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Mott-Peierls phase in deuterated copper-DCNQI systems: a comprehensive study of longitudinal and transverse conductivity and ageing effects
Marko Pinterić, Tomislav Vuletić, Martin Lončarić, Konstantin Petukhov, Boris Gorshunov, Jost Ulrich von Schütz, Silvija Tomić, Martin Dressel, 2003, izvirni znanstveni članek

Opis: Transport and low-frequency optical reflection measurements (180-380 GHz) are reported for the quasi-three-dimensional conducting alloy ▫$Cu[(2,5(CH_3)_2-DCNQI)_{0.70}(2,5(CD_3)_2-DCNQI)_{0.30}]_2$▫ between room temperature and 35 K. The optical properties of the system are strongly anisotropic. It is metallic down to 60 K where a Mott-Peierls phase transition occurs. While the transverse conductivity remains practically unchanged, the longitudinal conductivity abruptly drops at the phase transition. Comparing our latest results with previous dc data and measurements of the microwave conductivity also reported here, we find indications of an ageing effect in these samples.
Ključne besede: optical reflection measurements, organic conductors, microwave conductivity, phase transitions metal-insulator, ageing effect
Objavljeno: 01.06.2012; Ogledov: 763; Prenosov: 46
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Finite-size effects on order reconstruction around nematic defects
Samo Kralj, Riccardo Rosso, Epifanio G. Virga, 2010, izvirni znanstveni članek

Opis: By use of the Landau-de Gennes phenomenological theory, we study the texture of a nematic liquid crystal confined within a hybrid cell. Precisely, we consider cylindrically symmetric solutions containing topological defects dictated by appropriate boundary conditions. We focus our attention on cells whose dimensions are comparable with the biaxial correlation length ▫$xi_b$▫. For such severe confinements the order reconstruction (OR) configuration could be stable. Its structural details reflect the balance among boundary-enforced frustration, elastic penalties, and finite-size effects. In particular, we analyze the interplay between finite-size effects and topological defects. We show that defects are always pinned to the negatively (planar) uniaxial sheet of the OR structure. The presence of a ring defect can dramatically increase the critical threshold below which the OR structure is stable.
Ključne besede: physics, liquid crystals, nematic crystals, nematic defects, structural transitions
Objavljeno: 07.06.2012; Ogledov: 1037; Prenosov: 60
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Influence of a random field on particle fractionation and solidification in liquid-crystal colloid mixtures
Vlad Popa-Nita, Paul van der Schoot, Samo Kralj, 2006, izvirni znanstveni članek

Opis: The influence of a random-anisotropy (RA) type disorder on the phase separation of nematogen-colloid mixtures is studied theoretically by combiningthe phenomenological Landau-de Gennes, Carnahan-Starling, and hard-sphere crystal theories. We assume that the colloids enforce the RA disorder on the surrounding thermotropic liquid-crystal (LC) molecules. We adopt the Imry-Ma argument according to which the lower-temperature phase exhibits a domain-type pattern. The colloids impose a finite degree of orientational ordering even in the isotropic (paranematic) phase. In the ordered phase they give rise to a domain-type structure, resulting in the distorted nematic (speronematic) phase. The RA field opposes the phase separation tendency. With increasing disorder the difference between the paranematic and speronematic ordering decreases. Consequently there is a critical disorder, above which both phases become identical from the orientation point of view, but have different concentrations of colloids. We have also estimated another characteristic value of disorder above which the isotropic phase can exist only in a liquid state, the crystal phase being suppressed completely.
Ključne besede: liquid crystals, transitions, segregation, mixing, random fields, solidification, complex fluids
Objavljeno: 07.06.2012; Ogledov: 758; Prenosov: 59
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Directional learning and the provisioning of public goods
Heinrich H. Nax, Matjaž Perc, 2015, izvirni znanstveni članek

Opis: We consider an environment where players are involved in a public goods game and must decide repeatedly whether to make an individual contribution or not. However, players lack strategically relevant information about the game and about the other players in the population. The resulting behavior of players is completely uncoupled from such information, and the individual strategy adjustment dynamics are driven only by reinforcement feedbacks from each player's own past. We show that the resulting "directional learning" is sufficient to explain cooperative deviations away from the Nash equilibrium. We introduce the concept of k-strong equilibria, which nest both the Nash equilibrium and the Aumann-strong equilibrium as two special cases, and we show that, together with the parameters of the learning model, the maximal k-strength of equilibrium determines the stationary distribution. The provisioning of public goods can be secured even under adverse conditions, as long as players are sufficiently responsive to the changes in their own payoffs and adjust their actions accordingly. Substantial levels of public cooperation can thus be explained without arguments involving selflessness or social preferences, solely on the basis of uncoordinated directional (mis)learning.
Ključne besede: cooperation, public goods, directional learning, phase transitions, physics of social systems
Objavljeno: 23.06.2017; Ogledov: 275; Prenosov: 187
.pdf Celotno besedilo (1,40 MB)
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Vortices determine the dynamics of biodiversity in cyclical interactions with protection spillovers
Attila Szolnoki, Matjaž Perc, 2015, izvirni znanstveni članek

Opis: If rock beats scissors and scissors beat paper, one might assume that rock beats paper too. But this is not the case for intransitive relationships that make up the famous rock-paper-scissors game. However, the sole presence of paper might prevent rock from beating scissors, simply because paper beats rock. This is the blueprint for the rock-paper-scissors game with protection spillovers, which has recently been introduced as a new paradigm for biodiversity in well-mixed microbial populations. Here we study the game in structured populations, demonstrating that protection spillovers give rise to spatial patterns that are impossible to observe in the classical rock-paper-scissors game.Weshow that the spatiotemporal dynamics of the system is determined by the density of stable vortices, which may ultimately transform to frozen states, to propagating waves, or to target waves with reversed propagation direction, depending further on the degree and type of randomness in the interactions among the species. If vortices are rare, the fixation to waves and complex oscillatory solutions is likelier. Moreover, annealed randomness in interactions favors the emergence of target waves, while quenched randomness favors collective synchronization. Our results demonstrate that protection spillovers may fundamentally change the dynamics of cyclic dominance in structured populations, and they outline the possibility of programming pattern formation in microbial populations.
Ključne besede: cyclical interactions, pattern formation, vortices, phase transitions, selforganization, biodiversity
Objavljeno: 03.07.2017; Ogledov: 310; Prenosov: 245
.pdf Celotno besedilo (1,32 MB)
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Controlled nanoparticle targeting and nanoparticle-driven nematic structural transition
Alexander Dubtsov, Sergey V. Pasechnik, Dina V. Shmeliova, Samo Kralj, Robert Repnik, 2015, izvirni znanstveni članek

Opis: We study experimentally and theoretically controlled targeting of specific nanoparticles (NPs) to different regions within nematic liquid crystal. Using a simple mesoscopic Landau-de Gennes-type model in terms of a tensor nematic order parameter, we demonstrate a general mechanism which could be exploited for controlled targeting of NPs within a spatially nonhomogeneous nematic texture. Furthermore, we experimentally demonstrate using polarising microscopy that even a relatively low concentration of localised appropriate NPs could trigger a nematic structural transition. A simple estimate is derived to account for the observed transition.
Ključne besede: liquid crystals, nanoparticles, structural transitions
Objavljeno: 14.06.2017; Ogledov: 657; Prenosov: 201
.pdf Celotno besedilo (4,26 MB)
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Different reactions to adverse neighborhoods in games of cooperation
Chunyan Zhang, Jianlei Zhang, Franz Weissing, Matjaž Perc, Guangming Xie, Long Wang, 2012, izvirni znanstveni članek

Opis: In social dilemmas, cooperation among randomly interacting individuals is often difficult to achieve. The situation changes if interactions take place in a network where the network structure jointly evolves with the behavioral strategies of the interacting individuals. In particular, cooperation can be stabilized if individuals tend to cut interaction links when facing adverse neighborhoods. Here we consider two different types of reaction to adverse neighborhoods, and all possible mixtures between these reactions. When faced with a gloomy outlook, players can either choose to cut and rewire some of their links to other individuals, or they can migrate to another location and establish new links in the new local neighborhood. We find that in general local rewiring is more favorable for the evolution of cooperation than emigration from adverse neighborhoods. Rewiring helps to maintain the diversity in the degree distribution of players and favors the spontaneous emergence of cooperative clusters. Both properties are known to favor the evolution of cooperation on networks. Interestingly, a mixture of migration and rewiring is even more favorable for the evolution of cooperation than rewiring on its own. While most models only consider a single type of reaction to adverse neighborhoods, the coexistence of several such reactions may actually be an optimal setting for the evolution of cooperation.
Ključne besede: cooperation, public goods, phase transitions, social dilemmas, coevolution, physics of social systems
Objavljeno: 19.06.2017; Ogledov: 486; Prenosov: 220
.pdf Celotno besedilo (481,90 KB)
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Understanding recurrent crime as system-immanent collective behavior
Matjaž Perc, Karsten Donnay, Dirk Helbing, 2013, izvirni znanstveni članek

Opis: Containing the spreading of crime is a major challenge for society. Yet, since thousands of years, no effective strategy has been found to overcome crime. To the contrary, empirical evidence shows that crime is recurrent, a fact that is not captured well by rational choice theories of crime. According to these, strong enough punishment should prevent crime from happening. To gain a better understanding of the relationship between crime and punishment, we consider that the latter requires prior discovery of illicit behavior and study a spatial version of the inspection game. Simulations reveal the spontaneous emergence of cyclic dominance between criminals, inspectors, and ordinary people as a consequence of spatial interactions. Such cycles dominate the evolutionary process, in particular when the temptation to commit crime or the cost of inspection are low or moderate. Yet, there are also critical parameter values beyond which cycles cease to exist and the population is dominated either by a stable mixture of criminals and inspectors or one of these two strategies alone. Both continuous and discontinuous phase transitions to different final states are possible, indicating that successful strategies to contain crime can be very much counter-intuitive and complex. Our results demonstrate that spatial interactions are crucial for the evolutionary outcome of the inspection game, and they also reveal why criminal behavior is likely to be recurrent rather than evolving towards an equilibrium with monotonous parameter dependencies.
Ključne besede: crime, evolutionary games, collective phenomena, phase transitions, statistical physics
Objavljeno: 19.06.2017; Ogledov: 518; Prenosov: 220
.pdf Celotno besedilo (433,51 KB)
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Beyond pairwise strategy updating in the prisoner's dilemma game
Xiaofeng Wang, Matjaž Perc, Yongkui Liu, Xiaojie Chen, Long Wang, 2012, izvirni znanstveni članek

Opis: In spatial games players typically alter their strategy by imitating the most successful or one randomly selected neighbor. Since a single neighbor is taken as reference, the information stemming from other neighbors is neglected, which begets the consideration of alternative, possibly more realistic approaches. Here we show that strategy changes inspired not only by the performance of individual neighbors but rather by entire neighborhoods introduce a qualitatively different evolutionary dynamics that is able to support the stable existence of very small cooperative clusters. This leads to phase diagrams that differ significantly from those obtained by means of pairwise strategy updating. In particular, the survivability of cooperators is possible even by high temptations to defect and over a much wider uncertainty range. We support the simulation results by means of pair approximations and analysis of spatial patterns, which jointly highlight the importance of local information for the resolution of social dilemmas.
Ključne besede: cooperation, social dilemma, wisdom of crowds, phase transitions, physics of social systems
Objavljeno: 23.06.2017; Ogledov: 270; Prenosov: 208
.pdf Celotno besedilo (825,96 KB)
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