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• 1.
Umeå University, Faculty of Science and Technology, Department of Physics.
Université Montpellier II, France. Umeå University, Faculty of Science and Technology, Department of Physics. Physikalisches Institut, Universität Stuttgart, Germany. Université Montpellier II, France. Physikalisches Institut, Universität Stuttgart, Germany. Umeå University, Faculty of Science and Technology, Department of Physics.
Electronic properties of Cs-intercalated single-walled carbon nanotubes derived from nuclear magnetic resonance2011In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, p. 053045 (1)-(9)Article in journal (Refereed)

We report on the electronic properties of Cs-intercalated singlewalled carbon nanotubes (SWNTs). A detailed analysis of the 13C and133Cs nuclear magnetic resonance (NMR) spectra reveals an increased metallization of the pristine SWNTs under Cs intercalation. The ‘metallization’ of CsxC materials where x =0–0.144 is evidenced from the increased local electronic density of states (DOS) n(EF)at the Fermi level of the SWNTs as determined from spin–lattice relaxation measurements. In particular, there are two distinct electronic phases called α and β and the transition between these occurs around x = 0.05. The electronic DOS at the Fermi level increases monotonically at low intercalation levels x <0.05 (α-phase), whereas it reaches a plateau in the range 0.05 < x < 0.143 at high intercalation levels (β-phase). The new β-phase is accompanied by a hybridization of Cs(6s) orbitals with C(sp2)orbitals of the SWNTs. In both phases, two types of metallic nanotubes are found with a low and a high local n(EF), corresponding to different local electronic band structures of the SWNTs.

• 2.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Many-body effects on tracer particle diffusion with applications for single-protein dynamics on DNA2015In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 17, article id 043036Article in journal (Refereed)

30% of the DNA in E. coli bacteria is covered by proteins. Such a high degree of crowding affects the dynamics of generic biological processes (e.g. gene regulation, DNA repair, protein diffusion etc) in ways that are not yet fully understood. In this paper, we theoretically address the diffusion constant of a tracer particle in a one-dimensional system surrounded by impenetrable crowder particles. While the tracer particle always stays on the lattice, crowder particles may unbind to a surrounding bulk and rebind at another, or the same, location. In this scenario we determine how the long time diffusion constant D (after many unbinding events) depends on (i) the unbinding rate of crowder particles k(off), and (ii) crowder particle line density rho, from simulations (using the Gillespie algorithm) and analytical calculations. For small k(off), we find D similar to k(off)/rho(2) when crowder particles do not diffuse on the line, and D similar to root Dk(off)/rho when they are diffusing; D is the free particle diffusion constant. For large k(off), we find agreement with mean-field results which do not depend on k(off). From literature values of k(off) and D, we show that the small k(off) -limit is relevant for in vivo protein diffusion on crowded DNA. Our results apply to single-molecule tracking experiments.

• 3.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Semi-relativistic effects in spin-1/2 quantum plasmas2012In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 14, p. 073042-Article in journal (Refereed)

Emerging possibilities for creating and studying novel plasma regimes, e. g. relativistic plasmas and dense systems, in a controlled laboratory environment also require new modeling tools for such systems. This motivates theoretical studies of the kinetic theory governing the dynamics of plasmas for which both relativistic and quantum effects occur simultaneously. Here, we investigate relativistic corrections to the Pauli Hamiltonian in the context of a scalar kinetic theory for spin-1/2 quantum plasmas. In particular, we formulate a quantum kinetic theory for the collective motion of electrons that takes into account effects such as spin-orbit coupling and Zitterbewegung. We discuss the implications and possible applications of our findings.

• 4.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Zipf's law unzipped2011In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, p. 043004-Article in journal (Refereed)

Why does Zipf's law give a good description of data from seemingly completely unrelated phenomena? Here it is argued that the reason is that they can all be described as outcomes of a ubiquitous random group division: the elements can be citizens of a country and the groups family names, or the elements can be all the words making up a novel and the groups the unique words, or the elements could be inhabitants and the groups the cities in a country, and so on. A Random Group Formation (RGF) is presented from which a Bayesian estimate is obtained based on minimal information: it provides the best prediction for the number of groups with $k$ elements, given the total number of elements, groups, and the number of elements in the largest group. For each specification of these three values, the RGF predicts a unique group distribution $N(k)\propto \exp(-bk)/k^{\gamma}$, where the power-law index $\gamma$ is a unique function of the same three values. The universality of the result is made possible by the fact that no system specific assumptions are made about the mechanism responsible for the group division. The direct relation between $\gamma$ and the total number of elements, groups, and the number of elements in the largest group, is calculated. The predictive power of the RGF model is demonstrated by direct comparison with data from a variety of systems. It is shown that $\gamma$ usually takes values in the interval $1\leq\gamma\leq 2$ and that the value for a given phenomena depends in a systematic way on the total size of the data set. The results are put in the context of earlier discussions on Zipf's and Gibrat's laws, $N(k)\propto k^{-2}$ and the connection between growth models and RGF is elucidated.

• 5.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea.
The Ten Thousand Kims2011In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, p. 073036-Article in journal (Refereed)

In the Korean culture the family members are recorded in special familybooks. This makes it possible to follow the distribution of Korean familynames far back in history. It is here shown that these name distributionsare well described by a simple null model, the random group formation (RGF)model. This model makes it possible to predict how the name distributionschange and these predictions are shown to be borne out. In particular, theRGF model predicts that, for married women entering a collection of familybooks in a certain year, the occurrence of the most common family nameKim'' should be directly proportional the total number of married womenwith the same proportionality constant for all the years. This prediction isalso borne out to high degree. We speculate that it reflects some inherentsocial stability in the Korean culture. In addition, we obtain an estimate ofthe total population of the Korean culture down to year 500 AD, based on theRGF model and find about ten thousand Kims.

• 6.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
The meta book and size-dependent properties of written language2009In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 11, article id 123015Article in journal (Refereed)

Evidence is given for a systematic text-length dependence of the power-law index $\gamma$ of a single book. The estimated $\gamma$ values are consistent with a monotonic decrease from 2 to 1 with increasing length of a text. A direct connection to an extended Heap's lawis explored. The infinite book limit is, as a consequence, proposed to be given by $\gamma = 1$ instead of the value $\gamma=2$ expected if the Zipf's law was ubiquitously applicable. In addition we explore the idea that the systematic text-length dependence can be described by a meta book concept, which is an abstract representation reflecting the word-frequency structure of a text. According to this concept the word-frequency distribution of a text, with a certain length written by a single author, has the same characteristics as a text of the same length pulled out from an imaginary complete infinite corpus written by the same author.

• 7.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Selective pressure on metabolic network structures as measured from the random blind-watchmaker network2010In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 12, no 103047Article in journal (Refereed)

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

• 8.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Nonlinear wave interaction and spin models in the magnetohydrodynamic regime2011In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, no August, p. 083017-08331Article in journal (Refereed)

Here we consider the influence on the electron spin in the magnetohydrodynamic (MHD) regime. Recently developed models that include spin-velocity correlations are taken as the starting point. A theoretical argument is presented, suggesting that in the MHD regime a single-fluid electron model with spin correlations is equivalent to a model with spin-up and spin-down electrons constituting different fluids, but where the spin-velocity correlations are omitted. Three-wave interaction of two shear Alfven waves and a compressional Alfven wave is then taken as a model problem to evaluate the asserted equivalence. The theoretical argument turns out to be supported, because the predictions of the two models agree completely. Furthermore, the three-wave coupling coefficients obey the Manley-Rowe relations, which further support the soundness of the models and the validity of the assumptions made in the derivation. Finally, we point out that the proposed two-fluid model can be incorporated in standard particle-in-cell schemes with only minor modifications.

• 9.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
On the possibility of metamaterial properties in spin plasmas2008In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 10, no 11, p. 115031-Article in journal (Refereed)

The fluid theory of plasmas is extended to include the properties of electron spin. The linear theory of waves in a magnetized plasma is presented, and it is shown that the spin effects cause a change of the magnetic permeability. Furthermore, by changing the direction of the external magnetic field, the magnetic permeability may become negative. This leads to instabilities in the long wavelength regimes. If these can be controlled, however, the spin plasma becomes a metamaterial for a broad range of frequencies, i.e. above the ion cyclotron frequency but below the electron cyclotron frequency. The consequences of our results are discussed.

• 10.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Spin magnetohydrodynamics2007In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 9, p. 277-Article in journal (Refereed)

Starting from the non-relativistic Pauli description of spin-½ particles, a set of fluid equations, governing the dynamics of such particles interacting with external fields and other particles, is derived. The equations describe electrons, positrons, holes and similar conglomerates. In the case of electrons, the magnetohydrodynamic limit of an electron–ion plasma is investigated. The results should be of interest and relevance both to laboratory and astrophysical plasmas.

• 11.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Dispersion relation for electromagnetic wave propagation in a strongly magnetized plasma2006In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 8, no January, p. 16-Article in journal (Refereed)

A dispersion relation for electromagnetic wave propagation in a strongly magnetized cold plasma is deduced, taking photon–photon scattering into account. It is shown that the combined plasma and quantum electrodynamic effect is important for understanding the mode-structures in magnetar and pulsar atmospheres. The implications of our results are discussed.

• 12.
Lund Univ, Dept Phys, SE-22100 Lund, Sweden.
Umeå University, Faculty of Science and Technology, Department of Physics. Lund Univ, Dept Phys, SE-22100 Lund, Sweden. Lund Univ, Dept Phys, SE-22100 Lund, Sweden. Lund Univ, Dept Phys, SE-22100 Lund, Sweden. Univ Strathclyde, SUPA Dept Phys, Glasgow G4 0NG, Lanark, Scotland. Univ Strathclyde, SUPA Dept Phys, Glasgow G4 0NG, Lanark, Scotland. Umeå University, Faculty of Science and Technology, Department of Physics. Lund Univ, Dept Phys, SE-22100 Lund, Sweden.
Hollow microspheres as targets for staged laser-driven proton acceleration2011In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, p. 013030-Article in journal (Refereed)

A coated hollow core microsphere is introduced as a novel targetin ultra-intense laser–matter interaction experiments. In particular, it facilitates staged laser-driven proton acceleration by combining conventional target normal sheath acceleration (TNSA), power recycling of hot laterally spreading electrons and staging in a very simple and cheap target geometry. During TNSA of protons from one area of the sphere surface, laterally spreading hot electrons form a charge wave. Due to the spherical geometry, this wave refocuses on the opposite side of the sphere, where an opening has been laser micromachined.This leads to a strong transient charge separation field being set up there, which can post-accelerate those TNSA protons passing through the hole at the right time. Experimentally, the feasibility of using such targets is demonstrated. A redistribution is encountered in the experimental proton energy spectra, as predicted by particle-in-cell simulations and attributed to transient fields set up by oscillating currents on the sphere surface.

• 13. Dieckmann, ME
Umeå University, Faculty of Science and Technology, Department of Physics. Institute of Theoretical Physics IV, Ruhr-University Bochum, D-44780 Bochum, Germany.
On the ultrarelativistic two-stream instability, electrostatic turbulence and Brownian motion2006In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 8, article id 40Article in journal (Refereed)

Experimental evidence indicates that bulk plasma flow at ultrarelativistic speeds is common in astrophysical settings, e. g. the collimated jets of active galactic nuclei and gamma ray bursts. The low-plasma density of such flows implies their collisionless relaxation by means of wave-particle interactions. Such processes are not well understood in the ultrarelativistic regime. The thermalization of two interpenetrating equally dense electron-proton (e(-)p) beams in the absence of a magnetic field is examined here by means of 1.5D particle-in-cell simulations. The relative beam speeds correspond to Lorentz factors in the range 200-1000. The constraint to one spatial simulation dimension, which is aligned with the beam velocity vectors, implies that only the two-stream (TS) instability and the Weibel-type instability can grow, while filamentation instabilities are excluded. With this constraint and for our plasma parameters, the TS instability dominates. The electrostatic waves grow, saturate by the trapping of electrons, and collapse. The interaction of the electrons with the electric fields after the wave collapse represents a relativistic Wiener process. In response, the electrons are rapidly thermalized. The final electron distribution can be interpreted as a relativistic Maxwellian distribution with a high-energy tail arising from ultrarelativistic phase space holes.

• 14.
Umeå University, Faculty of Science and Technology, Department of Physics.
Department of Physics, University of Maryland. Department of Physics, University of Maryland. Department of Physics, University of Maryland. Institut für Theoretische Physik IV, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, D-44780 Bochum, Germany.
Laser acceleration of monoenergetic protons via a double layer emerging from an ultra-thin foil2009In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 11, p. 073006-073025Article in journal (Refereed)

We present theoretical and numerical studies of the acceleration of monoenergetic protons in a double layer formed by the laser irradiation of an ultra-thin film. The ponderomotive force of the laser light pushes the electrons forward, and the induced space charge electric field pulls the ions and makes the thin foil accelerate as a whole. The ions trapped by the combined electric field and inertial force in the accelerated frame, together with the electrons trapped in the well of the ponderomotive and ion electric field, form a stable double layer. The trapped ions are accelerated to monoenergetic energies up to 100 MeV and beyond, making them suitable for cancer treatment. We present an analytic theory for the laser-accelerated ion energy and for the amount of trapped ions as functions of the laser intensity, foil thickness and the plasma number density. We also discuss the underlying physics of the trapped and untrapped ions in a double layer. The analytical results are compared with those obtained from direct Vlasov simulations of the fully nonlinear electron and ion dynamics that is controlled by the laser light.

• 15.
Universidade do Vale do Rio dos Sinos, Brazil.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Fluid moment hierarchy equations derived from gauge invariant quantum kinetic theory2010In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 12, p. 073027-073039Article in journal (Refereed)

The gauge invariant electromagnetic Wigner equation is taken as the basis of a fluid-like system describing quantum plasmas, derived from the moments of the gauge invariant Wigner function. The use of the standard, gauge-dependent Wigner function is shown to produce inconsistencies if a direct correspondence principle is applied. The propagation of linear transverse waves is considered and it is shown to be in agreement with the kinetic theory in the long-wavelength approximation, provided that an adequate closure is chosen for the macroscopic equations. A general recipe to solve the closure problem is suggested.

• 16.
Umeå University, Faculty of Science and Technology, Department of Physics.
FOI, Swedish Defence Research Agency, Tumba SE-14725, Sweden. Umeå University, Faculty of Science and Technology, Department of Physics. Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea. Umeå University, Faculty of Science and Technology, Department of Physics.
Neutral theory of chemical reaction networks2012In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 14, p. 033032-Article in journal (Refereed)

To what extent do the characteristic features of a chemical reaction network reflect its purpose and function? In general, one argues that correlations between specific features and specific functions are key to understanding a complex structure. However, specific features may sometimes be neutral and uncorrelated with any system-specific purpose, function or causal chain. Such neutral features are caused by chance and randomness. Here we compare two classes of chemical networks: one that has been subjected to biological evolution (the chemical reaction network of metabolism in living cells) and one that has not (the atmospheric planetary chemical reaction networks). Their degree distributions are shown to share the very same neutral system-independent features. The shape of the broad distributions is to a large extent controlled by a single parameter, the network size. From this perspective, there is little difference between atmospheric and metabolic networks; they are just different sizes of the same random assembling network. In other words, the shape of the degree distribution is a neutral characteristic feature and has no functional or evolutionary implications in itself; it is not a matter of life and death.

• 17. Mendonca, JT
Umeå University, Faculty of Science and Technology, Department of Physics. Centre for Fundamental Physics (CfFP), STFC Rutherford Appleton Laboratory, Chilton, Didcot, UK; Institut für Theoretische Physik IV, Ruhr-Universität Bochum, D-44780 Bochum, Germany; SUPA Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK .
Nonlinear excitation of zonal flows by Rossby wave turbulence2009In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 11, article id 073038Article in journal (Refereed)

We apply the wave-kinetic approach to study nonlinearly coupled Rossby wave-zonal flow fluid turbulence in a two-dimensional rotating fluid. Specifically, we consider for the first time nonlinear excitations of zonal flows by a broad spectrum of Rossby wave turbulence. Short-wavelength Rossby waves are described here as a fluid of quasi-particles, and are referred to as the 'Rossbyons'. It is shown that Reynolds stresses of Rossbyons can generate large-scale zonal flows. The result should be useful in understanding the origin of large-scale planetary and near-Earth atmospheric circulations. It also provides an example of a turbulent wave background driving a coherent structure.

• 18. Moslem, W. M.
Umeå University, Faculty of Science and Technology, Department of Physics. Ruhr Univ Bochum, Inst Theoret Phys 4, Fak Phys & Astron, D-44780 Bochum, Germany; Max Planck Inst Extraterr Phys, D-85741 Garching, Germany; Inst Super Tecn, GoLP, P-1049001 Lisbon, Portugal; Rutherford Appleton Lab, CCLRC Ctr Fundamental Phys, Didcot 0X11 0QX, Oxon, England; Univ Strathclyde, SUPA Dept Phys, Glasgow G4 0NG, Lanark, Scotland; Univ Kwazulu Natal, Fac Sci & Agr, Sch Phys, ZA-4000 Durban, South Africa.
Electrostatic structures associated with dusty electronegative magnetoplasmas2010In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 12, article id 073010Article in journal (Refereed)

By using the hydrodynamic equations of positive and negative ions, the Boltzmann electron density distribution and the Poisson equation with stationary dust, a three-dimensional (3D) Zakharov-Kuznetsov (ZK) equation is derived for small but finite amplitude ion-acoustic waves. However, the ZK equation is not appropriate to describe the system either at critical plasma compositions or in the vicinity of the critical plasma compositions. Therefore, the modified ZK (MZK) and extended ZK (EZK) equations are derived. The generalized expansion method is used to analytically solve the ZK, MZK and EZK equations. A new class of solutions that admits a train of well-separated bell-shaped periodic pulses is obtained. In certain conditions, the latter degenerates to either solitary or shock wave solutions. The effects of the physical parameters on the nonlinear structures are examined in many plasma environments having different negative ion species, such as D- and F-regions of the Earth's ionosphere, as well as in laboratory plasma experiments. Numerical analysis of the solutions revealed that the profile of the nonlinear pulses suffers amplitude and width modifications due to enhancement of the dust practices, negative ions, positive-to-negative ion mass ratio and positive/negative ion cyclotron frequency. Furthermore, the necessary conditions for both solitons and shocks propagation as well as their polarity are examined.

• 19. Moslem, WM
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; Max-Planck-Institut für extraterrestrische Physik, D-85741 Garching, Germany; GoLP/Instituto Superior Técnico, 1049-001 Lisbon, Portugal; CCLRC Centre for Fundamental Physics, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon 0X11 0QX, UK; SUPA Department of Physics, University of Strathclyde, Glasgow G 40NG, UK; School of Physics, Faculty of Science and Agriculture, University of Kwazulu-Natal, Durban 4000, South Africa; Department of Physics, CITT, Islamabad, Pakistan.
Solitary and blow-up electrostatic excitations in rotating magnetized electron-positron-ion plasmas2009In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 11, article id 033028Article in journal (Refereed)

The nonlinear dynamics of a rotating magnetoplasma consisting of electrons, positrons and stationary positive ions is considered. The basic set of hydrodynamic and Poisson equations are reduced to a Zakharov-Kuznetsov (ZK) equation for the electric potential. The ZK equation is solved by applying an improved modified extended tanh-function method (2008 Phys. Lett. A 372 5691) and its characteristics are investigated. A set of new solutions are derived, including localized solitary waves, periodic nonlinear waveforms and divergent (explosive) pulses. The characteristics of these nonlinear excitations are investigated in detail.

• 20.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
A simple method to calculate first-passage time densities with arbitrary initial conditions2016In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 18, article id 063019Article in journal (Refereed)

Numerous applications all the way from biology and physics to economics depend on the density of first crossings over a boundary. Motivated by the lack of general purpose analytical tools for computing first-passage time densities (FPTDs) for complex problems, we propose a new simple method based on the independent interval approximation (IIA). We generalise previous formulations of the IIA to include arbitrary initial conditions as well as to deal with discrete time and non-smooth continuous time processes. Wederive a closed form expression for the FPTD in z and Laplace-transform space to a boundary in one dimension. Two classes of problems are analysed in detail: discrete time symmetric random walks (Markovian) and continuous time Gaussian stationary processes (Markovian and non-Markovian). Our results are in good agreement with Langevin dynamics simulations.

• 21. Sanders, Lloyd P.
Umeå University, Faculty of Science and Technology, Department of Physics.
Severe slowing-down and universality of the dynamics in disordered interacting many-body systems: ageing and ultraslow diffusion2014In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 16, p. 113050-Article in journal (Refereed)

Low-dimensional, many-body systems are often characterized by ultraslow dynamics. We study a labelled particle in a generic system of identical particles with hard-core interactions in a strongly disordered environment. The disorder is manifested through intermittent motion with scale-free sticking times at the single particle level. While for a non-interacting particle we find anomalous diffusion of the power-law form < x(2)(t)> similar or equal to t(alpha) of the mean squared displacement with 0 < alpha < 1, we demonstrate here that the combination of the disordered environment with the many-body interactions leads to an ultraslow, logarithmic dynamics < x(2)(t)> similar or equal to log(1/2)t with a universal 1/2 exponent. Even when a characteristic sticking time exists but the fluctuations of sticking times diverge we observe the mean squared displacement < x(2)(t)> similar or equal to t(gamma) with 0 < gamma < 1/2, that is slower than the famed Harris law < x(2)(t)> similar or equal to t(1/2) without disorder. We rationalize the results in terms of a subordination to a counting process, in which each transition is dominated by the forward waiting time of an ageing continuous time process.

• 22. Shaikh, Dastgeer
Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Ruhr-Universität Bochum, D-44780 Bochum, Germany; SUPA Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK; Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal.
3D electron fluid turbulence at nanoscales in dense plasmas2008In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 10, article id 083007Article in journal (Refereed)

We have performed three-dimensional (3D) nonlinear fluid simulations of electron fluid turbulence at nanoscales in an unmagnetized warm dense plasma in which mode coupling between wave function and electrostatic (ES) potential associated with underlying electron plasma oscillations (EPOs) lead to nonlinear cascades in inertial range. While the wave function cascades towards smaller length scales, ES potential follows an inverse cascade. We find from our simulations that the quantum diffraction effect associated with a Bohm potential plays a critical role in determining the inertial range turbulent spectrum and the subsequent transport level exhibited by the 3D EPOs.

• 23.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Spin-induced nonlinearities in the electron magnetohydrodynamic regime2010In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 12Article in journal (Refereed)

The influence of electron spin on the nonlinear propagation of whistler waves is studied in this paper. For this purpose, a recently developed electron two-fluid model, where the spin-up and spin-down populations are treated as different fluids, is adapted to the electron magnetohydrodynamic (MHD) regime. A nonlinear Schrödinger equation is then derived for the whistler waves and the coefficients of nonlinearity with and without spin effects are compared. The relative importance of spin effects depends on the plasma density and temperature as well as the external magnetic field strength and wave frequency. The significance of our results for various plasmas is discussed.

• 24.
Umeå University, Faculty of Science and Technology, Department of Physics. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China.
Umeå University, Faculty of Science and Technology, Department of Physics. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China. Umeå University, Faculty of Science and Technology, Department of Physics. Faculty of Natural Sciences, National University of Laos (NUOL), Vientiane, Laos. Umeå University, Faculty of Science and Technology, Department of Physics. Borås, Sweden. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China.
AC impedance of A4C60 fullerides under pressure2015In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 17, no 2, article id 023010Article in journal (Refereed)

Three A4C60 compounds, with A = Li, Na and K, have been studied by impedance spectroscopy between 100 K and 293 K at pressures up to 2 GPa. The results are in very good agreement with earlier DC resistance studies and with data from the literature. For all three materials the measured conductivity can be fitted by a sum of at least two Arrhenius terms. The band gaps derived from the resistance data, 0.3 eV for Na4C60 and 0.5 eV for K4C60, are in excellent agreement with data measured by other methods. For Li4C60, our results disagree with a recent suggestion that the conductivity is dominated by ionic conduction. Although a certain ionic component probably exists we suggest that electronic transport dominates in our samples at and below room temperature because the derived “activation energy” decreases under pressure, the derived “activation volume” is negative, and we observe neither a significant electrode blocking capacitance nor any significant metal transport under DC conditions.

• 25. Yi, Su Do
Umeå University, Faculty of Science and Technology, Department of Physics.
Allometric exponent and randomness2013In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 15, p. 043001-Article in journal (Refereed)

An allometric height-mass exponent gamma gives an approximative power-law relation < M > proportional to H-gamma between the average mass < M > and the height H for a sample of individuals. The individuals in the present study are humans but could be any biological organism. The sampling can be for a specific age of the individuals or for an age interval. The body mass index is often used for practical purposes when characterizing humans and it is based on the allometric exponent gamma = 2. It is shown here that the actual value of gamma is to a large extent determined by the degree of correlation between mass and height within the sample studied: no correlation between mass and height means gamma = 0, whereas if there was a precise relation between mass and height such that all individuals had the same shape and density then gamma = 3. The connection is demonstrated by showing that the value of gamma can be obtained directly from three numbers characterizing the spreads of the relevant random Gaussian statistical distributions: the spread of the height and mass distributions together with the spread of the mass distribution for the average height. Possible implications for allometric relations, in general, are discussed.

• 26.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Dynamics of a dusty plasma with intrinsic magnetization2009In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 11, no July, p. 073017-Article in journal (Refereed)

We consider a dusty plasma where dust particles have a magnetic dipole moment. A Hall-MHD type of model, generalized to account for the intrinsic magnetization, is derived. The model is shown to be energy conserving, and the energy density and flux are derived. The general dispersion relation is then derived, and we show that kinetic dust-Alfvén waves exhibit instability for a low dust and ion temperature and high dust density. We discuss the implication of our results.

• 27.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Scalar quantum kinetic theory for spin-1/2 particles: mean field theory2010In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 12, p. 043019-Article in journal (Refereed)

Starting from the Pauli Hamiltonian operator, we derive scalar quantum kinetic equations for spin-1/2 systems. Here, the regular Wigner two-state matrix is replaced by a scalar distribution function in extended phase space. Apart from being a formulation of significant interest, such a scalar quantum kinetic equation makes the comparison with classical kinetic theory straightforward and lends itself naturally to currently available numerical Vlasov and Boltzmann schemes. Moreover, while the quasi-distribution is a Wigner function in regular phase space, it is given by a Q-function in spin space. As such, nonlinear and dynamical quantum plasma problems are readily handled. Moreover, the issue of gauge invariance is treated.

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