| 1. |
- Kim, Joon Tae, et al.
(författare)
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Dual antiplatelet Use for extended period taRgeted to AcuTe ischemic stroke with presumed atherosclerotic OrigiN (DURATION) trial : Rationale and design
- 2023
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Ingår i: International Journal of Stroke. - : SAGE Publications. - 1747-4930 .- 1747-4949. ; 18:8, s. 1015-1020
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Tidskriftsartikel (refereegranskat)abstract
- Rationale: The optimal duration of dual antiplatelet therapy (DAPT) with clopidogrel-aspirin for the large artery atherosclerotic (LAA) stroke subtype has been debated. Aims: To determine whether the 1-year risk of recurrent vascular events could be reduced by a longer duration of DAPT in patients with the LAA stroke subtype. Methods and study design: A total of 4806 participants will be recruited to detect a statistically significant relative risk reduction of 22% with 80% power and a two-sided alpha error of 0.05, including a 10% loss to follow-up. This is a registry-based, multicenter, prospective, randomized, open-label, blinded end point study designed to evaluate the efficacy and safety of a 12-month duration of DAPT compared with a 3-month duration of DAPT in the LAA stroke subtype. Patients will be randomized (1:1) to either DAPT for 12 months or DAPT for 3 months, followed by monotherapy (either aspirin or clopidogrel) for the remaining 9 months. Study outcomes: The primary efficacy outcome of the study is a composite of stroke (ischemic or hemorrhagic), myocardial infarction, and all-cause mortality for 1 year after the index stroke. The secondary efficacy outcomes are (1) stroke, (2) ischemic stroke or transient ischemic attack, (3) hemorrhagic stroke, and (4) all-cause mortality. The primary safety outcome is major bleeding. Discussion: This study will help stroke physicians determine the appropriate duration of dual therapy with clopidogrel-aspirin for patients with the LAA stroke subtype. Trial registration: URL: https://cris.nih.go.kr/cris. CRIS Registration Number: KCT0004407.
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| 2. |
- Jo, Min-Seung, et al.
(författare)
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Aligned CuO nanowire array for a high performance visible light photodetector
- 2022
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- Recently, copper oxide (CuO) has drawn much attention as a promising material in visible light photodetection with its advantages in ease of nanofabrication. CuO allows a variety of nanostructures to be explored to enhance the optoelectrical performance such as photogenerated carriers scattering and bandgap engineering. However, previous researches neglect in-depth analysis of CuO's light interaction effects, restrictively using random orientation such as randomly arranged nanowires, single nanowires, and dispersed nanoparticles. Here, we demonstrate an ultra-high performance CuO visible light photodetector utilizing perfectly-aligned nanowire array structures. CuO nanowires with 300 nm-width critical dimension suppressed carrier transport in the dark state and enhanced the conversion of photons to carriers; additionally, the aligned arrangement of the nanowires with designed geometry improved the light absorption by means of the constructive interference effect. The proposed nanostructures provide advantages in terms of dark current, photocurrent, and response time, showing unprecedentedly high (state-of-the-art) optoelectronic performance, including high values of sensitivity (S = 172.21%), photo-responsivity (R = 16.03 A/W, lambda = 535 nm), photo-detectivity (D* = 7.78 x 10(11) Jones), rise/decay time (tau(r)/tau(d) = 0.31 s/1.21 s).
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| 3. |
- Kim, Beom Jun, et al.
(författare)
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Self organized scale-free networks from merging and regeneration
- 2005
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Ingår i: European Physical Journal B. - Berlin / Heidelberg : Springer. - 1434-6028 .- 1434-6036. ; 43:3, s. 369-372
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Tidskriftsartikel (refereegranskat)abstract
- We propose that the ubiquitous scale free nature of many real world networks may emerge from a steady state process where nodes are created and merged randomly. The merging may be viewed as an optimization of efficiency by minimizing redundancy.
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| 4. |
- Zhang, Tiankai, et al.
(författare)
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Ion-modulated radical doping of spiro-OMeTAD for more efficient and stable perovskite solar cells
- 2022
-
Ingår i: Science. - : AMER ASSOC ADVANCEMENT SCIENCE. - 0036-8075 .- 1095-9203. ; 377:6605, s. 495-501
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Tidskriftsartikel (refereegranskat)abstract
- Record power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have been obtained with the organic hole transporter 2,2,7,7-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9-spirobifluorene (spiro-OMeTAD). Conventional doping of spiro-OMeTAD with hygroscopic lithium salts and volatile 4-tert-butylpyridine is a time-consuming process and also leads to poor device stability. We developed a new doping strategy for spiro-OMeTAD that avoids post-oxidation by using stable organic radicals as the dopant and ionic salts as the doping modulator (referred to as ion-modulated radical doping). We achieved PCEs of >25% and much-improved device stability under harsh conditions. The radicals provide hole polarons that instantly increase the conductivity and work function (WF), and ionic salts further modulate the WF by affecting the energetics of the hole polarons. This organic semiconductor doping strategy, which decouples conductivity and WF tunability, could inspire further optimization in other optoelectronic devices.
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| 5. |
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| 6. |
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| 7. |
- Baek, Seung Ki, 1979-, et al.
(författare)
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Comment on `Six-state clock model on the square lattice: Fisher zeroapproach with Wang-Landau sampling'
- 2010
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 81:6, s. 3101-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 8. |
- Baek, Seung Ki, et al.
(författare)
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Critical condition of the water-retention model
- 2012
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : American Physical Society. - 1539-3755 .- 1550-2376. ; 85:3, s. 032103-
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Tidskriftsartikel (refereegranskat)abstract
- We study how much water can be retained without leaking through boundarieswhen each unit square of a two-dimensional lattice is randomly assigned a blockof unit bottom area but with different heights from zero to $n-1$.As more blocks are put into the system,there exists a phase transition beyond whichthe system retains a macroscopic volume of water.We locate the critical points and verify that the criticalitybelongs to the two-dimensional percolation universality class.If the height distribution can be approximated as continuous for large $n$,the system is always close to a critical point and the fraction of the areabelow the resulting water level is given by the percolation threshold.This provides a universal upper bound ofareas that can be covered by water in a random landscape.
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| 9. |
- Baek, Seung Ki, et al.
(författare)
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Critical temperatures of the three- and four-state Potts models on the kagome lattice
- 2011
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : American Physical Society. - 1063-651X .- 1095-3787. ; 83:6, s. 061104-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Baek, Seung Ki, 1979-, et al.
(författare)
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Curvature-induced frustration in the XY model on hyperbolic surfaces
- 2009
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Ingår i: Physical Review E - Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755. ; 79:6, s. 060106(R)-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 11. |
- Baek, Seung Ki, et al.
(författare)
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Dworkin's Paradox
- 2012
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Ingår i: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 7:6, s. e38529-
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Tidskriftsartikel (refereegranskat)abstract
- How to distribute welfare in a society is a key issue in the subject of distributional justice, which is deeply involved with notions of fairness. Following a thought experiment by Dworkin, this work considers a society of individuals with different preferences on the welfare distribution and an official to mediate the coordination among them. Based on a simple assumption that an individual's welfare is proportional to how her preference is fulfilled by the actual distribution, we show that an egalitarian preference is a strict Nash equilibrium and can be favorable even in certain inhomogeneous situations. These suggest how communication can encourage and secure a notion of fairness.
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| 12. |
- Baek, Seung Ki, 1979-, et al.
(författare)
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Flow improvement caused by agents who ignore traffic rules
- 2009
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Ingår i: Physical Review E - Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755. ; 80:1, s. 016111-
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Tidskriftsartikel (refereegranskat)abstract
- 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), and the traffic flow, resulting from the interaction between these two types of agents, which 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 small fraction of ignorers are present within a majority of abiders. The characteristic features for the spatial structure of the flow pattern are obtained and discussed.
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| 13. |
- Baek, Seung Ki, et al.
(författare)
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Ising model on a hyperbolic plane with a boundary
- 2011
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : American Physical Society. - 1539-3755 .- 1550-2376. ; 84, s. 032103-
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Tidskriftsartikel (refereegranskat)abstract
- A hyperbolic plane can be modeled by a structure called the enhanced binarytree. We study the ferromagnetic Ising model on top of the enhanced binarytree using the renormalization-group analysis in combination withtransfer-matrix calculations. We find a reasonable agreement with MonteCarlo calculations on the transition point, and the resulting criticalexponents suggest the mean-field surface critical behavior.
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| 14. |
- Baek, Seung Ki, et al.
(författare)
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Kosterlitz-Thouless transition of magnetic dipoles on thetwo-dimensional plane
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : APS. - 1098-0121 .- 1550-235X. ; 83:18, s. 184409-5 p
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Tidskriftsartikel (refereegranskat)abstract
- The universality class of a phase transition is often determined by factorslike dimensionality and inherent symmetry. We study the magneticdipole system in which the ground-state symmetry and the underlying latticestructure are coupled to each other in an intricate way. A two-dimensional(2D)square-lattice system of magnetic dipoles undergoes an order-disorder phasetransition belonging to the 2D Ising universality class.According to Prakash and Henley [Phys. Rev. B {\bf 42}, 6572 (1990)], this can berelated to the fourfold-symmetric ground states which suggests a similarityto the four-state clock model. Provided that this type ofsymmetry connection holds true, the magnetic dipoles on a honeycomb lattice,which possess sixfold-symmetric ground states, should exhibit aKosterlitz-Thouless transition in accordance with the six-state clock model.This is verified through numerical simulations in the present investigation.However, it is pointed out that this symmetry argument does not alwaysapply, which suggests that factors other than symmetry can be decisive forthe universality class of the magnetic dipole system.
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| 15. |
- Baek, Seung Ki, 1979-, et al.
(författare)
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Numerical Study of Game Theory
- 2010
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Ingår i: New Physics. - Seoul : The Korean Physical Society. - 0374-4914. ; 60:9, s. 943-951
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Game-theoretic approach has been providing a powerful tool in qualitative understanding of macroscopic social phenomena in social sciences, e.g., in economics and political science. Recently, researchers in physics, especially in statistical physics, use these game-theoretic approaches but in more quantitative way and have been producing a variety of interesting results in the new research area called ’sociophysics’ by studying human society as a complex system. This work introduces recent works that have tackled combinatorial complexities arising in game-theoretic studies with the aid of simplified assumptions and numerical computations. We first show how cooperation emerges in the prisoner’s dilemma game when each player’s memory capacity is enhanced and suggest that the intelligent tit-for-tat strategy plays a crucial role in the history of cooperation. And then it is numerically shown that there is a certain case of simultaneous coordination among many players where the system has a high risk of failure when everyone is willing to follow the coordination, which is actually higher than when some are not concerned about it. Lastly, we discuss mathematical treatment of an equilibrium solution for a reverse auction game, which is a variant of the minority game, and its computational approach.
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| 16. |
- Baek, Seung Ki, et al.
(författare)
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Percolation in hyperbolic lattices
- 2009
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : American Physical Society. - 1063-651X .- 1095-3787. ; 79:1, s. 011124-011130
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 17. |
- Baek, Seung Ki, et al.
(författare)
-
Quantum Monte Carlo study of the transverse-field quantum Ising modelon infinite-dimensional structures
- 2011
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 84, s. 174419-
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Tidskriftsartikel (refereegranskat)abstract
- In a number of classical statistical-physical models, there exists acharacteristic dimensionality called the upper critical dimension abovewhich one observes the mean-field critical behavior. Instead of constructinghigh-dimensional lattices, however, one can also considerinfinite-dimensional structures, and the question is whether this mean-fieldcharacter extends to quantum-mechanical cases as well. We thereforeinvestigate the transverse-field quantum Ising model on the globally couplednetwork and on the Watts-Strogatz small-world network by means of quantum MonteCarlo simulations and the finite-size scaling analysis. We confirm that bothof the structures exhibit critical behavior consistent with the mean-fielddescription. In particular, we show that the existing cumulant methodhas difficulty in estimating the correct dynamic critical exponent andsuggest that an order parameter based on the quantum-mechanicalexpectation value can be a practically useful numerical observable todetermine critical behavior when there is no well-defined dimensionality.
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| 18. |
- Baek, Seung Ki, et al.
(författare)
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Residual discrete symmetry of the five-state clock model
- 2013
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 88:1
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Tidskriftsartikel (refereegranskat)abstract
- It is well known that the q-state clock model can exhibit a Kosterlitz-Thouless (KT) transition if q is equal to or greater than a certain threshold, which has been believed to be five. However, recent numerical studies indicate that helicity modulus does not vanish in the high-temperature phase of the five-state clock model as predicted by the KT scenario. By performing Monte Carlo calculations under the fluctuating twist boundary condition, we show that it is because the five-state clock model does not have the fully continuous U(1) symmetry even in the high-temperature phase while the six-state clock model does. We suggest that the upper transition of the five-state clock model is actually a weaker cousin of the KT transition so that it is q≥6 that exhibits the genuine KT behavior
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| 19. |
- Baek, Seung Ki, 1979-, et al.
(författare)
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Surface and bulk criticality in midpoint percolation
- 2010
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 81:4, s. 1108-6 sidor
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 20. |
- Baek, Seung Ki, et al.
(författare)
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The Ten Thousand Kims
- 2011
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Ingår i: New Journal of Physics. - : Institute of Physics. - 1367-2630. ; 13, s. 073036-
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Tidskriftsartikel (refereegranskat)abstract
- 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 name``Kim'' 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.
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| 21. |
- Baek, Seung Ki, et al.
(författare)
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True and quasi long-range order in the generalized q-state clock model
- 2009
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 80, s. 060101-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 22. |
- Do Yi, Su, et al.
(författare)
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Human bipedalism and body-mass index
- 2017
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 7
-
Tidskriftsartikel (refereegranskat)abstract
- Body-mass index, abbreviated as BMI and given by M/H2 with the mass M and the height H, has been widely used as a useful proxy to measure a general health status of a human individual. We generalise BMI in the form of M/Hp and pursue to answer the question of the value of p for populations of animal species including human. We compare values of p for several different datasets for human populations with the ones obtained for other animal populations of fish, whales, and land mammals. All animal populations but humans analyzed in our work are shown to have p ≈ 3 unanimously. In contrast, human populations are different: As young infants grow to become toddlers and keep growing, the sudden change of p is observed at about one year after birth. Infants younger than one year old exhibit significantly larger value of p than two, while children between one and five years old show p ≈ 2, sharply different from other animal species. The observation implies the importance of the upright posture of human individuals. We also propose a simple mechanical model for a human body and suggest that standing and walking upright should put a clear division between bipedal human (p ≈ 2) and other animals (p ≈ 3).
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| 23. |
- Festin, Örjan, et al.
(författare)
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Vortex Fluctuations in High-Tc Films : Flux Noise Spectrum and Complex Impedance
- 1999
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 83:26, s. 5567-5570
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Tidskriftsartikel (refereegranskat)abstract
- The flux noise spectrum and complex impedance for a 500 Å thick YBCO film are measured and comparedwith predictions for two-dimensional vortex fluctuations. It is verified that the complex impedanceand the flux noise spectra are proportional to each other, that the logarithm of the flux noise spectrafor different temperatures has a common tangent with slope 21, and that the amplitude of the noisedecreases as d23, where d is the height above the film at which the magnetic flux is measured. Acrossover from normal to anomalous vortex diffusion is indicated by the measurements and is discussedin terms of a two-dimensional decoupling.
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| 24. |
- Grönlund, Andreas, et al.
(författare)
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Fractal Profit Landscape of the Stock Market
- 2012
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Ingår i: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 7:4, s. e33960-
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the structure of the profit landscape obtained from the most basic, fluctuation based, trading strategy applied for the daily stock price data. The strategy is parameterized by only two variables, p and q Stocks are sold and bought if the log return is bigger than p and less than -q, respectively. Repetition of this simple strategy for a long time gives the profit defined in the underlying two-dimensional parameter space of p and q. It is revealed that the local maxima in the profit landscape are spread in the form of a fractal structure. The fractal structure implies that successful strategies are not localized to any region of the profit landscape and are neither spaced evenly throughout the profit landscape, which makes the optimization notoriously hard and hypersensitive for partial or limited information. The concrete implication of this property is demonstrated by showing that optimization of one stock for future values or other stocks renders worse profit than a strategy that ignores fluctuations, i.e., a long-term buy-and-hold strategy.
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| 25. |
- Holme, Petter, et al.
(författare)
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Attack vulnerability of complex networks
- 2002
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 65:5, s. 056109-
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Tidskriftsartikel (refereegranskat)abstract
- We study the response of complex networks subject to attacks on vertices and edges. Several existing complex network models as well as real-world networks of scientific collaborations and Internet traffic are numerically investigated, and the network performance is quantitatively measured by the average inverse geodesic length and the size of the largest connected subgraph. For each case of attacks on vertices and edges, four different attacking strategies are used: removals by the descending order of the degree and the betweenness centrality, calculated for either the initial network or the current network during the removal procedure. It is found that the removals by the recalculated degrees and betweenness centralities are often more harmful than the attack strategies based on the initial network, suggesting that the network structure changes as important vertices or edges are removed. Furthermore, the correlation between the betweenness centrality and the degree in complex networks is studied.
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| 26. |
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| 27. |
- Holme, Petter, 1973-
(författare)
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Form and function of complex networks
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Networks are all around us, all the time. From the biochemistry of our cells to the web of friendships across the planet. From the circuitry of modern electronics to chains of historical events. A network is the result of the forces that shaped it. Thus the principles of network formation can be, to some extent, deciphered from the network itself. All such information comprises the structure of the network. The study of network structure is the core of modern network science. This thesis centres around three aspects of network structure: What kinds of network structures are there and how can they be measured? How can we build models for network formation that give the structure of networks in the real world? How does the network structure affect dynamical systems confined to the networks? These questions are discussed using a variety of statistical, analytical and modelling techniques developed by physicists, mathematicians, biologists, chemists, psychologists, sociologists and anthropologists. My own research touches all three questions. In this thesis I present works trying to answer: What is the best way to protect a network against sinister attacks? How do groups form in friendship networks? Where do traffic jams appear in a communication network? How is cellular metabolism organised? How do Swedes flirt on the Internet? . . . and many other questions.
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| 28. |
- Holme, Petter, et al.
(författare)
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Growing scale-free networks with tunable clustering
- 2002
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 65:2, s. 026107-
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Tidskriftsartikel (refereegranskat)abstract
- We extend the standard scale-free network model to include a "triad formation step." We analyze the Geometric properties of networks Generated by this algorithm both analytically and by numerical calculations, and find that our model possesses the same characteristics as the standard scale-free networks such as the power-law degree distribution and the small average geodesic length, but with the high clustering at the same time. In our model, the clustering coefficient is also shown to be tunable simply by changing a control parameter-the average number of triad formation trials per time step.
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| 29. |
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| 30. |
- Holme, Petter, 1973-, et al.
(författare)
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Heterogeneous attachment strategies optimize the topology of dynamic wireless networks
- 2010
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 73:4, s. 597-604
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Tidskriftsartikel (refereegranskat)abstract
- In optimizing the topology of wireless networks built of a dynamic set of spatially embedded agents, there are many trade-offs to be dealt with. The network should preferably be as small (in the sense that the average, or maximal, pathlength is short) as possible, it should be robust to failures, not consume too much power, and so on. In this paper, we investigate simple models of how agents can choose their neighbors in such an environment. In our model of attachment, we can tune from one situation where agents prefer to attach to others in closest proximity, to a situation where agents attach to random others regardless of distance (which thus are, on average, further away than the connections to the spatial neighbors). We evaluate this scenario with several performance measures and find that the optimal topologies, for most of the quantities, is obtained for strategies resulting in a mix of most local and a few random connections.
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| 31. |
- Holme, Petter, et al.
(författare)
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Network bipartivity
- 2003
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 68:5 Pt 2
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Tidskriftsartikel (refereegranskat)abstract
- Systems with two types of agents with a preference for heterophilous interaction produce networks that are more or less close to bipartite. We propose two measures quantifying the notion of bipartivity. The two measures-one well known and natural, but computationally intractable, and the other computationally less complex, but also less intuitive-are examined on model networks that continuously interpolate between bipartite graphs and graphs with many odd circuits. We find that the bipartivity measures increase as we tune the control parameters of the test networks to intuitively increase the bipartivity, and thus conclude that the measures are quite relevant. We also measure and discuss the values of our bipartivity measures for empirical social networks (constructed from professional collaborations, Internet communities, and field surveys). Here we find, as expected, that networks arising from romantic online interaction have high, and professional collaboration networks have low, bipartivity values. In some other cases, probably due to low average degree of the network, the bipartivity measures cannot distinguish between romantic and friendship oriented interaction.
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| 32. |
- Holme, Petter, et al.
(författare)
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Network Bipartivity
- 2003
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Ingår i: Physical Review E. - 1063-651X. ; 68:5, s. 056107-
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Tidskriftsartikel (refereegranskat)
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| 33. |
- Holme, Petter, et al.
(författare)
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Phase transitions in the two-dimensional random gauge XY model
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:10
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Tidskriftsartikel (refereegranskat)abstract
- The two-dimensional random gauge XY model, where the quenched random variables are magnetic bond angles uniformly distributed within [(-)rpi,rpi](0less than or equal torless than or equal to1), is studied via Monte Carlo simulations. We investigate the phase diagram in the plane of the temperature T and the disorder strength r and infer, in contrast to a prevailing conclusion in many earlier studies, that the system is superconducting at any disorder strength r for sufficiently low T. It is also argued that the superconducting to normal transition has different nature at weak disorder and strong disorder: termed Kosterlitz-Thouless (KT) type and non-KT type, respectively. The results are compared to earlier works.
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| 34. |
- Holme, Petter, et al.
(författare)
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Prisoners' dilemma in real-world acquaintance networks : Spikes and quasiequilibria induced by the interplay between structure and dynamics
- 2003
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 68:3, s. 030901-
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Tidskriftsartikel (refereegranskat)abstract
- We study Nowak and May's spatial prisoners' dilemma game driven by mutations (random choices of suboptimal strategies) on empirical social networks. The time evolution of the cooperation level is highly complex containing spikes and steps between quasistable levels. A statistical characterization of the quasistable states and a study of the mechanisms behind the steps are given. We argue that the crucial structural ingredients causing the observed behavior is an inhomogeneous degree distribution and that the connections within vertices of highest degree are rather sparse. Based on these observations we construct model networks with a similar complex time evolution of the cooperation level.
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| 35. |
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| 36. |
- Holme, Petter, et al.
(författare)
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Vertex overload breakdown in evolving networks
- 2002
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 65:6, s. 066109-
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Tidskriftsartikel (refereegranskat)abstract
- We study evolving networks based on the Barabasi-Albert scale-free network model with vertices sensitive to overload breakdown. The load of a vertex is defined as the betweenness centrality of the vertex. Two cases of load limitation are considered, corresponding to the fact that the average number of connections per vertex is increasing with the network's size ("extrinsic communication activity"), or that it is constant ("intrinsic communication activity"). Avalanchelike breakdowns for both load limitations are observed. In order to avoid such avalanches we argue that the capacity of the vertices has to grow with the size of the system. An interesting irregular dynamics of the formation of the giant component (for the intrinsic communication activity case) is also studied. Implications on the growth of the Internet are discussed.
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| 37. |
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| 38. |
- Kim, Beom Jun, et al.
(författare)
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Dynamic instabilities induced by asymmetric influence : Prisoners' dilemma game in small-world networks
- 2002
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 66:2, s. 021907-
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Tidskriftsartikel (refereegranskat)abstract
- A two-dimensional small-world-type network, subject to spatial prisoners' dilemma dynamics and containing an influential node defined as a special node, with a finite density of directed random links to the other nodes in the network, is numerically investigated. It is shown that the degree of cooperation does not remain at a steady state level but displays a punctuated equilibrium-type behavior manifested by the existence of sudden breakdowns of cooperation. The breakdown of cooperation is linked to an imitation of a successful selfish strategy of the influential node. It is also found that while the breakdown of cooperation occurs suddenly, its recovery requires longer time. This recovery time may, depending on the degree of steady state cooperation, either increase or decrease with an increasing number of long-range connections.
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| 39. |
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| 40. |
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| 41. |
- Kim, Beom Jun, et al.
(författare)
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XY model in small-world networks
- 2001
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 64:5, s. 056135-
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Tidskriftsartikel (refereegranskat)abstract
- The phase transition in the XY model on one-dimensional small-world networks is investigated by means of Monte Carlo simulations. It is found that long-range order is present at finite temperatures, even for very small values of the rewiring probability, suggesting a finite-temperature transition for any nonzero rewiring probability. Nature of the phase transition is discussed in comparison with the globally coupled XY model.
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| 42. |
- Kim, Beom Jun, et al.
(författare)
-
XY model in small-world networks
- 2001
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Ingår i: Physical Review E. - 1063-651X. ; 64:5, s. 056135-
-
Tidskriftsartikel (refereegranskat)
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| 43. |
- Lee, Sang Hoon, 1982-, et al.
(författare)
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Neutral theory of chemical reaction networks
- 2012
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 14, s. 033032-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 44. |
- Lu, Xin, et al.
(författare)
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The sensitivity of respondent-driven sampling
- 2012
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Ingår i: Journal of the Royal Statistical Society. - 0964-1998 .- 1467-985X. ; 175:1, s. 191-216
-
Tidskriftsartikel (refereegranskat)abstract
- Researchers in many scientific fields make inferences from individuals to larger groups. For many groups, however, there is no list of members from which to draw a random sample. Respondent-driven sampling (RDS) is a relatively new sampling methodology that circumvents this difficulty by using the social networks of the groups under study. The RDS method has been shown to provide unbiased estimates of population proportions given certain conditions. The method is now widely used in human immunodeficiency virus related studies among high risk populations globally. We test the RDS methodology by simulating RDS studies on the social networks of a large Lesbian, gay, bisexual and transgender Web community. The robustness of the RDS method is tested by violating, one by one, the conditions under which the method provides unbiased estimates. Simulations indicate that the bias is large if networks are directed or respondents choose to invite people on the basis of characteristics that are correlated with the study outcomes. The bias and variance increase if participants invite close as opposed to more distant friends whereas sampling in denser networks sharply reduces variance. However, the RDS method shows strong resistance to sampling without replacement, low response rates and certain errors in the participants reporting of their network sizes, as well as the selection criteria of seeds. The effects of network structure and the number of seeds and coupons are also discussed.
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| 45. |
- Medvedyeva, Kateryna, et al.
(författare)
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Dynamic critical behavior of the XY model in small-world networks
- 2003
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : American Physical Society. - 1063-651X .- 1095-3787. ; 67:3, s. 036118-
-
Tidskriftsartikel (refereegranskat)abstract
- 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 Pgreater than or similar to0.03.
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| 46. |
- Minnhagen, Petter, et al.
(författare)
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Evidence of two distinct dynamic critical exponents in connection with vortex physics
- 2001
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 87:3
-
Tidskriftsartikel (refereegranskat)abstract
- The dynamic critical exponent z is determined from numerical simulations for the three-dimensional (3D) lattice Coulomb gas (LCG) and the 3D XY models with relaxational dynamics. It is suggested that the dynamics is characterized by two distinct dynamic critical indices z0 and z related to the divergence of the relaxation time τ by τ∝ξz0 and τ∝k-z, where ξ is the correlation length and k the wave vector. The values determined are z0≈1.5 and z≈1 for the 3D LCG and z0≈1.5 and z≈2 for the 3D XY model. Comparisons with other results are discussed.
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| 47. |
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| 48. |
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| 49. |
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| 50. |
- Minnhagen, Petter, 1946-, et al.
(författare)
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The groundstates and phases of the two-dimensional fully frustrated XY model
- 2010
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Ingår i: CONDENSED MATTER THEORIES, VOL 24. - : WORLD SCIENTIFIC. ; , s. 3-14
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Konferensbidrag (refereegranskat)abstract
- The 2D Fully Frustrated NY(FFXY) class of models is shown to contain a new ground-state in addition to the checkerboard groundstate of the standard 2D NY model. The spin configuration of this additional groundstate is obtained and its connection to a broken Z(2)-symmetry explained. This means that the class of 2D FEXY models belongs within a U(1) circle times Z(2) circle times Z(2)-symmetry phase-transition representation. The phase diagram is reviewed and the central charges of the four multicritical points described. The implications for the standard 2D FFXY-model are discussed and elucidated, in particular with respect to the long standing controversy concerning the phase transitions of the standard 2D FEXY-model.
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