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• 1.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Non-Kosterlitz-Thouless transitions for the q-state clock models2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 82, nr 3, s. 031102-Artikkel i tidsskrift (Fagfellevurdert)

The $q$-state clock model with the cosine potential has a single phasetransition for $q\leq4$ and two transitions for $q\geq5$. It is shown byMonteCarlo simulations that the helicity modulus for the five-state clock model($q=5$) does not vanish at the high-temperature transition. This is incontrast to the clock models with $q\geq6$ for which the helicity modulusvanishes. This means that the transition for the five-state clock modeldiffers from the Kosterlitz-Thouless (KT) transition. It is also shown thatthis change in the transition is caused by an interplay between the numberof angular directions and the interaction potential: by slightly modifyingthe interaction potential, the KT transition for $q=6$ turns into the samenon-KT transition. Likewise, the KT transition is recovered for $q=5$ whenthe Villain potential is used. Comparisons with other clock-model resultsare made and discussed.

• 2.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Gyeonggi Science High School. Department of Energy Science and BK21 Physics Research Division, Sungkyunkwan University, Suwon 440-746, Korea.
Flow improvement caused by agents who ignore traffic rules2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 80, nr 1, s. 016111-Artikkel i tidsskrift (Fagfellevurdert)

A system of agents moving along a road in both directions is studied numerically within a cellular-automata formulation. An agent steps to the right with probability $q$ or to the left with $1-q$ when encountering other agents. Our model is restricted to two agent types, traffic-rule abiders ($q=1$) and traffic-rule ignorers ($q=1/2$). The traffic flow, resulting from the interaction between these two types of agents, is obtained as a function of density and relative fraction. The risk for jamming at a fixed density, when starting from a disordered situation, is smaller when every agent abides by a traffic rule than when all agents ignore the rule. Nevertheless, the absolute minimum occurs when a smallfraction of ignorers are present within a majority of abiders. The characteristic features for the spatial structure of the flow pattern are obtained and discussed.

• 3.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Dept. of Physics, Sungkyunkwan University, Suwon, Korea.
Comment on `Six-state clock model on the square lattice: Fisher zeroapproach with Wang-Landau sampling'2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 81, nr 6, s. 3101-Artikkel i tidsskrift (Annet vitenskapelig)

Hwang in [Phys. Rev. E {\bf{80}}, 042103 (2009)] suggested that the two transitions of the six-state clock model on the square lattice are\emph{not} of the Kosterlitz-Thouless type. Here we show from simulations thatat the upper transition, the helicity modulus does make a discontinuous jumpto zero. This gives strong evidence for a Kosterlitz-Thoulesstransition.

• 4.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Department of Physics, BK21 Physics Research Division, Sungkyunkwan University, Suwon 440-746, Korea.
Percolation in hyperbolic lattices2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 79, nr 1, s. 011124-011130Artikkel i tidsskrift (Fagfellevurdert)

The percolation transitions on hyperbolic lattices are investigated numerically using finite-size scaling methods. The existence of two distinct percolation thresholds is verified. At the lower threshold, an unbounded cluster appears and reaches from the middle to the boundary. This transition is of the same type and has the same finite-size scaling properties as the corresponding transition for the Cayley tree. At the upper threshold, on the other hand, a single unbounded cluster forms which overwhelms all the others and occupies a finite fraction of the volume as well as of the boundary connections. The finite-size scaling properties for this upper threshold are different from those of the Cayley tree and two of the critical exponents are obtained. The results suggest that the percolation transition for the hyperbolic lattices forms a universality class of its own.

• 5.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Dept. of Physics, Sungkyunkwan University, Suwon, Korea.
Surface and bulk criticality in midpoint percolation2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 81, nr 4, s. 1108-6 sidorArtikkel i tidsskrift (Fagfellevurdert)

The concept of midpoint percolation has recently been applied tocharacterize the double percolation transitions in negatively curvedstructures. Regular $d$-dimensional hypercubic lattices are in the presentwork investigated using the same concept.Specifically, the site-percolation transitions at the critical thresholds areinvestigated for dimensions up to $d=10$ by means of the Leath algorithm.It is shown that the explicit inclusion of the boundariesprovides a straightforward way to obtain critical indices, both for thebulk and surface parts. At and above the critical dimension $d=6$, it isfound that the percolation cluster contains only a finite number of surfacepoints in the infinite-size limit. This is in accordance with theexpectation from studies of lattices with negative curvature. It is alsofound that the number of surface points, reached by the percolation clusterin the infinite limit, approaches $2d$ for large dimensions $d$. We alsonote that the size dependence in proliferation of percolatingclusters for $d\ge 7$ can be obtained by solely counting surface pointsof the midpoint cluster.

• 6.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Dept. of Physics, Sungkyunkwan University, Korea.
True and quasi long-range order in the generalized q-state clock model2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 80, s. 060101-Artikkel i tidsskrift (Fagfellevurdert)

From consideration of the order-parameter distribution,we propose an observable which makes a clear distinction betweentrue and quasi long-range orders in the two-dimensionalgeneralized $q$-state clock model.Measuring this quantity by Monte Carlo simulations for $q=8$, weconstruct a phase diagram and identify critical properties acrossthe phase-separation lines among the true long-range order, quasilong-range order, and disorder. Our result supports the theoreticalprediction that there appears a discontinuous order-disorder transition assoon as the two phase-separation lines merge.

• 7.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea.
Critical temperatures of the three- and four-state Potts models on the kagome lattice2011Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 83, nr 6, s. 061104-Artikkel i tidsskrift (Fagfellevurdert)

The value of the internal energy per spin is independent of the strip widthfor a certain class of spin systems on two dimensional infinite strips. Itis verified that the Ising model on the kagome lattice belongs to this classthrough an exact transfer-matrix calculation of the internal energy for thetwo smallest widths. More generally, one can suggest an upper bound forthe critical coupling strength $K_c(q)$ for the $q$-state Potts model fromexact calculations of the internal energy for the two smallest stripwidths. Combining this with the corresponding calculation for the duallattice and using an exact duality relation enables us to conjecture thecritical coupling strengths for the three- and four-state Pottsmodels on the kagome lattice. The values are $K_c(q=3)=1.056~509~426~929~0$and $K_c(q=4) = 1.149~360~587~229~2$,and the values can, in principle, be obtained to an arbitrary precision. Wediscuss the fact that these values are in the middle of earlierapproximate results and furthermore differ from earlier conjecturesfor the exact values.

• 8.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Department of Applied Physics, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan. BK21 Physics Research Division and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea.
Curvature-induced frustration in the XY model on hyperbolic surfaces2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 79, nr 6, s. 060106(R)-Artikkel i tidsskrift (Fagfellevurdert)

We study low-temperature properties of the XY spin model on a negatively curved surface. Geometric curvature of the surface gives rise to frustration in local spin configuration, which results in the formation of high-energy spin clusters scattered over the system. Asymptotic behavior of the spin-glass susceptibility suggests a zero-temperature glass transition, which is attributed to multiple optimal configurations of spin clusters due to nonzero surface curvature of the system. It implies that a constant ferromagnetic spin interaction on a regular lattice can exhibit glasslike behavior without possessing any disorder if the lattice is put on top of a negatively curved space such as a hyperbolic surface.

• 9. Baek, Yongjoo
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Market behavior and performance of different strategy evaluation schemes2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 82, nr 026109Artikkel i tidsskrift (Fagfellevurdert)

Strategy evaluation schemes are a crucial factor in any agent-based market model, as they determine the agents’ strategy preferences and consequently their behavioral pattern. This study investigates how the strategy evaluation schemes adopted by agents affect their performance in conjunction with the market circumstances. We observe the performance of three strategy evaluation schemes, the history-dependent wealth game, the trend-opposing minority game, and the trend-following majority game, in a stock market where the price is exogenously determined. The price is either directly adopted from the real stock market indices or generated with a Markov chain of order ≤2. Each scheme’s success is quantified by average wealth accumulated by the traders equipped with the scheme. The wealth game, as it learns from the history, shows relatively good performance unless the market is highly unpredictable. The majority game is successful in a trendy market dominated by long periods of sustained price increase or decrease. On the other hand, the minority game is suitable for a market with persistent zigzag price patterns. We also discuss the consequence of implementing finite memory in the scoring processes of strategies. Our findings suggest under which market circumstances each evaluation scheme is appropriate for modeling the behavior of real market traders.

• 10.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Models and average properties of scale-free directed networks2006Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 74, s. 026104-026104-7Artikkel i tidsskrift (Fagfellevurdert)

We extend the merging model for undirected networks by Kim et al. [Eur. Phys. J. B 43, 369 (2004)] to directed networks and investigate the emerging scale-free networks. Two versions of the directed merging model, friendly and hostile merging, give rise to two distinct network types. We uncover that some nontrivial features of these two network types resemble two levels of a certain randomization/nonspecificity in the link reshuffling during network evolution. Furthermore, the same features show up, respectively, in metabolic networks and transcriptional networks. We introduce measures that single out the distinguishing features between the two prototype networks, as well as point out features that are beyond the prototypes.

• 11.
Niels Bohr Institute, Blegdansvej 17, DK 2100. Copenhagen, Denmark.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Niels Bohr Institute, Blegdansvej 17, DK 2100. Copenhagen, Denmark. Niels Bohr Institute, Blegdansvej 17, DK 2100. Copenhagen, Denmark.
Degree landscapes in scale-free networks2006Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 74, s. 036119-Artikkel i tidsskrift (Fagfellevurdert)

We generalize the degree-organizational view of real-world networks with broad degree distributions in a landscape analog with mountains (high-degree nodes) and valleys (low-degree nodes). For example, correlated degrees between adjacent nodes correspond to smooth landscapes (social networks), hierarchical networks to one-mountain landscapes (the Internet), and degree-disassortative networks without hierarchical features to rough landscapes with several mountains. To quantify the topology, we here measure the widths of the mountains and the separation between different mountains. We also generate ridge landscapes to model networks organized under constraints imposed by the space the networks are embedded in, associated to spatial or in molecular networks to functional localization.

• 12.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Ferromagnetic behavior in magnetized plasmas2007Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 76, nr 055403(R), s. 4-Artikkel i tidsskrift (Fagfellevurdert)

We consider a low-temperature plasma within a newly developed magnetohydrodynamic fluid model. Inaddition to the standard terms, the electron spin, quantum particle dispersion, and degeneracy effects areincluded. It turns out that the electron spin properties can give rise to ferromagnetic behavior in certainregimes. If additional conditions are satisfied, a homogeneous magnetized plasma can even be unstable. Thishappens in the low-temperature high-density regime, when the magnetic properties associated with the spin canovercome the stabilizing effects of the thermal and Fermi pressure, to cause a Jeans-like instability.

• 13.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Explosion triggering by an accelerating flame2006Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 73, nr 6, s. 066305-Artikkel i tidsskrift (Fagfellevurdert)

The analytical theory of explosion triggering by an accelerating flame is developed. The theory describes the structure of a one-dimensional isentropic compression wave pushed by the flame front. The condition of explosion in the gas mixture ahead of the flame front is derived; the instant of the explosion is determined provided that a mechanism of chemical kinetics is known. As an example, it is demonstrated how the problem is solved in the case of a single reaction of Arrhenius type, controlling combustion both inside the flame front and ahead of the flame. The model of an Arrhenius reaction with a cutoff temperature is also considered. The limitations of the theory due to the shock formation in the compression wave are found. Comparison of the theoretical results to the previous numerical simulations shows good agreement.

• 14.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Princeton University. Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Princeton University.
Role of Compressibility in Moderating Flame Acceleration in Tubes2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 81, nr 2, s. 026309-Artikkel i tidsskrift (Fagfellevurdert)

The effect of gas compression on spontaneous flame acceleration leading to deflagration-to-detonation transition is studied theoretically for small Reynolds number flame propagation from the closed end of a tube. The theory assumes weak compressibility through expansion in small Mach number. Results show that the flame front accelerates exponentially during the initial stage of propagation when the Mach number is negligible. With continuous increase in the flame velocity with respect to the tube wall, the flame-generated compression waves subsequently moderate the acceleration process by affecting the flame shape and velocity, as well as the flow driven by the flame.

• 15.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Theory and modeling of accelerating flames in tubes2005Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 72, nr 4, s. 046307-Artikkel i tidsskrift (Fagfellevurdert)

The analytical theory of premixed laminar flames accelerating in tubes is developed, which is an important part of the fundamental problem of flame transition to detonation. According to the theory, flames with realistically large density drop at the front accelerate exponentially from a closed end of a tube with nonslip at the walls. The acceleration is unlimited in time; it may go on until flame triggers detonation. The analytical formulas for the acceleration rate, for the flame shape and the velocity profile in the flow pushed by the flame are obtained. The theory is validated by extensive numerical simulations. The numerical simulations are performed for the complete set of hydrodynamic combustion equations including thermal conduction, viscosity, diffusion, and chemical kinetics. The theoretical predictions are in a good agreement with the numerical results. It is also shown how the developed theory can be used to understand acceleration of turbulent flames.

• 16.
CERMICS, École National des Ponts et Chaussées.
CERMICS, École National des Ponts et Chaussées.
Spectral method for the time-dependent Gross-Pitaevskii equation with a harmonic trap2003Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 67, nr 4, s. 046706-Artikkel i tidsskrift (Fagfellevurdert)

We study the numerical resolution of the time-dependent Gross-Pitaevskii equation, a nonlinear Schrödinger equation used to simulate the dynamics of Bose-Einstein condensates. Considering condensates trapped in harmonic potentials, we present an efficient algorithm by making use of a spectral-Galerkin method, using a basis set of harmonic-oscillator functions, and the Gauss-Hermite quadrature. We apply this algorithm to the simulation of condensate breathing and scissor modes.

• 17.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Relativistic kinetic theory for spin-1/2 particles: Conservation laws, thermodynamics, and linear waves2019Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 100, nr 2, artikkel-id 023201Artikkel i tidsskrift (Fagfellevurdert)

We study a recently derived fully relativistic kinetic model for spin-1/2 particles. First, the full set of conservation laws for energy, momentum, and angular momentum are given together with an expression for the (nonsymmetric) stress-energy tensor. Next, the thermodynamic equilibrium distribution is given in different limiting cases. Furthermore, we address the analytical complexity that arises when the spin and momentum eigenfunctions are coupled in linear theory by calculating the linear dispersion relation for such a case. Finally, we discuss the model and give some context by comparing with potentially relevant phenomena that are not included, such as radiation reaction and vacuum polarization.

• 18.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Networking genetic regulation and neural computation: directed network topology and its effect on the dynamics2004Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 70, nr 6, s. 061908-Artikkel i tidsskrift (Fagfellevurdert)
• 19.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Networking the seceder model: group formation in social and economic systems2004Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 70, nr 3, s. 036108-Artikkel i tidsskrift (Fagfellevurdert)

The seceder model illustrates how the desire to be different from the average can lead to formation of groups in a population. We turn the original, agent based, seceder model into a model of network evolution. We find that the structural characteristics of our model closely match empirical social networks. Statistics for the dynamics of group formation are also given. Extensions of the model to networks of companies are also discussed.

• 20.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Laboratoire de Physique de la Matière Condensée, CNRS UMR 6622, Université de Nice–Sophia Antipolis, Parc Valrose, F-06108 Nice Cedex 2, France.
Directed transport with real-time external steering and drifts along pre-designed paths using a Brownian motor2011Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 83, nr 2Artikkel i tidsskrift (Fagfellevurdert)

We have realized real-time steering of the directed transport in a Brownian motor based on cold atoms in optical lattices and demonstrate drifts along predesigned paths. The transport is induced by spatiotemporal asymmetries in the system, where we can control the spatial part, and we show that the response to changes in asymmetry is very fast. In addition to directional steering, a real-time control of the magnitude of the average drift velocity and an on-off switching of the motor are also demonstrated. We use a noninvasive real-time detection of the transport, enabling feedback control of the system.

• 21.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Multiscaling in an YX model of networks2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 80, s. 036120-Artikkel i tidsskrift (Fagfellevurdert)

Weinvestigate a Hamiltonian model of networks. The model is amirror formulation of the XY model (hence the name)—instead ofletting the XY spins vary, keeping the coupling topology static,we keep the spins conserved and sample different underlying networks.Our numerical simulations show complex scaling behaviors with various exponentsas the system grows and temperature approaches zero, but nofinite-temperature universal critical behavior. The ground-state and low-order excitations forsparse, finite graphs are a fragmented set of isolated networkclusters. Configurations of higher energy are typically more connected. Theconnected networks of lowest energy are stretched out giving thenetwork large average distances. For the finite sizes we investigate,there are three regions—a low-energy regime of fragmented networks, anintermediate regime of stretched-out networks, and a high-energy regime ofcompact, disordered topologies. Scaling up the system size, the bordersbetween these regimes approach zero temperature algebraically, but different network-structuralquantities approach their T=0 values with different exponents. We arguethis is a, perhaps rare, example of a statistical-physics modelwhere finite sizes show a more interesting behavior than thethermodynamic limit.

• 22.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
Department of Physics, AlbaNova University Center, KTH, SE-106 91 Stockholm, Sweden. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik. Department of Physics, AlbaNova University Center, KTH, SE-106 91 Stockholm, Sweden. Department of Physics, AlbaNova University Center, KTH, SE-106 91 Stockholm, Sweden. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
Computation of the Ising partition function for two-dimensional square grids2004Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 69, nr 4, s. 19-, artikkel-id 046104Artikkel i tidsskrift (Fagfellevurdert)

An improved method for obtaining the Ising partition function for $n \times n$ square grids with periodic boundary is presented. Our method applies results from Galois theory in order to split the computation into smaller parts and at the same time avoid the use of numerics. Using this method we have computed the exact partition function for the $320 \times 320$-grid, the $256 \times 256$-grid, and the $160 \times 160$-grid, as well as for a number of smaller grids. We obtain scaling parameters and compare with what theory prescribes.

• 23.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Finite-size scaling in random K-satisfiability problems2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 82, nr 061109Artikkel i tidsskrift (Fagfellevurdert)

We provide a comprehensive view of various phase transitions in random K-satisfiability problems solved by stochastic-local-search algorithms. In particular, we focus on the finite-size scaling (FSS) exponent, which is mathematically important and practically useful in analyzing finite systems. Using the FSS theory of nonequilibrium absorbing phase transitions, we show that the density of unsatisfied clauses clearly indicates the transition from the solvable (absorbing) phase to the unsolvable (active) phase as varying the noise parameter and the density of constraints. Based on the solution clustering (percolation-type) argument, we conjecture two possible values of the FSS exponent, which are confirmed reasonably well in numerical simulations for 2≤K≤3.

• 24.
Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea.
Effects of substrate network topologies on competition dynamics2006Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 74, nr 026118Artikkel i tidsskrift (Fagfellevurdert)

We study a competition dynamics, based on the minority game, endowed with various substrate network structures. We observe the effects of the network topologies by investigating the volatility of the system and the structure of follower networks. The topology of substrate structures significantly influences the system efficiency represented by the volatility and such substrate networks are shown to amplify the herding effect and cause inefficiency in most cases. The follower networks emerging from the leadership structure show a power-law incoming degree distribution. This study shows the emergence of scale-free structures of leadership in the minority game and the effects of the interaction among players on the networked version of the game.

• 25.
Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea.
Random field Ising model on networks with inhomogeneous connections2006Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 74, nr 031118Artikkel i tidsskrift (Fagfellevurdert)

We study a zero-temperature phase transition in the random field Ising model on scale-free networks with the degree exponent γ. Using an analytic mean-field theory, we find that the spins are always in the ordered phase for γ<3. On the other hand, the spins undergo a phase transition from an ordered phase to a disordered phase as the dispersion of the random fields increases for γ>3. The phase transition may be either continuous or discontinuous depending on the shape of the random field distribution. We derive the condition for the nature of the phase transition. Numerical simulations are performed to confirm the results.

• 26.
Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea.
Critical behavior of the Ising model in annealed scale-free networks2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 80, nr 051127Artikkel i tidsskrift (Fagfellevurdert)

We study the critical behavior of the Ising model in annealed scale-free (SF) networks of finite system size with forced upper cutoff in degree. By mapping the model onto the weighted fully connected Ising model, we derive analytic results for the finite-size scaling (FSS) near the phase transition, characterized by the cutoff-dependent two-parameter scaling with four distinct scaling regimes, in highly heterogeneous networks. These results are essentially the same as those found for the nonequilibrium contact process in annealed SF networks, except for an additional complication due to the trivial critical point shift in finite systems. The discrepancy of the FSS theories between annealed and quenched SF networks still remains in the equilibrium Ising model, like some other nonequilibrium models. All of our analytic results are confirmed reasonably well by numerical simulations.

• 27.
Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea.
Statistical properties of sampled networks2006Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 73, nr 016102Artikkel i tidsskrift (Fagfellevurdert)

We study the statistical properties of the sampled scale-free networks, deeply related to the proper identification of various real-world networks. We exploit three methods of sampling and investigate the topological properties such as degree and betweenness centrality distribution, average path length, assortativity, and clustering coefficient of sampled networks compared with those of original networks. It is found that the quantities related to those properties in sampled networks appear to be estimated quite differently for each sampling method. We explain why such a biased estimation of quantities would emerge from the sampling procedure and give appropriate criteria for each sampling method to prevent the quantities from being overestimated or underestimated.

• 28.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Linearized kinetic theory of spin-1/2 particles in magnetized plasmas2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 82, nr 5, s. 054607-10 pagesArtikkel i tidsskrift (Fagfellevurdert)

We have considered linear kinetic theory, including the electron-spin properties in a magnetized plasma. The starting point is a mean-field Vlasov-like equation, derived from a fully quantum-mechanical treatment, where effects from the electron-spin precession and the magnetic dipole force are taken into account. The general conductivity tensor is derived, including both the free current contribution and the magnetization current associated with the spin contribution. We conclude the paper with an extensive discussion of the quantummechanical boundary where we list parameter conditions that must be satisfied for various quantum effects to be influential.

• 29. Mamun, A. A.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Solitary waves and double layers in a dusty electronegative plasma2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 80, nr 4, s. 046406-Artikkel i tidsskrift (Fagfellevurdert)

A dusty electronegative plasma containing Boltzmann electrons, Boltzmann negative ions, cold mobile positive ions, and negatively charged stationary dust has been considered. The basic features of arbitrary amplitude solitary waves (SWs) and double layers (DLs), which have been found to exist in such a dusty electronegative plasma, have been investigated by the pseudopotential method. The small amplitude limit has also been considered in order to study the small amplitude SWs and DLs analytically. It has been shown that under certain conditions, DLs do not exist, which is in good agreement with the experimental observations of Ghim and Hershkowitz [Y. Ghim (Kim) and N. Hershkowitz, Appl. Phys. Lett. 94, 151503 (2009)].

• 30.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Magnetosonic solitons in a fermionic quantum plasma2007Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 76, nr 6:2, artikkel-id 067401Artikkel i tidsskrift (Fagfellevurdert)

Starting from the governing equations for a quantum magnetoplasma including the quantum Bohm potential and electron spin-1/2 effects, we show that the system of quantum magnetohydrodynamic (QMHD) equations admits rarefactive solitons due to the balance between nonlinearities and quantum diffraction and tunneling effects. It is found that the electron spin-1/2 effect introduces a pressurelike term with negative sign in the QMHD equations, which modifies the shape of the solitary magnetosonic waves and makes them wider and shallower. Numerical simulations of the time-dependent system shows the development of rarefactive QMHD solitary waves that are modified by the spin effects.

• 31. Masoud, Waqas
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Institut für Theoretische Physik IV, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, Germany .
Electromagnetic wave equations for relativistically degenerate quantum magnetoplasmas2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 81, nr 6, s. 066401-5 pagesArtikkel i tidsskrift (Fagfellevurdert)

A generalized set of nonlinear electromagnetic quantum hydrodynamic (QHD) equations is derived for a magnetized quantum plasma, including collisional, electron spin-1/2, and relativistically degenerate electron pressure effects that are relevant for dense astrophysical systems, such as white dwarfs. For illustrative purposes, linear dispersion relations are derived for one-dimensional magnetoacoustic waves for a collisionless nonrelativistic degenerate gas in the presence of the electron spin-1/2 contribution and for magnetoacoustic waves in a plasma containing relativistically degenerate electrons. It is found that both the spin and relativistic degeneracy at high densities tend to slow down the magnetoacoustic wave due to the Pauli paramagnetic effect and relativistic electron mass increase. The present study outlines the theoretical framework for the investigation of linear and nonlinear behaviors of electromagnetic waves in dense astrophysical systems. The results are applied to calculate the magnetoacoustic speeds for both the nonrelativistic and relativistic electron degeneracy cases typical for white dwarf stars.

• 32.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Department of Molecular Science and Technology, Ajou University, Suwon 442-749, Korea.
Dynamic critical behavior of the XY model in small-world networks2003Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 67, nr 3, s. 036118-Artikkel i tidsskrift (Fagfellevurdert)

The critical behavior of the XY model on small-world network is investigated by means of dynamic Monte Carlo simulations. We use the short-time relaxation scheme, i.e., the critical behavior is studied from the nonequilibrium relaxation to equilibrium. Static and dynamic critical exponents are extracted through the use of the dynamic finite-size scaling analysis. It is concluded that the dynamic universality class at the transition is of the mean-field nature. We also confirm numerically that the value of dynamic critical exponent is independent of the rewiring probability P for P≳0.03.

• 33.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Analytic results for the percolation transitions of the enhancedbinary tree2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 82, s. 011113-Artikkel i tidsskrift (Fagfellevurdert)

Percolation for a planar lattice has a single percolation threshold, whereaspercolation for a negatively curved lattice displays two separate thresholds.The enhanced binary tree (EBT) can be viewed as aprototype model displaying two separate percolation thresholds.We present an analytic result for the EBT model which givestwo critical percolation threshold probabilities,$p_{c1}=\frac{1}{2}\sqrt{13}-\frac{3}{2}$ and $p_{c2}=1/2$,and yields a size-scaling exponent $\Phi =\ln\left[\frac{p(1+p)}{1-p(1-p)}\right]/\ln 2$. It is inferred that the twothresholdvalues give exact upper limits and that $p_{c1}$ is furthermore exact. Inaddition, we argue that $p_{c2}$ is also exact. The physics of the model andthe results are described within themidpoint-percolation concept: Monte Carlo simulations are presented for thenumber of boundary points which are reached from the midpoint, and theresults are compared to the number of routes from the midpoint to theboundary given by the analytic solution. These comparisonsprovide a more precise physical picture of what happens at the transitions.Finally, the results are compared to related works, in particular, the Cayleytree and Monte Carlo results for hyperbolic lattices as well as earlierresults for the EBT model. It disproves a conjecture that the EBT has anexact relation to the thresholds of its dual lattice.

• 34.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Localized whistlers in magnetized spin quantum plasmas2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 82, s. 056406-Artikkel i tidsskrift (Fagfellevurdert)

The nonlinear propagation of electromagnetic (EM) electron-cyclotron waves (whistlers) along an external magnetic field, and their modulation by electrostatic small but finite amplitude ion-acoustic density perturbations are investigated in a uniform quantum plasma with intrinsic spin of electrons. The effects of the quantum force associated with the Bohm potential and the combined effects of the classical as well as the spin-induced ponderomotive forces (CPF and SPF, respectively) are taken into consideration. The latter modify the local plasma density in a self-consistent manner. The coupled modes of wave propagation is shown to be governed by a modified set of nonlinear Schrödinger-Boussinesq-like equations which admit exact solutions in form of stationary localized envelopes. Numerical simulation reveals the existence of large-scale density fluctuations that are self-consistently created by the localized whistlers in a strongly magnetized high density plasma. The conditions for the modulational instability (MI) and the value of its growth rate are obtained. Possible applications of our results, e.g., in strongly magnetized dense plasmas and in the next generation laser-solid density plasma interaction experiments are discussed.

• 35.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Asymmetric velocity correlations in shearing media2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 82, nr 3, s. 031303-Artikkel i tidsskrift (Fagfellevurdert)

A model of soft frictionless disks in two dimensions at zero temperature is  simulated with a shearing dynamics to study various kinds of asymmetries in  sheared systems. We examine both single particle properties, the spatial  velocity correlation function, and a correlation function designed to  separate clockwise and counter-clockwise rotational fields from one  another. Among the rich and interesting behaviors we find that the velocity  correlation along the two different diagonals corresponding to compression and  dilation, respectively, are almost identical and, furthermore, that a feature  in one of the correlation functions is directly related to irreversible  plastic events.

• 36.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Diffusion and velocity auto-correlation at the jamming transition2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 81, nr 4, s. 040301-Artikkel i tidsskrift (Fagfellevurdert)

We perform numerical simulations to examine particle diffusion at steady shear  in a soft-disk model in two dimensions and zero temperature around the jamming  density. We find that the diffusion constant depends on shear rate as  D\sim\dot\gamma below jamming and as D\sim\dot\gamma^{q_D} with q_D<1 at the  transition, and set out to relate this to properties of the velocity  auto-correlation function. It is found that this correlation function is  governed by two processes with different time scales. The first time scale,  the inverse of the externally applied shear rate, controls an exponential  decay of the correlations whereas the second time scale, equal to the inverse  shear stress, governs an algebraic decay with time. The obtained value of  q_D is related to these properties of the correlation function.

• 37. Rong, Zhi-Hai
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Emergence of cooperation through coevolving time scale in spatial prisoner's dilemma2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 82, nr 2, s. 026101-Artikkel i tidsskrift (Fagfellevurdert)

We study evolutionary prisoner’s dilemma game by considering adaptive strategy-selection time scale among individuals according to a “win-slower, lose-faster” rule: if an individual successfully resists the invasion of an opponent, she is prone to hold her strategy for longer time through decreasing her strategy-selection time scale; otherwise, she increases the time scale because of losing. We find that the greater the losers increase their strategy-selection time scales, the better for cooperation. Interestingly, optimal cooperation can be induced by proper adaptive rate in the strategy-selection time scale. Our results may have potential implications in the design of consensus protocol in multiagent systems.

• 38.
Niels Bohr Institute.
Niels Bohr Institute. Niels Bohr Institute. Niels Bohr Institute.
Network models of phage-bacteria coevolution2006Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 74, s. 066105-Artikkel i tidsskrift (Fagfellevurdert)

Bacteria and their bacteriophages are the most abundant, widespread, and diverse groups of biological entities on the planet. In an attempt to understand how the interactions between bacteria, virulent phages, and temperate phages might affect the diversity of these groups, we developed a stochastic network model for examining the coevolution of these ecologies. In our approach, nodes represent whole species or strains of bacteria or phages, rather than individuals, with “speciation” and extinction modeled by duplication and removal of nodes. Phage-bacteria links represent host-parasite relationships and temperate-virulent phage links denote prophage-encoded resistance. The effect of horizontal transfer of genetic information between strains was also included in the dynamical rules. The observed networks evolved in a highly dynamic fashion but the ecosystems were prone to collapse (one or more entire groups going extinct). Diversity could be stably maintained in the model only if the probability of speciation was independent of the diversity. Such an effect could be achieved in real ecosystems if the speciation rate is primarily set by the availability of ecological niches.

• 39.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. NORDITA, Blegdamsvej 17, Dk 2100, Copenhagen, Denmark.
Searchability of networks2005Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 72, nr 4, s. 046117-046125Artikkel i tidsskrift (Fagfellevurdert)

We investigate the searchability of complex systems in terms of their interconnectedness. Associating searchability with the number and size of branch points along the paths between the nodes, we find that scale-free networks are relatively difficult to search, and thus that the abundance of scale-free networks in nature and society may reflect an attempt to protect local areas in a highly interconnected network from nonrelated communication. In fact, starting from a random node, real-world networks with higher order organization like modular or hierarchical structure are even more difficult to navigate than random scale-free networks. The searchability at the node level opens the possibility for a generalized hierarchy measure that captures both the hierarchy in the usual terms of trees as in military structures, and the intrinsic hierarchical nature of topological hierarchies for scale-free networks as in the Internet.

• 40.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Navigating networks with limited information2005Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 71, nr 6, s. 066111-Artikkel i tidsskrift (Fagfellevurdert)

We study navigation with limited information in networks and demonstrate that many real-world networks have a structure which can be described as favoring communication at short distance at the cost of constraining communication at long distance. This feature, which is robust and more evident with limited than with complete information, reflects both topological and possibly functional design characteristics. For example, the characteristics of the networks studied derived from a city and from the Internet are manifested through modular network designs. We also observe that directed navigation in typical networks requires remarkably little information on the level of individual nodes. By studying navigation or specific signaling, we take a complementary approach to the common studies of information transfer devoted to broadcasting of information in studies of virus spreading and the like.

• 41.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Modeling self-organization of communication and topology in social networks2006Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 74, nr 1, s. 016108-Artikkel i tidsskrift (Fagfellevurdert)

This paper introduces a model of self-organization between communication and topology in social networks, with a feedback between different communication habits and the topology. To study this feedback, we let agents communicate to build a perception of a network and use this information to create strategic links. We observe a narrow distribution of links when the communication is low and a system with a broad distribution of links when the communication is high. We also analyze the outcome of chatting, cheating, and lying, as strategies to get better access to information in the network. Chatting, although only adopted by a few agents, gives a global gain in the system. Contrary, a global loss is inevitable in a system with too many liars.

• 42.
Department of Biology, University of Washington, Seattle, Washington 98195-1800, USA.
Niels Bohr Institute.
Reinforced communication and social navigation generate groups in model networks2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 79, nr 2, s. 026111-026118Artikkel i tidsskrift (Fagfellevurdert)

To investigate the role of information flow in group formation, we introduce a model of communication and social navigation. We let agents gather information in an idealized network society and demonstrate that heterogeneous groups can evolve without presuming that individuals have different interests. In our scenario, individuals’ access to global information is constrained by local communication with the nearest neighbors on a dynamic network. The result is reinforced interests among like-minded agents in modular networks; the flow of information works as a glue that keeps individuals together. The model explains group formation in terms of limited information access and highlights global broadcasting of information as a way to counterbalance this fragmentation. To illustrate how the information constraints imposed by the communication structure affects future development of real-world systems, we extrapolate dynamics from the topology of four social networks.

• 43. Salimullah, M.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Dust-lower-hybrid drift instabilities with dust charge fluctuations in an inhomogeneous dusty magnetoplasma2004Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 70, s. 26404-Artikkel i tidsskrift (Fagfellevurdert)
• 44.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Department of Applied Physics, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan. BK21 Physics Research Division and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea.
Phase transition of q-state clock models on heptagonal lattices2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 80, nr 1, s. 011133-Artikkel i tidsskrift (Fagfellevurdert)

We study the q-state clock models on heptagonal lattices assigned on a negatively curved surface. We show that the system exhibits three classes of equilibrium phases; in between ordered and disordered phases, an intermediate phase characterized by a diverging susceptibility with no magnetic order is observed at every q ≥ 2. The persistence of the third phase for all q is in contrast with the disappearance of the counterpart phase in a planar system for small q, which indicates the significance of nonvanishing surface-volume ratio that is peculiar in the heptagonal lattice. Analytic arguments based on Ginzburg-Landau theory and generalized Cayley trees make clear that the two-stage transition in the present system is attributed to an energy gap of spin-wave excitations and strong boundary-spin contributions. We further demonstrate that boundary effects breaks the mean-field character in the bulk region, which establishes the consistency with results of clock models on boundary-free hyperbolic lattices.

• 45.
Department of Physics and Center for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville, Huntsville, Alabama 35899, USA.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Numerical study of the magnetic electron drift vortex mode turbulence in a nonuniform magnetoplasma2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 79, nr 6, s. 066404-066410Artikkel i tidsskrift (Fagfellevurdert)

Asimulation study of the magnetic electron drift vortex (MEDV) modeturbulence in a magnetoplasma in the presence of inhomogeneities inthe plasma temperature and density, as well as in theexternal magnetic field, is presented. The study shows that theinfluence of the magnetic field inhomogeneity is to suppress streamerlikestructures observed in previous simulation studies without background magnetic fields.The MEDV mode turbulence exhibits nonuniversal (non-Kolmogorov-type) spectra for differentsets of the plasma parameters. In the presence of aninhomogeneous magnetic field, the spectrum changes to a 7/3 powerlaw, which is flatter than without magnetic field gradients. Therelevance of this work to laser-produced plasmas in the laboratoryis briefly mentioned.

• 46.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Institut für Theoretische Physik IV, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, D-44780 Bochum, Germany. Department of Physics, Linköping University,SE-58183 Linköping, Sweden.
Magnetization of a warm plasma by the nonstationary ponderomotive force of an electromagnetic wave2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 80, s. 027401-Artikkel i tidsskrift (Fagfellevurdert)

It is shown that magnetic fields can be generated in a warm plasma by the nonstationary ponderomotive force of a large-amplitude electromagnetic wave. In the present Brief Report, we derive simple and explicit results that can be useful for understanding the origin of the magnetic fields that are produced in intense laser-plasma interaction experiments.

• 47.
Joint Institute of High Temperatures, Russian Academy of Sciences, Moscow, Russian Federation.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Superluminal tunneling of microwaves in smoothly varying transmission lines2008Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 78, nr 1, s. 016601-Artikkel i tidsskrift (Fagfellevurdert)

Tunneling of microwaves through a smooth barrier in a transmission line is considered. In contrast to standard wave barriers, we study the case where the dielectric permittivity is positive, and the barrier is caused by the inhomogeneous dielectric profile. It is found that reflectionless, superluminal tunneling can take place for waves with a finite spectral width. The consequences of these findings are discussed, and an experimental setup testing our predictions is proposed.

• 48.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Chalmers, Dept Appl Mech, S-41296 Gothenburg, Sweden.
Different stages of flame acceleration from slow burning to Chapman-Jouguet deflagration2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 80, nr 3, s. 036317-Artikkel i tidsskrift (Fagfellevurdert)

Numerical simulations of spontaneous flame acceleration are performed within the problem of flame transition to detonation in two-dimensional channels. The acceleration is studied in the extremely wide range of flame front velocity changing by 3 orders of magnitude during the process. Flame accelerates from realistically small initial velocity (with Mach number about 10−3) to supersonic speed in the reference frame of the tube walls. It is shown that flame acceleration undergoes three distinctive stages: (1) initial exponential acceleration in the quasi-isobaric regime, (2) almost linear increase in the flame speed to supersonic values, and (3) saturation to a stationary high-speed deflagration velocity. The saturation velocity of deflagration may be correlated with the Chapman-Jouguet deflagration speed. The acceleration develops according to the Shelkin mechanism. Results on the exponential flame acceleration agree well with previous theoretical and numerical studies. The saturation velocity is in line with previous experimental results. Transition of flame acceleration regime from the exponential to the linear one, and then to the constant velocity, happens because of gas compression both ahead and behind the flame front.

• 49.
Institute of Theoretical Physics, Lanzhou University, Lanzhou Gansu, China.
Department of Mathematics, College of Science, Shanghai University, Shanghai, China. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Institute of Theoretical Physics, Lanzhou University, Lanzhou Gansu, China.
Influence of synaptic interaction on firing synchronization and spike death in excitatory neuronal networks2008Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 78, nr 6, s. 061906-Artikkel i tidsskrift (Fagfellevurdert)

We investigate the influence of efficacy of synaptic interaction on firing synchronization in excitatoryneuronal networks. We find spike death phenomena: namely, the state of neurons transits from the limit cycleto a fixed point or transient state. The phenomena occur under the perturbation of an excitatory synapticinteraction, which has a high efficacy. We show that the decrease of synaptic current results in spike deaththrough depressing the feedback of the sodium ionic current. In the networks with the spike death property thedegree of synchronization is lower and insensitive to the heterogeneity of neurons. The mechanism of theinfluence is that the transition of the neuron state disrupts the adjustment of the rhythm of the neuronsoscillation and prevents a further increase of the firing synchronization.

• 50.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Effects of strategy-migration direction and noise in the evolutionary spatial prisoner's dilemma2009Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 80, nr 2, s. 026108-Artikkel i tidsskrift (Fagfellevurdert)

Spatialgames are crucial for understanding patterns of cooperation in nature(and to some extent society). They are known to bemore sensitive to local symmetries than, e.g., spin models. Thispaper concerns the evolution of the prisoner's dilemma game onregular lattices with three different types of neighborhoods—the von Neumann,Moore, and kagomé types. We investigate two kinds of dynamicsfor the players to update their strategies (that can beunconditional cooperator or defector). Depending on the payoff difference, anindividual can adopt the strategy of a random neighbor [avoter-model-like dynamics (VMLD)] or impose its strategy on a randomneighbor, i.e., invasion-process-like dynamics (IPLD). In particular, we focus onthe effects of noise, in combination with the strategy dynamics,on the evolution of cooperation. We find that VMLD, comparedto IPLD, better supports the spreading and sustaining of cooperation.We see that noise has nontrivial effects on the evolutionof cooperation: maximum cooperation density can be realized either ata medium noise level, in the limit of zero noiseor in both these regions. The temptation to defect andthe local interaction structure determine the outcome. Especially, in thelow noise limit, the local interaction plays a crucial rolein determining the fate of cooperators. We elucidate these bothby numerical simulations and mean-field cluster approximation methods.

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