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PubMed1980

Evaluation of cooperativity for phase transitions in two- and three-dimensional systems.

Use of the so-called "cooperative unit" (readily obtainable from the midpoint slope of phase transition curves) is discussed for the determination of cluster sizes and cooperative interaction energies. This quantity has been commonly employed in a rather empirical way since its correct interpretation is known only for some special cases (linear systems, all-or-none transitions). It is shown in the framework of a lattice model (Ising model) that the cooperative unit may be interpreted in terms of correlation functions and that it defines an average cluster corresponding to the patch size as obtained from scattering experiments. Relations between the cooperative unit and a cooperativity parameter are given for various lattices. Different types of transition curves are discussed using a simple analytical formalism, the quasichemical approximation. Some important nonideality effects are investigated which may lead to a "smearing-out" of first-order transitions.

Journal Article↗

PubMed1984

Effects of pressure on the phase transition of bilayers in liposomes. Influence of cholesterol and alpha-tocopherol.

The effects of pressure on the gel-to-liquid crystalline phase transition temperature of dimyristoylphosphatidylcholine (DMPC) bilayers containing cholesterol, alpha-tocopherol, and alpha-tocopheryl acetate were studied by fluorescence depolarization. The transition temperature of cholesterol mixtures (greater than 7.5 mol%) was lower than that of 100% DMPC at atmospheric pressure, but it became higher than the latter on increase in pressure. The thermodynamic parameters of the transition (delta V, delta S, delta H) were estimated and the functions of cholesterol and alpha-tocopherols in the bilayers are discussed.

Cholesterol↗

PubMed2000

Field theory of absorbing phase transitions with a nondiffusive conserved field

We investigate the critical behavior of a reaction-diffusion system exhibiting a continuous absorbing-state phase transition. The reaction-diffusion system strictly conserves the total density of particles, represented as a nondiffusive conserved field, and allows an infinite number of absorbing configurations. Numerical results show that it belongs to a wide universality class that also includes stochastic sandpile models. We derive microscopically the field theory representing this universality class.

Journal Article↗

PubMed2001

Temperature dependence of the refractive index near the reentrant-isotropic--calamitic-nematic phase transition.

The laser-induced nonlinear optical response of a lyotropic liquid crystal system in the reentrant-isotropic and calamitic-nematic phases is investigated by the use of the thermal lens technique. The occurrence of an inversion in the temperature coefficient of the ordinary refractive index, dn(radially)/dT, near the reentrant-isotropic-calamitic-nematic phase transition, is discussed. This effect is attributed to the behavior of the electronic polarizability due to the change in micelle shape near the isotropic-nematic transition, and correlated with the results obtained near the nematic-isotropic transition, previously reported.

Journal Article↗

PubMed2004

Study of an order-disorder phase transition on an atomic scale: the example of decagonal Al-Ni-Co quasicrystals.

In the present paper we succeeded in studying structural phase transitions from an atomistic point of view by positron annihilation Doppler broadening. This differs and is complementary to conventionally used diffraction experiments with large coherence lengths. In the exemplary case of the 1140 K order-disorder transition in decagonal Al71.5Ni14Co14.5 quasicrystals the importance of this atomistic approach and its wide scope of application is demonstrated.

Journal Article↗

PubMed2005

Quantum phase transitions in the sub-Ohmic spin-boson model: failure of the quantum-classical mapping.

The effective theories for many quantum phase transitions can be mapped onto those of classical transitions. Here we show that the naive mapping fails for the sub-Ohmic spin-boson model which describes a two-level system coupled to a bosonic bath with power-law spectral density, J(omega) proportional, variantomega(s). Using an epsilon expansion we prove that this model has a quantum transition controlled by an interacting fixed point at small s, and support this by numerical calculations. In contrast, the corresponding classical long-range Ising model is known to display mean-field transition behavior for 0 < s < 1/2, controlled by a noninteracting fixed point. The failure of the quantum-classical mapping is argued to arise from the long-ranged interaction in imaginary time in the quantum model.

Journal Article↗

PubMed2006

Structures and phase transitions in the ordered double perovskites Ba2BiIIIBiVO6 and Ba2BiIIISbVO6.

High-resolution neutron powder diffraction has been used to investigate the structures and phase transitions in the double perovskites Ba(2)Bi(3+)Bi(5+)O(6) (dibarium dibismuth hexaoxide) and Ba(2)BiSbO(6) (dibarium bismuth antimony hexaoxide) in the temperature ranges 4.2-973 and 4.2-625 K, respectively. The charge-ordered bismuthate adopts four structures in the temperature range - monoclinic in P2(1)/n, monoclinic in I2/m, rhombohedral in R3, and finally cubic in Fm3m. The low-temperature monoclinic structure has been determined for the first time. The transitions from P2(1)/n to I2/m, at 132 K, and R3 to Fm3m, at 820 K, are tricritical in nature; the transition from I2/m to R3 at ca 430 K is discontinuous. The behaviour of Ba(2)BiSbO(6) is very similar, except that the transition temperatures are lower - 250 K for I2/m to R3 and 515 K for R3 to Fm3m - and the low-temperature structure is not formed at all. The R3 to Fm3m transition in this compound is closer to second order in nature, although there is evidence for some contribution from higher-order terms.

Journal Article↗

PubMed2003

Direct fit of a theoretical model of phase transition in oscillatory finger motions.

This paper presents a general method to fit the Schöner-Haken-Kelso (SHK) model of human movement phase transitions directly to time series data. A robust variant of the extended Kalman filter technique is applied to the data of a single subject. The options of covariance resetting and iteration within recursion were used to obtain time-dependent estimates of both the alpha and beta parameters in the SHK model. Comparison between transition onset time and the time at which /beta(t/T)/alpha(t/T)/ becomes critical indicates that the transitions are advanced by noise. The method can be extended to handle non-normal data and generalization across subjects and/or experimental conditions.

Adult↗

PubMed2003

[Influences of R2O-Al2O3-B2O3-SiO2 system glass and superfine alpha-Al2O3 on the sintering and phase transition of hydroxyapatite ceramics].

The effects of R2O-Al2O3-B2O3-SiO2 system glass and superfine alpha-Al2O3 on the sintering and phase transition of hydroxyapatite (HAP) ceramics were assessed. The results showed that alpha-Al2O3 impeded the sintering of HAP and raised the sintering temperature. When glass and alpha-Al2O3 were used together to reinforce HAP ceramics, better results could be obtained; the bending strength of multiphase HAP ceramics approached 106 MPa when 10% (wt) alpha-Al2O3 and 20%(wt) glass were used and sintered at 1200 for 1 h.

Aluminum Oxide↗

PubMed1974

Physical and physiological evidence for two phase transitions in cytoplasmic membranes of animal cells.

Electron spin resonance analysis of suspensions of animal cell plasma membranes consistently reveals four characteristic temperatures for lateral phase separations in the membrane lipids. Similar analysis of an aqueous dispersion of lipids extracted from these membranes reveals only two characteristic temperatures, indicating that some aspect of lipid organization in membranes is destroyed by the extraction procedure. The characteristic temperatures for surface membranes from two different species of homeothermic animals were nearly identical and were approximately 37 degrees , 31 degrees , 21 degrees , and 15 degrees . A treatment of the physical data revealed that these temperatures could identify independent phase transitions for two hydrocarbon compartments of approximately equal size with lower and upper characteristic temperatures of 21 degrees and 37 degrees , and of 15 degrees and 31 degrees . The analysis of the effects of temperature on a number of physiological parameters indicates that 21 degrees and 37 degrees are likely to define the boundaries for lateral phase separations in the inner monolayer and 15 degrees and 31 degrees the boundaries for lateral phase separations in the outer monolayer.

Adenosine Triphosphatases↗

PubMed2000

Absence of thermodynamic phase transition in a model glass former

The glass transition can be viewed simply as the point at which the viscosity of a structurally disordered liquid reaches a universal threshold value. But this is an operational definition that circumvents fundamental issues, such as whether the glass transition is a purely dynamical phenomenon. If so, ergodicity gets broken (the system becomes confined to some part of its phase space), but the thermodynamic properties of the liquid remain unchanged across the transition, provided they are determined as thermodynamic equilibrium averages over the whole phase space. The opposite view claims that an underlying thermodynamic phase transition is responsible for the pronounced slow-down in the dynamics at the liquid-glass boundary. Such a phase transition would trigger the dynamic standstill, and then be masked by it. Here we perform Monte Carlo simulations of a two-dimensional system of polydisperse hard disks far within its glassy phase. The approach allows for non-local moves in a way that preserves micro-reversibility. We find no evidence for a thermodynamic phase transition up to very high densities; the glass is thus indistinguishable from the liquid on purely thermodynamic grounds.

Journal Article↗

PubMed2006

Pressure-dependent Ni-O phase transitions and Ni oxide formation on Pt(111): an in situ STM study at elevated temperatures.

Growth, atomic structure and O2 partial pressure dependent phase transitions of Ni-O structures and thin NiO films on Pt(111) have been studied using scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). In situ STM experiments were performed during film growth by reactive metal deposition at elevated temperatures (400-550 K) and variable O2 pressure. Depending on the substrate temperature, one-dimensional network-like Ni-O structures and islands with (7x1) and (4x2) reconstructions are formed during the initial stages of growth. These structures transform reversibly to a (2x2) reconstruction in a narrow O2 pressure range of 1.5-2x10(-6) mbar and can be monitored by in situ STM. Upon reduction of the O2 pressure to <10(-10) mbar pseudomorphic Ni monolayers are obtained. The defect-free ordering of Ni atoms on Pt(111) in a single stacking domain indicates an O-surfactant induced growth mode. The structural properties of the O2 pressure-dependent Ni-O phases are discussed in a simple model assuming NiO(001)-like atomic arrangements in the adsorbate overlayer. At higher coverage stable (111)-oriented NiO islands grow in a three-dimensional mode.

Journal Article↗

PubMed2006

Critical behavior at the isotropic-to-nematic phase transition in a bent-core liquid crystal.

Magnetic birefringence and dynamic light scattering measurements of orientational order parameter fluctuations at the isotropic-nematic phase transition of a bent-core liquid crystal reveal a pretransitional temperature dependence consistent with the standard Landau-deGennes mean field theory. However, as follows: the transition in the bent-core compound is more weakly first order (TNI-T* approximately 0.4 degrees C), the leading Landau coefficient is approximately 30 times lower, the viscosity associated with nematic order fluctuations is approximately 10 times higher, and the density change is approximately 10 times lower, than typically observed in calamitic (rod-shaped) liquid crystals. One consistent explanation for these anomalies is an optically isotropic phase composed of microscopic complexes or "clusters" of bent-core molecules.

Journal Article↗

PubMed1981

Analysis of entrainment of circadian oscillators by skeleton photoperiods using phase transition curves.

A skeleton photoperiod consists of two short pulses which are applied on the circadian oscillator at times corresponding to the beginning and to the end of a continuous light stimulus. To study several problems in entrainment of circadian rhythms by skeleton photoperiods, we develop a simple diagrammatic solution of the steady state entrainment making use of phase transition curves which are directly gotten from phase response curves. The graphical method is simple and systematic to study entrainment by light cycles with various day lengths. As the method is also intuitive, we can easily examine three problems. (1) In Drosophila the phase relation (psi) between rhythm and light cycle is a continuous function of day length of skeleton photoperiods up to about 12 h, but a marked discontinuity (psi-jump) sets in between 13 and 14 h. By the diagrammatic method we find that psi-jump is mathematically a bifurcation phenomenon. (2) The action of photoperiods up to about 12 h is fully simulated by two 15-min skeleton pulses. Do 3-min skeleton pulses imitate the complete photoperiods? We find that pulse width is arbitrary to some extent. (3) Why skeleton photoperiods up to about 12 h are good models of complete photoperiods? The reason is the small amplitude and the nearly symmetrical form of phase response curves in the subjective day.

Animals↗

PubMed1991

Solution effects on the thermotropic phase transition of unilamellar liposomes.

We have investigated the effect of two monosaccharides, glucose and fructose, and two disaccharides, sucrose and trehalose, on the thermotropic phase transition of unilamellar extruded vesicles of DPPC. All the sugars investigated raise the main transition temperature (Tm) of some fraction of the lipid, but there are differences between the effect of glucose and the other three sugars. At low concentrations of glucose, Tm is lowered. At high concentrations of glucose there are two transitions, one with a low Tm and one with a high Tm. The data suggest that at low concentrations, all of the glucose present may bind to the bilayer and increase headgroup spacing by physical intercalation or increased hydration. The appearance of a Tm above that of pure hydrated DPPC suggests the possibility of the dehydration of some other population of phospholipid molecules. The other three sugars increase Tm, but at high concentrations of trehalose, sucrose, and fructose a second peak occurs at a low Tm. The other sugars appear to dehydrate the bilayer at low concentrations, but may show some binding or increased hydration of some portion of the lipid at very high concentrations. The sugar effects on unilamellar vesicles are strikingly different from the effects of these sugars on multilamellar vesicles.

1,2-Dipalmitoylphosphatidylcholine↗

PubMedDate not supplied

Determination of membrane lipid phase transition temperature from 13C-NMR intensities.

A new and simple approach for the determination of the temperature of gel-to-liquid crystalline phase transitions (Tc) of biological (chloroplast) membrane lipids from 13C-NMR resonance intensities is proposed. The variation of intensity of a temperature-sensitive NMR resonance is monitored by recording the spectra of the sample at a range of temperatures. From such a series of spectra recorded at different temperatures, a temperature-insensitive resonance is located. Then the ratio of the intensity of the temperature-sensitive to the intensity of the temperature-insensitive resonance is calculated from each spectrum to even out the procedural error, if any. The values of this ratio at different temperatures, when plotted against sample temperature, shows a break at Tc as confirmed by spin label ESR studies.

Chemical Phenomena↗

PubMed1999

Pretransitional behavior above the nematic-isotropic phase transition of an auxetic trimer liquid crystal.

Static light scattering and electric field-induced Kerr measurements were performed above the nematic-isotropic phase transition of a terminal-lateral-lateral-terminal negative Poisson ratio trimer. For both measurements the inverse susceptibility was observed to be nearly linear with temperature, a result inconsistent with our previously reported Kerr data [Phys. Rev. E 58, 2041 (1998)].

Journal Article↗

PubMed2002

Universal relationship between a quantum phase transition and instability points of classical systems.

The direct and universal relationship between the accumulation of exceptional points in the quantum spectrum at a phase transition and the singularity of the classical action at a homoclinic point of the separatrix is investigated. The particular common features are the analytic structure and, related to it, instability and high sensitivity leading generically to the onset of chaos in both cases under perturbation.

Journal Article↗