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PubMed2000

Phase transitions and steady-state microstructures in a two-temperature lattice-gas model with mobile active impurities

The nonequilibrium, steady-state phase transitions and the structure of the different phases of a two-dimensional system with two thermodynamic temperatures are studied via a simple lattice-gas model with mobile active impurities ("hot/cold spots") whose activity is controlled by an external drive. The properties of the model are calculated by Monte Carlo computer-simulation techniques. The two temperatures and the external drive on the system lead to a rich phase diagram including regions of microstructured phases in addition to macroscopically ordered (phase-separated) and disordered phases. Depending on the temperatures, microstructured phases of both lamellar and droplet symmetry arise, described by a length scale that is determined by the characteristic temperature controlling the diffusive motion of the active impurities.

Journal Article↗
PubMed2002

Scaling behavior of the order parameter and its conjugated field in an absorbing phase transition around the upper critical dimension.

We analyze numerically the critical behavior of an absorbing phase transition in a conserved lattice gas in an external field. The external field is realized as a spontaneous creation of active particles that drives the system away from criticality. Nevertheless, the order parameter obeys certain scaling laws for sufficiently small external fields. These scaling laws are investigated and the corresponding exponents are determined in various dimensions (D=2,3,4,5). At the so-called upper critical dimension D(c)=4 one has to modify the usual scaling laws by logarithmic corrections.

Journal Article↗
PubMed2002

Synchronization of oscillators with long range interaction: Phase transition and anomalous finite size effects.

Synchronization in a lattice of a finite population of phase oscillators with algebraically decaying, non-normalized coupling is studied by numerical simulations. A critical level of decay is found, below which full locking takes place if the population contains a sufficiently large number of elements. For large number of oscillators and small coupling constant, numerical simulations and analytical arguments indicate that a phase transition separating synchronization from incoherence appears at a decay exponent value equal to the number of dimensions of the lattice. In contrast with earlier results on similar systems with normalized coupling, we have indications that for the decay exponent less than the dimensions of the lattice and for large populations, synchronization is possible even if the coupling is arbitarily weak. This finding suggests that in organisms interacting through slowly decaying signals such as light or sound, collective oscillations can always be established if the population is sufficiently large.

Journal Article↗
PubMed2003

Magnetic-field-induced changes of the isotropic-nematic phase transition in side-chain polymer liquid crystals.

The isotropic-nematic (I-N) phase transition of side-chain polymer liquid crystals is intrinsically a weak first-order transition with a biphasic region spread over a wide temperature interval. In the presence of high magnetic fields we find the I-N transition to become a strong first order. The I-N biphasic region shrinks its temperature window as larger magnetic fields are applied, until it completely disappears and the transition completes at a fixed temperature. We interpret this behavior as a consequence of the nonlinear coupling of the magnetic field to the system free energy, via the suppression of the order fluctuations in the nematic mesophase at the I-N transition crossing.

Journal Article↗
PubMed2006

Nonequilibrium phase transitions and finite-size scaling in weighted scale-free networks.

We consider nonequilibrium phase transitions, such as epidemic spreading, in weighted scale-free networks, in which highly connected nodes have a relatively smaller ability to transfer infection. We solve the dynamical mean-field equations and discuss finite-size scaling theory. The theoretical predictions are confronted with the results of large scale Monte Carlo simulations on the weighted Barabási-Albert network. Local scaling exponents are found different at a typical site and at a node with very large connectivity.

Journal Article↗
PubMed2001

Ferromagnetic phase transition in a Heisenberg fluid: Monte Carlo simulations and Fisher corrections to scaling.

The magnetic phase transition in a Heisenberg fluid is studied by means of the finite size scaling technique. We find that even for larger systems, considered in an ensemble with fixed density, the critical exponents show deviations from the expected lattice values similar to those obtained previously. This puzzle is clarified by proving the importance of the leading correction to the scaling that appears due to Fisher renormalization with the critical exponent equal to the absolute value of the specific heat exponent alpha. The appearance of such new corretions to scaling is a general feature of systems with constraints.

Journal Article↗
PubMed2002

Universality class of nonequilibrium phase transitions with infinitely many absorbing states.

We consider systems whose steady states exhibit a nonequilibrium phase transition from an active state to one-among an infinite number-absorbing state, as some control parameter is varied across a threshold value. The pair contact process, stochastic fixed-energy sandpiles, activated random walks, and many other cellular automata or reaction-diffusion processes are covered by our analysis. We argue that the upper-critical dimension below which anomalous fluctuation driven scaling appears is d(c)=6, in contrast to a widespread belief. We provide the exponents governing the critical behavior close to or at the transition point to first order in an epsilon =6-d expansion.

Journal Article↗
PubMed2004

Commensurate-incommensurate magnetic phase transition in magnetoelectric single crystal LiNiPO(4).

Neutron scattering studies of single crystal LiNiPO4 reveal a spontaneous first-order commensurate-incommensurate magnetic phase transition. Short- and long-range incommensurate phases are intermediate between the high temperature paramagnetic and the low temperature antiferromagnetic phases. The modulated structure has a predominant antiferromagnetic component, giving rise to satellite peaks in the vicinity of the fundamental antiferromagnetic Bragg reflection, and a ferromagnetic component, giving rise to peaks at small momentum transfers around the origin at (0,+/-Q,0). The wavelength of the modulated magnetic structure varies continuously with temperature. It is argued that the incommensurate short- and long-range phases are due to spin-dimensionality crossover from a continuous to the discrete Ising state.

Journal Article↗
PubMed2002

Nonlinear dielectric spectroscopy in the smectic-A-smectic-C(*)(alpha) phase transition.

We performed nonlinear dielectric spectroscopy to study the dynamic properties near the smectic-A-smectic-C(*)(alpha) phase transition point of an antiferroelectric liquid crystal 4-(1-methyl-heptyloxycar- bonyl)phenyl 4-octylcarbonyloxybiphenyl-4-carboxylate (MHPOCBC). A Landau-type theory was developed in order to analyze the experimentally obtained frequency dispersions. From the experimental and theoretical results we successfully obtained the temperature dependence of the relaxation frequency of the soft mode in both the smectic-A and smectic-C(*)(alpha) phases, which gave direct evidence that the transition is brought about by soft mode condensation. Significant features of this transition are also discussed.

Journal Article↗
PubMed1988

Cholesterol affects divalent cation-induced fusion and isothermal phase transitions of phospholipid membranes.

The influence of cholesterol on divalent cation-induced fusion and isothermal phase transitions of large unilamellar vesicles composed of phosphatidylserine (PS) was investigated. Vesicle fusion was monitored by the terbium/dipicolinic acid assay for the intermixing of internal aqueous contents, in the temperature range 10-40 degrees C. The fusogenic activity of the cations decreases in the sequence Ca2+ greater than Ba2+ greater than Sr2+ much greater than Mg2+ for cholesterol concentrations in the range 20-40 mol%, and at all temperatures. Increasing the cholesterol concentration decreases the initial rate of fusion in the presence of Ca2+ and Ba2+ at 25 degrees C, reaching about 50% of the rate for pure PS at a mole fraction of 0.4. From 10 to 25 degrees C, Mg2+ is ineffective in causing fusion at all cholesterol concentrations. However, at 30 degrees C, Mg2+-induced fusion is observed with vesicles containing cholesterol. At 40 degrees C, Mg2+ induces slow fusion of pure PS vesicles, which is enhanced by the presence of cholesterol. Increasing the temperature also causes a monotonic increase in the rate of fusion induced by Ca2+, Ba2+ and Sr2+. The enhancement of the effect of cholesterol at high temperatures suggests that changes in hydrogen bonding and interbilayer hydration forces may be involved in the modulation of fusion by cholesterol. The phase behavior of PS/cholesterol membranes in the presence of Na+ and divalent cations was studied by differential scanning calorimetry. The temperature of the gel-liquid crystalline transition (Tm) in Na+ is lowered as the cholesterol content is increased, and the endotherm is broadened. Addition of divalent cations shifts the Tm upward, with a sequence of effectiveness Ba2+ greater than Sr2+ greater than Mg2+. The Tm of these complexes decreases as the cholesterol content is increased. Although the transition is not detectable for cholesterol concentrations of 40 and 50 mol% in the presence of Na+, Sr2+ or Mg2+, the addition of Ba2+ reveals endotherms with Tm progressively lower than that observed at 30 mol%. Although the presence of cholesterol appears to induce an isothermal gel-liquid crystalline transition by decreasing the Tm, this change in membrane fluidity does not enhance the rate of fusion, but rather decreases it. The effect of cholesterol on the fusion of PS/phosphatidylethanolamine (PE) vesicles was investigated by utilizing a resonance energy transfer assay for lipid mixing. The initial rate of fusion of PS/PE and PS/PE/cholesterol vesicles is saturated at high Mg2+ concentrations. With Ca2+, saturation is not observed for cholesterol-containing vesicles.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2000

[The molecular structure and the arrangement changes of stilbazoles in the phase transition process studied by temperature-variable FTIR spectroscopy].

Hydrogen bonding is one of the principal intermolecular forces to enable the construction of supramolecular structures. By choosing an appropriate proton acceptor and a proton donor, stable intermolecular hydrogen bonds can form and affect or change the phase behavior of the donor and acceptor. We have obtained two intermolecular hydrogen bonded liquid crystals using the stilbazoles as the proton acceptor. In this paper we will research the molecular structure and the arrangement changes during the phase transition process of the stilbazoles by temperature variable FTIR spectroscopy. The results show that the number of the gauche conformation is increasing as the temperature increasing. When it is coming to the isotropic phase, the alkyl chain is in disorder. The frequency shift of carbonyl stretching bands has been found to be sensitive to the change of molecular conformation, inter and intramolecular interactions.

Hydrogen Bonding↗
PubMed2000

Topology and phase transitions: paradigmatic evidence

We report upon the numerical computation of the Euler characteristic chi (a topologic invariant) of the equipotential hypersurfaces Sigma(v) of the configuration space of the two-dimensional lattice varphi(4) model. The pattern chi(Sigma(v)) versus v (potential energy) reveals that a major topology change in the family Sigma(v)(vinR) is at the origin of the phase transition in the model considered. The direct evidence given here-of the relevance of topology for phase transitions-is obtained through a general method that can be applied to any other model.

Journal Article↗
PubMed2002

Influence of extended dynamics on phase transitions in a driven lattice gas.

Monte Carlo simulations and dynamical mean-field approximations are performed to study the phase transition in a driven lattice gas with nearest-neighbor exclusion on a square lattice. A slight extension of the microscopic dynamics with allowing the next-nearest-neighbor hops results in dramatic changes. Instead of the phase separation into high- and low-density regions in the stationary state the system exhibits a continuous transition belonging to the Ising universality class for any driving. The relevant features of the phase diagram are reproduced by an improved mean-field analysis.

Journal Article↗
PubMed2006

Thermally induced solid-state phase transition of bis(triisopropylsilylethynyl) pentacene crystals.

Bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) is a functionalized pentacene derivative designed to enhance both the solution solubility and solid-state packing of pentacene. In this paper, we report our observations of a solid-state phase transition in TIPS pentacene crystals upon heating or cooling. Evidence from differential scanning calorimetry (DSC), hot-stage optical microscopy, as well as high-temperature X-ray and electron diffraction are presented. A reasonable match with experimental data is obtained with molecular modeling. Our results reveal that the transition is associated with a conformational reorganization of the TIPS side groups, accompanied by a slight decrease in the acene-to-acene spacing and a shift of the overlap between the neighboring pentacene units. The observed cracking should be avoided or minimized in TIPS pentacene-based thin film transistors to maintain their relatively high charge carrier mobility.

Calorimetry, Differential Scanning↗
PubMed2005

Evolution kinetics and phase transitions of complex adaptive systems.

Motivated by the fact that a population of competing agents never set up a true stationary distribution, we propose a theory of evolution kinetics for complex adaptive systems. The formula derived for the survival probability is used to describe different phases in the population evolution when the prize-to-fine ratio as well as time changes. A kinetic phase diagram is obtained to show the phase boundaries for the self-segregating region, the intermediate region, and the clustering region. The kinetic evolutionary equations of strategy distribution are also established and used to give the root-mean-square separation of strategy distribution. It is revealed that the rootstock of the phase transitions is ascribed to the cooperation and competition among agents with different gene values for a limited resource.

Adaptation, Physiological↗
PubMed2006

Real time scanning force microscopy observation of a structural phase transition in self-assembled alkanethiols.

We present here a new approach based on high resolution scanning force microscopy that permits molecular determination and frictional discrimination between coexisting molecular configurations of alkanethiols on Au(111). The method has allowed visualizing in real time a phase transition between two differently ordered self-assembled configurations, which spontaneously takes place with time at constant coverage and under ambient conditions. Frictional force measurements indicate that the molecular rearrangement observed occurs via an order-disorder-order process.

Alkanes↗