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  • 1.
    Baek, Seung Ki
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Minnhagen, Petter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hierarchical renormalization-group study on the planar bond-percolation problem2011In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 83, no 5, p. 055601-Article in journal (Refereed)
    Abstract [en]

    For certain hierarchical structures, one can study the percolation problemusing the renormalization-group method in a very precise way.We show that the idea can be alsoapplied to two-dimensional planar lattices by regarding them as hierarchicalstructures. Either a lower bound or an exact critical probability can beobtained with this method and the correlation-length critical exponent isapproximately estimated as $\nu \approx 1$.

    Download full text (pdf)
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  • 2.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Virtual issue to honour Lennart Stenflo on his 75th birthday: foreword2015In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 90, no 6, p. 1-2, article id 060303Article in journal (Other academic)
  • 3.
    Brodin, Gert
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stenflo, L.
    A new decay channel for upper-hybrid waves2016In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 91, no 10, article id 104005Article in journal (Refereed)
    Abstract [en]

    We look here at a three-wave interaction process involving only electrostatic waves in an electron plasma with stationary ions. Special attention is given to the case with an upper-hybrid wave as a pump wave, where a new decay channel is pointed out. The corresponding growth rate is calculated.

  • 4.
    Brodin, Gert
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stenflo, Lennart
    Three-wave coupling coefficients for a magnetized plasma2012In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 85, no 3, p. 035504-Article in journal (Refereed)
    Abstract [en]

    The resonant interaction between three waves in a uniform magnetized plasma is reconsidered. Starting from previous kinetic expressions, which contain a general but too little used result, we are able to improve the formulae. This leads to an explicit expression for the three-wave coupling coefficient which applies for arbitrary wave propagation in a magnetized Vlasov plasma.

  • 5.
    Brodin, Gert
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stenflo, Lennart
    Linköping University, Sweden.
    Juul Rasmussen, Jens
    Focus issue to honour Hans L Pecseli on his 70th birthday2017In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 92, no 1, article id 010301Article in journal (Other academic)
  • 6.
    Brodin, Gert
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zamanian, Jens
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mendonca, J. T.
    The transition from the classical to the quantum regime in nonlinear Landau damping2015In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 90, no 6, article id 068020Article in journal (Other academic)
    Abstract [en]

    Starting from the Wigner–Moyal equation coupled to Poisson’s equation, a simplified set of equations describing nonlinear Landau damping of Langmuir waves is derived. This system is studied numerically, with a particular focus on the transition from the classical to the quantum regime. In the quantum regime several new features are found. This includes a quantum modified bounce frequency, and the discovery that bounce-like amplitude oscillations can take place even in the absence of trapped particles. The implications of our results are discussed.

  • 7. Eliasson, Bengt
    et al.
    Liu, Chuan S
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, D-44780 Bochum, Germany; Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK.
    Kumar, Naveen
    Dynamics of relativistic laser pulses in plasmas2006In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 73, no 6, p. 632-638Article in journal (Refereed)
    Abstract [en]

    The dynamics of intense laser pulses in plasmas are investigated both theoretically and numerically. The linear growth and nonlinear saturation of relativistic stimulated Raman scattering of plasmons are investigated by means of a nonlinear dispersion relation and via direct Vlasov simulations. We observe acceleration of electrons up to ultra-relativistic energies by a positive electrostatic potential that is created by intense short laser pulses.

  • 8.
    Grönlund, Andreas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sneppen, Kim
    Minnhagen, Petter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Correlations in networks associated to preferential growth2005In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 71, no 6, p. 680-Article in journal (Refereed)
  • 9. Mamun, A. A.
    et al.
    Shukla , P.K.
    Umeå University, Faculty of Science and Technology, Physics.
    Radiative Drift-Alfvén Wave Instability in a Nonuniform Dusty Magnetoplasma2004In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 69, p. 41-Article in journal (Refereed)
  • 10.
    Marklund, Mattias
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shukla, Padma K.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stenflo, Lennart
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundin, Joakim
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nonlinear propagation of partially coherent dispersive Alfvén waves.2006In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Physica Scripta, Vol. 74, no 3, p. 373-376Article in journal (Refereed)
    Abstract [en]

    The effects of partial coherence on the propagation of dispersive Alfvén waves in a magnetoplasma are investigated. In particular, nonlinear interactions between dispersive Alfvén waves and ion-acoustic perturbations are considered by means of a Wigner formalism. A set of governing equations consisting of a kinetic equation for dispersive Alfvén waves coupled nonlinearly to a ponderomotive force driven ion-acoustic wave equation is obtained. The governing nonlinear equations are used to derive a nonlinear dispersion relation that is appropriate for investigating the modulational instability of broadband Alfvén wavepackets. The spectral broadening of the Alfvén waves gives rise to new regimes for the growth rate of the modulational instability.

  • 11. Mendonca, J. T.
    et al.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Weibel instability in relativistic quantum plasmas2015In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 90, no 8, article id 088003Article in journal (Other academic)
    Abstract [en]

    Generation of quasi-static magnetic fields, due to the Weibel instability is studied in a relativistic quantum plasma. This instability is induced by a temperature anisotropy. The dispersion relation and growth rates for low frequency electromagnetic perturbations are derived using a wavekinetic equation which describes the evolution of the electron Wigner quasi-distribution. The influence of parallel kinetic effects is discussed in detail.

  • 12. Moodley, Mervlyn
    et al.
    Paul, S.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nzundu, T. Nsio
    Stochastic wave-function unravelling of the generalized Lindblad equation using correlated states2012In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 85, no 4, p. 045002-Article in journal (Refereed)
    Abstract [en]

    We perform a stochastic wave-function unravelling of the generalized Lindblad master equation using correlated states, a combination of the system state vectors and the environment population. The time-convolutionless projection operator method using correlated projection superoperators is applied to a two-state system, a qubit, that is coupled to an environment consisting of two energy bands which are both populated. These results are compared to the data obtained from Monte Carlo wave-function simulations based on the unravelling of the master equation. We also show a typical quantum trajectory and the average time evolution of the state vector on the Bloch sphere.

  • 13. Robinson, Nathaniel D.
    et al.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Chhowalla, Manish
    Graphene electrodes for organic metal-free light-emitting devices2012In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T146, p. 014023-Article in journal (Refereed)
    Abstract [en]

    In addition to its fascinating electrical and mechanical properties, graphene is also an electrochemically stable and transparent electrode material. We demonstrate its applicability as both anode and cathode in a light-emitting electrochemical cell (LEC), an electrochemical analogue to a polymer organic light-emitting diode. Specifically, we summarize recent progress in carbon-based metal-free light-emitting devices enabled by chemically derived graphene cathodes on quartz and plastic substrates, and explain the advantages of using LECs in manufacturing large-area devices.

  • 14.
    Shukla, P. K.
    Umeå University, Faculty of Science and Technology, Physics.
    Stopping power of a charged dust projectile in the presence of the dust acoustic wave in dusty plasmas2007In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 76, p. 165-Article in journal (Refereed)
  • 15.
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Faculty of Physics and Astronomy, Ruhr University Bochum, D-44780 Bochum, Germany; GoLP/Instituto de Plasmas e Fusao Nuclear, Instituto Superior Técnico, 1049-001 Lisboa, Portugal; SUPA Department of Physics, University of Strathclyde, Glasgow, Scotland; School of Physics, University of KwaZulu-Natal, Durban, South Africa; Department of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan.
    A note on the formation of large-scale structures in the Universe2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 77, no 6, article id 068201Article in journal (Refereed)
    Abstract [en]

    A general dispersion relation for low-frequency electrostatic waves in a self-gravitating pair ion-dust plasma is derived. The dispersion relation admits two classes of the Jeans instability. The growth rates and thresholds of the latter are obtained. The implication of the present work to the formation of large-scale structures in the dusty universe is highlighted.

  • 16.
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Ruhr-Universität Bochum, D-44780 Bochum, Germany; Scottish Universities Physics Alliance (SUPA), Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK; GoLP/Instituto de Plasmas Fusão Nuclear, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal; School of Physics, University of KwaZulu-Natal, 4000 Durban, South Africa; Max-Planck Institut für extraterrestrische Physik, D-85740 Garching, Germany.
    Instability of short wavelength electrostatic electron-cyclotron modes in the presence of an ion density ripple in plasmas2009In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 80, no 3, article id 038201Article in journal (Refereed)
    Abstract [en]

    It is shown that short wavelength (in comparison with the ion thermal gyroradius), high-frequency (HF) electrostatic electron-cyclotron (EEC) modes can be excited by the pre-existing ion density ripple across the external magnetic field in a dusty magnetoplasma. For this purpose, we use the Boltzmann distributed ion density perturbation, together with the electron continuity and momentum equations, and derive the standard Mathieu equation. The latter admits unstable solutions, demonstrating that the short wavelength HF-EEC-like modes in dusty magnetoplasmas can be driven due to the free energy in the ion density ripple.

  • 17. Staun Olsen, J.
    et al.
    Steenstrup, S.
    Gerward, L.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    High pressure studies up to 50 GPa of Bi-based high-Tc superconductors1991In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 44, no 2, p. 211-213Article in journal (Refereed)
    Abstract [en]

    The high-Tc superconductor with nominal composition BiSrCaCu2Ox has been studied at high pressure, i.e. up to 50 GPa. A tetragonal structure was compatible with the measurements at all pressures, and no phase change was observed. The bulk modulus, B0 = 62.5 GPa, obtained has a somewhat smaller value than the one estimated earlier.

  • 18.
    Stenflo, Lennart
    et al.
    Department of Physics, Linkoping University, SE-581 83 Linkoping, Sweden.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Parametric decay of whistler waves in electron magnetohydrodynamics2011In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 83, no 3, p. 035503-035503Article in journal (Refereed)
    Abstract [en]

    The nonlinear interaction between three whistler waves is reconsidered. We argue that the squared growth rate of parametric decay instability has to be expressed in a form that is always positive.

1 - 18 of 18
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