nr 14, s. 140406-Artikel i tidskrift (Refereegranskat)
  • 15. Glampedakis, K.
    et al.
    Jones, D. I.
    Samuelsson, Lars
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gravitational Waves from Color-Magnetic "Mountains" in Neutron Stars2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, nr 8, s. 081103-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Neutron stars may harbor the true ground state of matter in the form of strange quark matter. If present, this type of matter is expected to be a color superconductor, a consequence of quark pairing with respect to the color and flavor degrees of freedom. The stellar magnetic field threading the quark core becomes a color-magnetic admixture and, in the event that superconductivity is of type II, leads to the formation of color-magnetic vortices. In this Letter, we show that the volume-averaged color-magnetic vortex tension force should naturally lead to a significant degree of nonaxisymmetry in systems such as radio pulsars. We show that gravitational radiation from such color-magnetic "mountains" in young pulsars, such as the Crab and Vela, could be observable by the future Einstein Telescope, thus, becoming a probe of paired quark matter in neutron stars. The detectability threshold can be pushed up toward the sensitivity level of Advanced LIGO if we invoke an interior magnetic field about a factor ten stronger than the surface polar field.

  • 16. Gonoskov, Arkady
    et al.
    Bashinov, A
    Gonoskov, Ivan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Harvey, C
    Ilderton, A
    Kim, A
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Mourou, G
    Sergeev, M
    Anomalous radiative trapping in laser fields of extreme intensity2014Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 113, s. 014801-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate that charged particles in a suciently intense standing wave are compressed toward, and oscillate synchronously at, the antinodes of the electric eld. We call this unusualbehaviour `anomalous radiative trapping' (ART). We show using dipole pulses, which oer a pathto increased laser intensity, that ART opens up new possibilities for the generation of radiationand particle beams, both of which are high-energy, directed and collimated. ART also provides a mechanism for particle control in high-intensity quantum-electrodynamics experiments.

  • 17.
    Gonoskov, Arkady
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gonoskov, Ivan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Harvey, Christopher
    Ilderton, Antony
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Kim, Arkady
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Mourou, Gérard
    Sergeev, Alexander
    Probing nonperturbative QED with optimally focused laser pulses2013Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, nr 6, s. 060404-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study nonperturbative pair production in intense, focused laser fields called e-dipole pulses. We address the conditions required, such as the quality of the vacuum, for reaching high intensities without initiating beam-depleting cascades, the number of pairs which can be created, and experimental detection of the created pairs. We find that e-dipole pulses offer an optimal method of investigating nonperturbative QED.

  • 18.
    Gonoskov, Arkady
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Russian Acad Sci, Inst Appl Phys, Nizhnii Novgorod 603950, Russia.
    Korzhimanov, A. V.
    Russian Acad Sci, Inst Appl Phys, Nizhnii Novgorod 603950, Russia.
    Eremin, V. I.
    Russian Acad Sci, Inst Appl Phys, Nizhnii Novgorod 603950, Russia.
    Kim, A. V.
    Russian Acad Sci, Inst Appl Phys, Nizhnii Novgorod 603950, Russia.
    Sergeev, A. M.
    Russian Acad Sci, Inst Appl Phys, Nizhnii Novgorod 603950, Russia.
    Multicascade proton acceleration by a superintense laser pulse in the regime of relativistically induced slab transparency2009Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 102, nr 18, s. 184801-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The regime of multicascade proton acceleration during the interaction of a 1021–1022 W=cm2 laserpulse with a structured target is proposed. The regime is based on the electron charge displacement under the action of laser ponderomotive force and on the effect of relativistically induced slab transparency which allows realization of the idea of multicascade acceleration. It is shown that a target comprising several thin foils properly spaced apart can optimize the acceleration process and give at the output aquasi-monoenergetic beam of protons with energies up to hundreds of MeV with an energy spread of just a few percent.

  • 19. Guillaume, E.
    et al.
    Döpp, A.
    Thaury, C.
    Ta Phuoc, K.
    Lifschitz, A.
    Grittani, G.
    Goddet, J.-P.
    Tafzi, A.
    Chou, S.-W.
    Veisz, László
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Malka, V.
    Electron Rephasing in a Laser-Wakefield Accelerator2015Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, nr 15, artikel-id 155002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An important limit for energy gain in laser-plasma wakefield accelerators is the dephasing length, after which the electron beam reaches the decelerating region of the wakefield and starts to decelerate. Here, we propose to manipulate the phase of the electron beam in the wakefield, in order to bring the beam back into the accelerating region, hence increasing the final beam energy. This rephasing is operated by placing an upward density step in the beam path. In a first experiment, we demonstrate the principle of this technique using a large energy spread electron beam. Then, we show that it can be used to increase the energy of monoenergetic electron beams by more than 50%.

  • 20.
    Harvey, Chris
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Heinzl, Thomas
    Ilderton, Anton
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Intensity-dependent electron mass shift in a laser field: existence, universality, and detection2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, nr 10, s. 100402-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The electron mass shift in a laser field has long remained an elusive concept. We show that the mass shift can exist in pulses but that it is neither unique nor universal: it can be reduced by pulse shaping. We show also that the detection of mass shift effects in laser-particle scattering experiments is feasible with current technology, even allowing for the transverse structure of realistic beams.

  • 21.
    Ilderton, Anton
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Trident pair production in strong laser pulses2011Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 106, nr 2, s. 020404-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We calculate the trident pair production amplitude in a strong laser background.We allow for finite pulsedurations, while still treating the laser fields nonperturbatively in strong-field QED. Our approach revealsexplicitly the individual contributions of the one-step and two-step processes.We also expose the role gaugeinvariance plays in the amplitudes and discuss the relation between our results and the optical theorem.

  • 22.
    Jukimenko, Olexy
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Dion, Claude
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden.
    Bychkov, Vitaly
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Multidimensional instability and dynamics of spin-avalanches in crystals of nanomagnets2014Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 113, nr 21, artikel-id 217206Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We obtain a fundamental instability of the magnetization-switching fronts in superparamagnetic and ferromagnetic materials such as crystals of nanomagnets, ferromagnetic nanowires, and systems of quantum dots with large spin. We develop the instability theory for both linear and nonlinear stages. By using numerical simulations we investigate the instability properties focusing on spin avalanches in crystals of nanomagnets. The instability distorts spontaneously the fronts and leads to a complex multidimensional front dynamics. We show that the instability has a universal physical nature, with a deep relationship to a wide variety of physical systems, such as the Darrieus-Landau instability of deflagration fronts in combustion, inertial confinement fusion, and thermonuclear supernovae, and the instability of doping fronts in organic semiconductors.

  • 23. Khrennikov, K.
    et al.
    Wenz, J.
    Buck, A.
    Xu, Jiancai
    Heigoldt, M.
    Veisz, Laszlo
    MPI für Quantenoptik, Garching, Germany.
    Karsch, S.
    Tunable All-Optical Quasimonochromatic Thomson X-Ray Sourcein the Nonlinear Regime2015Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, nr 19, artikel-id 195003Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present an all-laser-driven, energy-tunable, and quasimonochromatic x-ray source based on Thomson scattering from laser-wakefield-accelerated electrons. One part of the laser beam was used to drive a few-fs bunch of quasimonoenergetic electrons, while the remainder was backscattered off the bunch at weakly relativistic intensity. When the electron energy was tuned from 17–50 MeV, narrow x-ray spectra peaking at 5–42 keV were recorded with high resolution, revealing nonlinear features. We present a large set of measurements showing the stability and practicality of our source.

  • 24.
    Kobyakov, Dmitry
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Radiophysics Department, Nizhny Novgorod State University, Novgorod, Russia and The Niels Bohr International Academy, The Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.
    Pethick, Christopher J.
    The Niels Bohr International Academy, The Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark and NORDITA, KTH Royal Institute of Technology and Stockholm University, Stockholm, Sweden.
    Towards a metallurgy of neutron star crusts2014Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, nr 11, s. 112504-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the standard picture of the crust of a neutron star, matter there is simple: a body-centered-cubic (bcc) lattice of nuclei immersed in an essentially uniform electron gas. We show that at densities above that for neutron drip (~4×1011 g cm-3 or roughly one thousandth of nuclear matter density), the interstitial neutrons give rise to an attractive interaction between nuclei that renders the lattice unstable. We argue that the likely equilibrium structure is similar to that in displacive ferroelectric materials such as BaTiO3. As a consequence, properties of matter in the inner crust are expected to be much richer than previously appreciated and we mention possible consequences for observable neutron star properties.

  • 25.
    Korzhimanov, Artem V
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Efimenko, ES
    Golubev, SV
    Kim, AV
    Generating high-energy highly charged ion beams from petawatt-class laser interactions with compound targets2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, nr 24, s. 245008-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A new method of generation of high-energy highly charged ion beams is proposed. The method is based on the interaction of petawatt circularly polarized laser pulses with high-Z compound targets consisting of two species of different charge-to-mass ratio. It is shown that highly charged ions produced by field ionization can be accelerated up to tens of MeV/u with ion (actually with Z <= 25) beam parameters like density and total charge inaccessible in conventional accelerators. A possibility of further ionization of the accelerated ion bunches in foil is also discussed. DOI: 10.1103/PhysRevLett.109.245008

  • 26.
    Lee, Sang Hoon
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Holme, Petter
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Exploring maps with greedy navigators2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, nr 12, s. 8701-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    During the last decade of network research focusing on structural and dynamical properties of networks, the role of network users has been more or less underestimated from the bird’s-eye view of global perspective. In this era of global positioning system equipped smartphones, however, a user’s ability to access local geometric information and find efficient pathways on networks plays a crucial role, rather than the globally optimal pathways. We present a simple greedy spatial navigation strategy as a probe to explore spatial networks. These greedy navigators use directional information in every move they take, without being trapped in a dead end based on their memory about previous routes. We suggest that the centralities measures have to be modified to incorporate the navigators’ behavior, and present the intriguing effect of navigators’ greediness where removing some edges may actually enhance the routing efficiency, which is reminiscent of Braess’s paradox. In addition, using samples of road structures in large cities around the world, it is shown that the navigability measure we define reflects unique structural properties, which are not easy to predict from other topological characteristics. In this respect, we believe that our routing scheme significantly moves the routing problem on networks one step closer to reality, incorporating the inevitable incompleteness of navigators’ information.

  • 27.
    Lee, Sungmin
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Holme, Petter
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Wu, Zhi-Xi
    Lanzhou University.
    Emergent hierarchical structures in multiadaptive games2011Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 106, nr 2, s. 028702-4Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate a game-theoretic model of a social system where both the rules of the game and the interaction structure are shaped by the behavior of the agents. We call this type of model, with several types of feedback couplings from the behavior of the agents to their environment, a multiadaptive game. Our model has a complex behavior with several regimes of different dynamic behavior accompanied by different network topological properties. Some of these regimes are characterized by heterogeneous, hierarchical interaction networks, where cooperation and network topology coemerge from the dynamics.

  • 28.
    Lizana, Ludvig
    et al.
    Niels Bohr Institute, Blegdamsvej 17, DK-2100, Copenhagen, Denmark.
    Rosvall, Martin
    Umeå universitet. Niels Bohr Institute, Blegdamsvej 17, DK-2100, Copenhagen, Denmark.
    Sneppen, Kim
    Niels Bohr Institute.
    Time walkers and spatial dynamics of aging information2010Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 104, nr 4, artikel-id 040603Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The distribution of information is essential for a living system’s ability to coordinate and adapt. Random walkers are often used to model this distribution process and, in doing so, one effectively assumes that information maintains its relevance over time. But the value of information in social and biological systems often decays and must continuously be updated. To capture the spatial dynamics of aging information, we introduce time walkers. A time walker moves like a random walker, but interacts with traces left by other walkers, some representing older information, some newer. The traces form a navigable information landscape which we visualize as a river network. We quantify the dynamical properties of time walkers, and the quality of the information left behind, on a two-dimensional lattice. We show that searching in this landscape is superior to random searching.

  • 29. Lomholt, Michael A.
    et al.
    Lizana, Ludvig
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. NYU, Dept Phys, New York, NY 10003 USA and NYU, Ctr Soft Matter Res, New York, NY 10003 USA.
    Metzler, Ralf
    Ambjörnsson, Tobias
    Microscopic Origin of the Logarithmic Time Evolution of Aging Processes in Complex Systems2013Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, nr 20, s. 208301-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    There exists compelling experimental evidence in numerous systems for logarithmically slow time evolution, yet its full theoretical understanding remains elusive. We here introduce and study a generic transition process in complex systems, based on nonrenewal, aging waiting times. Each state n of the system follows a local clock initiated at t = 0. The random time tau between clock ticks follows the waiting time density psi (tau). Transitions between states occur only at local clock ticks and are hence triggered by the local forward waiting time, rather than by psi (tau). For power-law forms psi (tau) similar or equal to tau(-1-alpha) (0 < alpha < 1) we obtain a logarithmic time evolution of the state number < n(t)> similar or equal to log(t/t(0)), while for alpha > 2 the process becomes normal in the sense that < n(t)> similar or equal to t. In the intermediate range 1 < alpha < 2 we find the power-law growth < n(t)> similar or equal to t(alpha-1). Our model provides a universal description for transition dynamics between aging and nonaging states.

  • 30.
    Lomholt, Michael
    et al.
    MEMPHYS, Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Odense M, Denmark.
    Lizana, Ludvig
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Department of Physics and Center for Soft Matter Research, New York University, 4 Washington Place, New York, New York 10003, USA.
    Metzler, Ralf
    Department of Physics, Tampere University of Technology, Tampere, Finland and Institute for Physics and Astronomy, University of Potsdam, Potsdam-Golm, Germany.
    Ambjörnsson, Tobias
    Lund University.
    Microscopic Origin of the Logarithmic Time Evolution of Aging Processes in Complex Systems2013Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, nr 20, s. 208301-208306Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    There exists compelling experimental evidence in numerous systems for logarithmically slow time evolution, yet its full theoretical understanding remains elusive. We here introduce and study a generic transition process in complex systems, based on nonrenewal, aging waiting times. Each state n of the system follows a local clock initiated at t=0. The random time τ between clock ticks follows the waiting time density ψ(τ). Transitions between states occur only at local clock ticks and are hence triggered by the local forward waiting time, rather than by ψ(τ). For power-law formsψ(τ)≃τ-1-α (0<α<1) we obtain a logarithmic time evolution of the state number ⟨n(t)⟩≃log⁡(t/t0), while for α>2 the process becomes normal in the sense that ⟨n(t)⟩≃t. In the intermediate range 1<α<2 we find the power-law growth ⟨n(t)⟩≃tα-1. Our model provides a universal description for transition dynamics between aging and nonaging states.

  • 31.
    Lundström, E.
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Lundin, Joakim
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Bingham, R.
    Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, Oxfordshire, United Kingdom .
    Collier, J.
    Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, Oxfordshire, United Kingdom .
    Mendonça, J.T.
    Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, Oxfordshire, United Kingdom .
    Norreys, P.
    Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, Oxfordshire, United Kingdom .
    Using high-power lasers for detection of elastic photon-photon scattering.2006Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 96, nr 8, s. 083602-083605Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The properties of four-wave interaction via the nonlinear quantum vacuum is investigated. The effect of the quantum vacuum is to generate photons with new frequencies and wave vectors, due to elastic photon-photon scattering. An expression for the number of generated photons is derived, and using state-of-the-art laser data it is found that the number of photons can reach detectable levels. In particular, the prospect of using the high-repetition Astra Gemini system at the Rutherford Appleton Laboratory is discussed. The problem of noise sources is reviewed, and it is found that the noise level can be reduced well below the signal level. Thus, detection of elastic photon-photon scattering may for the first time be achieved.

  • 32. Mamun, A. A.
    et al.
    Shukla , P.K.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Morfill , G.E.
    Theory of Mach cones in magnetized dusty plasmas with strongly correlated charged dust grains2004Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 92, s. 95005-Artikel i tidskrift (Refereegranskat)
  • 33.
    Marklund, Mattias
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Dynamics of spin-1/2 quantum plasmas2007Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 98, nr 2, s. 025001-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The fully nonlinear governing equations for spin-1/2 quantum plasmas are presented. Starting from the Pauli equation, the relevant plasma equations are derived, and it is shown that nontrivial quantum spin couplings arise, enabling studies of the combined collective and spin dynamics. The linear response of the quantum plasma in an electron-ion system is obtained and analyzed. Applications of the theory to solid state and astrophysical systems as well as dusty plasmas are pointed out.

  • 34. Marschall, Theodore
    et al.
    Keta, Yann-Edwin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Département de Physique, École Normale Supérieure de Lyon, Lyon, France; Déepartement de Physique, Université Claude Bernard Lyon 1, Villeurbanne, France.
    Olsson, Peter
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Teitel, S.
    Orientational Ordering in Athermally Sheared, Aspherical, Frictionless Particles2019Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, nr 18, artikel-id 188002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We numerically simulate the uniform athermal shearing of bidisperse, frictionless, two-dimensional spherocylinders and three-dimensional prolate ellipsoids. We focus on the orientational ordering of particles as an asphericity parameter α → 0 and particles approach spherical. We find that the nematic order parameter S2 is nonmonotonic in the packing fraction ϕ and that, as α → 0, S2 stays finite at jamming and above. The approach to spherical particles thus appears to be singular. We also find that sheared particles continue to rotate above jamming and that particle contacts preferentially lie along the narrowest width of the particles, even as α → 0.

  • 35.
    Modestov, Mikhail
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Bychkov, Vitaly
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Ultrafast Spin Avalanches in Crystals of Nanomagnets in Terms of Magnetic Detonation2011Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, nr 20, s. 207208-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent experiments [W. Decelle et al., Phys. Rev. Lett. 102 027203 (2009)] have discovered ultrafast propagation of spin avalanches in crystals of nanomagnets, which is 3 orders of magnitude faster than the traditionally studied magnetic deflagration. The new regime has been hypothetically identified as magnetic detonation. Here we demonstrate unequivocally the possibility of magnetic detonation in the crystals, as a front consisting of a leading shock and a zone of Zeeman energy release. We study the key features of the process and find that the magnetic detonation speed only slightly exceeds the sound speed in agreement with the experimental observations. For combustion science, our results provide a unique physical example of extremely weak detonation.

  • 36.
    Olsson, Peter
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Dimensionality and Viscosity Exponent in Shear-driven Jamming2019Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, nr 10, artikel-id 108003Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Collections of bidisperse frictionless particles at zero temperature in three dimensions are simulated with a shear-driven dynamics with the aim to compare with the behavior in two dimensions. Contrary to the prevailing picture, and in contrast to results from isotropic jamming from compression or quench, we find that the critical exponents in three dimensions are different from those in two dimensions and conclude that shear-driven jamming in two and three dimensions belong to different universality classes.

  • 37.
    Olsson, Peter
    et al.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Teitel, S.
    Comment on "effects of point defects on the phase diagram of vortex states in high-Tc superconductors in the Bc axis"2005Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Physical Review Letters, Vol. 94, nr 21, s. 219703-Artikel i tidskrift (Refereegranskat)
  • 38.
    Olsson, Peter
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Teitel, S.
    Herschel-bulkley shearing rheology near the athermal jamming transition2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, nr 10, s. 108001-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider the rheology of soft-core frictionless disks in two dimensions in the neighborhood of the athermal jamming transition. From numerical simulations of bidisperse, overdamped particles, we argue that the divergence of the viscosity below jamming is characteristic of the hard-core limit, independent of the particular soft-core interaction. We develop a mapping from soft-core to hard-core particles that recovers all the critical behavior found in earlier scaling analyses. Using this mapping we derive a relation that gives the exponent of the nonlinear Herschel-Bulkley rheology above jamming in terms of the exponent of the diverging viscosity below jamming.

  • 39.
    Petchenko, Arkady
    et al.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Bychkov, Vitaly
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Akkerman, V'yacheslav
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Eriksson, Lars-Erik
    Violent folding of a flame front in a flame-acoustic resonance2006Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 97, nr 16, s. 164501-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The first direct numerical simulations of violent flame folding because of the flame-acoustic resonance are performed. Flame propagates in a tube from an open end to a closed one. Acoustic amplitude becomes extremely large when the acoustic mode between the flame and the closed tube end comes in resonance with intrinsic flame oscillations. The acoustic oscillations produce an effective acceleration field at the flame front leading to a strong Rayleigh-Taylor instability during every second half period of the oscillations. The Rayleigh-Taylor instability makes the flame front strongly corrugated with elongated jets of heavy fuel mixture penetrating the burnt gas and even with pockets of unburned matter separated from the flame front

  • 40.
    Sjölund, Peder
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Petra, Stefan J.H.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Dion, Claude
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Jonsell, Svante
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Nylén, Mats
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Sanchez-Palencia, L.
    Laboratoire Charles Fabry de l’Institut d’Optique, CNRS and Université Paris-Sud XI, France.
    Kastberg, Anders
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Demonstration of a controllable three-dimensional Brownian motor in symmetric potentials2006Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 96, nr 19, s. 190602-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate a Brownian motor, based on cold atoms in optical lattices, where isotropic random fluctuations are rectified in order to induce controlled atomic motion in arbitrary directions. In contrast to earlier demonstrations of ratchet effects, our Brownian motor operates in potentials that are spatially and temporally symmetric, but where spatiotemporal symmetry is broken by a phase shift between the potentials and asymmetric transfer rates between them. The Brownian motor is demonstrated in three dimensions and the noise-induced drift is controllable in our system.

  • 41. Sorasio, G.
    et al.
    Shukla, Padma K.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Stenflo, Lennart
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Comment on "Drift-Wave Instability Excited by Field-Aligned Ion Flow Velocity Shear in the Absence of Electron Current" (multiple letters)2004Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 92, nr 6, s. 069501-Artikel i tidskrift (Refereegranskat)
  • 42.
    Stasiewicz, Kristof
    et al.
    Swedish Institute of Space Physics Uppsala, Sweden.
    Ekeberg, Jonas
    Swedish Institute of Space Physics Uppsala, Sweden.
    [Reply to comment on heating of the solar corona by dissipative Alfvén solitons]2007Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 99, nr 8, s. 089502(1)-Artikel i tidskrift (Refereegranskat)
  • 43.
    Sundqvist, Bertil
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Resistivity saturation in fcc La under high pressure1992Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 69, nr 18, s. 2693-2696Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Data are presented for the electrical resistivity ρ of fcc La as a function of T and p in the range 70 to 700 K and 0 to 1.25 GPa. The data are analyzed using the phenomenological ‘‘parallel resistivity’’ formula for resistivity saturation, which describes very well the T dependences of both ρ and its pressure coefficient. The pressure coefficients of the electron-phonon interaction parameter and Tc are calculated from the p dependence of the electron-phonon component, and the p dependence of the parallel saturation resistivity is obtained from band-structure calculations using a simple Bloch-Boltzmann model. In both cases excellent agreement with experiment is found.

  • 44.
    Sundqvist, Bertil
    et al.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Andersson, Ove
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Soldatov, Alexander
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Comment on “Pressure-Induced Structural Metastability in Crystalline C601995Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 75, nr 15, s. 2906-Artikel i tidskrift (Refereegranskat)
  • 45.
    Trusina, Ala
    et al.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Maslow, Sergei
    Department of Physics, Brookhaven National Laboratory, Upton, New York 11973, USA.
    Minnhagen, Petter
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Sneppen, Kim
    NORDITA, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark.
    Hierarchy measures in complex networks2004Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 92, nr 17, s. 178702-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using each node’s degree as a proxy for its importance, the topological hierarchy of a complex network is introduced and quantified. We propose a simple dynamical process used to construct networks which are either maximally or minimally hierarchical. Comparison with these extremal cases as well as with random scale-free networks allows us to better understand hierarchical versus modular features in several real-life complex networks. For random scale-free topologies the extent of topological hierarchy is shown to smoothly decline with y, the exponent of a degree distribution, reaching its highest possible value for y ≤,2 and quickly approaching zero for y > 3.

  • 46. Trusina, Ala
    et al.
    Rosvall, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Sneppen, Kim
    Communication boundaries in networks2005Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 94, nr 23, s. 238701-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate and quantify the interplay between topology and the ability to send specific signals in complex networks. We find that in a majority of investigated real-world networks the ability to communicate is favored by the network topology at small distances, but disfavored at larger distances. We further suggest how the ability to locate specific nodes can be improved if information associated with the overall traffic in the network is available.

     

  • 47.
    Vågberg, Daniel
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Olsson, Peter
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    S., Teitel
    University of Rochester, Rochester NY 14627 USA.
    Dissipation and Rheology of Sheared Soft-Core Frictionless Disks Below Jamming2014Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, nr 20, artikel-id 208303Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We use numerical simulations to investigate the effect that different models of energy dissipation have on the rheology of soft-core frictionless disks, below jamming in two dimensions. We find that it is not necessarily the mass of the particles that determines whether a system has Bagnoldian or Newtonian rheology, but rather the presence or absence of large connected clusters of particles. We demonstrate the key role that tangential dissipation plays in the formation of such clusters and in several models find a transition from Bagnoldian to Newtonian rheology as the packing fraction. is varied. For each model, we show that appropriately scaled rheology curves approach a well defined limit as the mass of the particles decreases and collisions become strongly inelastic.

  • 48.
    Vågberg, Daniel
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Olsson, Peter
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    S., Teitel
    University of Rochester, Rochester NY 14627 USA.
    Universality of Jamming Criticality in Overdamped Shear-Driven Frictionless Disks2014Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 113, nr 14, artikel-id 148002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the criticality of the jamming transition for overdamped shear-driven frictionless disks in two dimensions for two different models of energy dissipation: (i) Durian’s bubble model with dissipation proportional to the velocity difference of particles in contact, and (ii) Durian’s “mean-field” approximation to (i), with dissipation due to the velocity difference between the particle and the average uniform shear flow velocity. By considering the finite-size behavior of pressure, the pressure analog of viscosity, and the macroscopic friction σ/p  , we argue that these two models share the same critical behavior.

  • 49. Yang, Xigui
    et al.
    Yao, Mingguang
    Wu, Xiangying
    Liu, Shijie
    Chen, Shuanglong
    Yang, Ke
    Liu, Ran
    Cui, Tian
    Sundqvist, Bertil
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. State Key Lab of Superhard Materials, Jilin University, Changchun, China.
    Liu, Bingbing
    Novel Superhard sp3 Carbon Allotrope from Cold-Compressed C70 Peapods2017Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 118, nr 24, artikel-id 245701Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Design and synthesis of new carbon allotropes have always been important topics in condensed matter physics and materials science. Here we report a new carbon allotrope, formed from cold-compressed C70 peapods, which most likely can be identified with a fully sp3-bonded monoclinic structure, here named V carbon, predicted from our simulation. The simulated x-ray diffraction pattern, near K-edge spectroscopy, and phonon spectrum agree well with our experimental data. Theoretical calculations reveal that V carbon has a Vickers hardness of 90 GPa and a bulk modulus ∼400  GPa, which well explains the "ring crack" left on the diamond anvils by the transformed phase in our experiments. The V carbon is thermodynamically stable over a wide pressure range up to 100 GPa, suggesting that once V carbon forms, it is stable and can be recovered to ambient conditions. A transition pathway from peapod to V carbon has also been suggested. These findings suggest a new strategy for creating new sp3-hybridized carbon structures by using fullerene@nanotubes carbon precursor containing odd-numbered rings in the structures.

  • 50.
    Zhu, Xiaomei
    et al.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Brändström, Erik
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Sundqvist, Bertil
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    Observation of the transverse force on moving vortices in YBCO films1997Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 78, nr 1, s. 122-125Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have designed a mechanical experiment to measure the total transverse force on moving vortices in a type II superconductor. The vortices are driven into motion by the vibration of a magnet above a superconducting YBCO film. The transverse force is measured by monitoring the motion of the superconductor in response. The advantage of this direct force measurement is that it is free from complications due to the vortex interaction. Our results are consistent with the Ao-Thouless theory.

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