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At least 1,081 records · Page 60Linked to original sources
PubMed2004

Quasidynamical symmetry in an interacting boson model phase transition.

The apparent persistence of symmetry in the face of strong symmetry-breaking interactions is examined in a many-boson model. The model exhibits a transition between two phases associated with U(5) and O(6) symmetries, respectively, as the value of a control parameter progresses from 0 to 1. The remarkable fact is that, in spite of strong mixing of the symmetries for intermediate values of the control parameter, the model continues to exhibit the characteristics of its closest symmetry limit for all but a relatively narrow transition region that becomes progressively narrower as the boson number increases. This phenomenon is explained in terms of quasidynamical symmetry.

Journal Article↗
PubMed2001

Nonequilibrium phase transition for a heavy particle in a granular fluid.

It is shown that the homogeneous cooling state (HCS) for a heavy impurity particle in a granular fluid supports two distinct phases. The order parameter straight phi;(s) is the mean square velocity of the impurity particle relative to that of a fluid particle, and the control parameter xi* is the fluid cooling rate relative to the impurity collision rate. For xi*<1 there is a "normal" phase for which straight phi;(s) scales as the fluid/impurity mass ratio, just as for a system with elastic collisions. For xi*>1 an "ordered" phase occurs in which straight phi;(s) is finite even for vanishingly small mass ratio, representing an extreme violation of energy equipartition. The phenomenon can be described in terms of a Landau-like free energy for a second order phase transition. The dynamics leading to the HCS is studied in detail using an asymptotic analysis of the Enskog-Lorentz kinetic equation near each phase and the critical domain. Critical slowing is observed with a divergent relaxation time at the critical point. The stationary velocity distributions are determined in each case, showing a crossover from Maxwellian in the normal phase to an exponential quartic function of the velocity that is sharply peaked about the nonzero straight phi;(s) for the ordered phase. It is shown that the diffusion coefficient in the normal phase diverges at the critical point and remains so in the ordered phase. This is interpreted as a transition from diffusive to ballistic dynamics between the normal and ordered phases.

Journal Article↗
PubMed2004

Soft perforation of planar bilayer lipid membranes of dipalmitoylphosphatidylcholine at the temperature of the phase transition from the liquid crystalline to the gel state.

In contrast to the widely used method of electroporation, the method of soft perforation of lipid bilayers is proposed. It is based on the structural rearrangement of the lipid bilayer formed from disaturated phospholipids at the temperature of the phase transition from the liquid crystalline state to the gel state. This allows us to obtain a lipid pore population without the use of a strong electric field. It is shown that the planar lipid bilayer membrane (pBLM) formed from dipalmitoylphosphatidylcholine in 1 M LiCl aqueous solution exhibits the appearance of up to 50 lipid pores per 1 mm(2) of membrane surface, with an average single pore conductivity of 31 +/- 13 nS. The estimation of a single pore radius carried out with water-soluble poly(ethylene glycol)s (PEGs) showed that the average pore radius ranged between 1.0-1.7 nm. It was found experimentally that PEG-1450, PEG-2000, and PEG-3350 should be in a position to block the single pore conductivity completely, while PEG-6000 fully restored the ionic conductivity. The similarity of these PEG effects to ionic conductivity in protein pores makes it possible to suggest that the partition of the PEG molecules between the pore and the bulk solution does not depend on the nature of the chemical groups located in the pore wall.

Crystallization↗
PubMedDate not supplied

[Evaluation of phase transition models in lipid-aqueous systems by the optical analogy method].

There are two models of phase transition in mesomorphic lipid-water systems. The first one describes the transition in terms of trans-gauche isomerisation, the second-by the appearance of gauche-trans-gauche isomers (kinks). Both were argumented theoretically, but experiments cannot as yet offer an unambiguous decision. We have tried to evaluate each of the assumptions based on the qualitative analysis of X-ray and optical diffraction pictures from models and real objects. We have had in mind the early proved reason in the application of optical modeling of liquid crystal structures in a range of which our dipalmitoyllecithin-water system could be included. Results of comparing optical diffraction of laser beam (gamma = 630 nm) on two-dimensional models with our X-ray pictures of the system mentioned in L alpha and L beta modifications have shown that high-temperature L alpha phase consists of molecules with gauche-conformation of aliphatic fragments. In view of these data the kink model seems less realistic.

Crystallography↗
PubMed2003

Phase transitions of the anisotropic Ashkin-Teller model on a family of diamond-type hierarchical lattices.

The phase transitions of the anisotropic Ashkin-Teller model on a family of diamond-type hierarchical lattices is studied by means of the transfer-matrix method and the real-space renormalization-group transformation. We find that the phase diagram, for the ferromagnetic case, consists of five phases, i.e., the fully disordered paramagnetic phase P, the fully ordered ferromagnetic phase F, and three partially ordered ferromagnetic phases F(s), F(sigma), and F(s sigma), as well as ten nontrivial fixed points. The correlation length critical exponents and the crossover exponents are also calculated. In addition, we also investigate the variations of the critical exponents with the fractal dimension d(f), the number of branches m, and the number of bonds per branch b of the generator of the family of diamond-type hierarchical lattices. Finally we give a brief discussion about universality.

Journal Article↗
PubMed2003

Redox regulation of the G1 to S phase transition in the mouse embryo fibroblast cell cycle.

The hypothesis that intracellular oxidation/reduction (redox) reactions regulate the G(0)-G(1) to S-phase transition in the mouse embryonic fibroblast cell cycle was investigated. Intracellular redox state was modulated with a thiol-antioxidant, N-acetyl-L-cysteine (NAC), and cell cycle progression was measured using BrdUrd pulse-chase and flow cytometric analysis. Treatment with NAC for 12 h resulted in an approximately 6-fold increase in intracellular low-molecular-weight thiols and a decrease in the MFI of an oxidation-sensitive probe, dihydrofluorescein diacetate, indicating a shift in the intracellular redox state toward a more reducing environment. NAC-induced alterations in redox state caused selective delays in progression from G(0)-G(1) to S phase in serum-starved cells that were serum stimulated to reenter the cell cycle as well as to inhibit progression from G(1) to S phase in asynchronous cultures with no significant alterations in S phase, and G(2)+M transits. NAC treatment also showed a 70% decrease in cyclin D1 protein levels and a 3-4-fold increase in p27 protein levels, which correlated with decreased retinoblastoma protein phosphorylation. Cells released from the NAC treatment showed a transient increase in dihydrofluorescein fluorescence and oxidized glutathione content between 0 and 8 h after release, indicating a shift in intracellular redox state to a more oxidizing environment. These changes in redox state were followed by an increase in cyclin D1, a decrease in p27, retinoblastoma protein hyperphosphorylation and subsequent entry into S phase by 8-12 h after the removal of NAC. These results support the hypothesis that a redox cycle within the mammalian cell cycle might provide a mechanistic link between the metabolic processes early in G(1) and the activation of G(1)-regulatory proteins in preparation for the entry of cells into S phase.

Acetylcysteine↗
PubMed2004

Transmembrane peptide-induced lipid sorting and mechanism of Lalpha-to-inverted phase transition using coarse-grain molecular dynamics.

Molecular dynamics results are presented for a coarse-grain model of 1,2-di-n-alkanoyl-sn-glycero-3-phosphocholine, water, and a capped cylindrical model of a transmembrane peptide. We first demonstrate that different alkanoyl-length lipids are miscible in the liquid-disordered lamellar (Lalpha) phase. The transmembrane peptide is constructed of hydrophobic sites with hydrophilic caps. The hydrophobic length of the peptide is smaller than the hydrophobic thickness of a bilayer consisting of an equal mixture of long and short alkanoyl tail lipids. When incorporated into the membrane, a meniscus forms in the vicinity of the peptide and the surrounding area is enriched in the short lipid. The meniscus region draws water into it. In the regions that are depleted of water, the bilayers can fuse. The lipid headgroups then rearrange to solvate the newly formed water pores, resulting in an inverted phase. This mechanism appears to be a viable pathway for the experimentally observed Lalpha-to-inverse hexagonal (HII) peptide-induced phase transition.

Computer Simulation↗
PubMed1997

Effects of triterpenoid glycosides of the dammaran series and their aglycons on phase transitions of dipalmitoylphosphatidylcholine.

The effects of triterpenoid glycosides from Korean red ginseng Panax C.A. Meyer and their aglycons on the phase transitions of model DPPC membranes were studied by microcalorimetry. Glycosides Rb1, Rg1, 20(S) protopanaxadiol only slightly interacted with the DPPC gel phase and caused no formation of new phases. At concentrations close to equimolar values, the glycosides slightly disturbed the packing of lipids and induced the formation of the second phases with more dense lipid packing but smaller sizes of the cooperative domains as compared with pure lipid. Cholesterol (3%) produced no effect on the DPPC interaction with Rb1, Rg1 and panaxatriol but affected the DPPC interaction with panaxadiol. At equimolar panaxadiol/DPPC ratio and in the presence of 3% cholesterol, panaxadiol completely eliminated the main transition.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2005

Deconfinement phase transition in a 3D nonlocal U(1) lattice gauge theory.

We introduce a 3D compact U(1) lattice gauge theory having nonlocal interactions in the temporal direction, and study its phase structure. The model is relevant for the compact QED3 and strongly correlated electron systems like the t-J model of cuprates. For a power-law decaying long-range interaction, which simulates the effect of gapless matter fields, a second-order phase transition takes place separating the confinement and deconfinement phases. For an exponentially decaying interaction simulating matter fields with gaps, the system exhibits no signals of a second-order transition.

Journal Article↗
PubMed1980

A statistical mechanical model of the lipid bilayer above its phase transition.

A statistical mechanical model of a bilayer of dipalmitoyl-3-sn-phosphatidylcholine molecules above their phase transition is presented. A molecular field approximation developed in previous work by Marcelja is extended by setting the molecular field at each depth in the bilayer in proportion to the average chain order at that depth. The free energy of the hydrocarbon/water interface and that due to the interaction of the polar headgroups is included in the evaluation of the statistical weights of the chain conformations. The model gives good agreement with several independent experimental results. It resolves the dilemma posed by the experimental evidence that there is (i) a considerable variation in order parameter along the lipid chain, but (ii) no collective tilt in the more ordered region of the chain. The model gives an explanation of how the lipid chains pack under these two constraints. The order parameter profile down the chain does not correspond to the profile across the bilayer.

Calorimetry↗
PubMed2003

Phase transition in the ABC model.

Recent studies have shown that one-dimensional driven systems can exhibit phase separation even if the dynamics is governed by local rules. The ABC model, which comprises three particle species that diffuse asymmetrically around a ring, shows anomalous coarsening into a phase separated steady state. In the limiting case in which the dynamics is symmetric and the parameter q describing the asymmetry tends to one, no phase separation occurs and the steady state of the system is disordered. In the present work, we consider the weak asymmetry regime q=exp(-beta/N), where N is the system size, and study how the disordered state is approached. In the case of equal densities, we find that the system exhibits a second-order phase transition at some nonzero beta(c). The value of beta(c)=2pi square root 3 and the optimal profiles can be obtained by writing the exact large deviation functional. For nonequal densities, we write down mean-field equations and analyze some of their predictions.

Journal Article↗
PubMed2000

Zero-temperature d-wave superconducting phase transition

The Ginzburg-Landau-Wilson theory that describes the disordered-metal- d-wave-superconductor phase transition at zero temperature is derived at weak coupling. The theory represents an interacting dissipative system of bosonic Cooper pairs in an effective random potential. I show that there exists a wide crossover regime in the theory controlled by a line of Gaussian fixed points, each of which in two dimensions is characterized by a different universal value of the dc critical conductivity. Relation to experiments on overdoped and underdoped cuprates is discussed.

Journal Article↗
PubMed2004

Phase transitions in a disordered system in and out of equilibrium.

The equilibrium and nonequilibrium disorder-induced phase transitions are compared in the random-field Ising model. We identify in the demagnetized state the correct nonequilibrium hysteretic counterpart of the T=0 ground state, and present evidence of universality. Numerical simulations in d=3 indicate that exponents and scaling functions coincide, while the location of the critical point differs, as corroborated by exact results for the Bethe lattice. These results are of relevance for optimization, and for the generic question of universality in the presence of disorder.

Journal Article↗
PubMed2005

Fluctuation conductivity of thin films and nanowires near a parallel-field-tuned superconducting quantum phase transition.

We calculate the fluctuation correction to the normal state conductivity in the vicinity of a quantum phase transition from a superconducting to a normal state, induced by applying a magnetic field parallel to a dirty thin film or a nanowire with thickness smaller than the superconducting coherence length. We find that at zero temperature, where the correction comes purely from quantum fluctuations, the positive "Aslamazov-Larkin" contribution, the negative "density of states" contribution, and the "Maki-Thompson" interference contribution are all of the same order and the total correction is negative. Further, we show that, based on how the quantum critical point is approached, there are three regimes that show different temperature and field dependencies which should be experimentally accessible.

Journal Article↗
PubMed2005

A 13C NMR study of the molecular dynamics and phase transition of confined benzene inside titanate nanotubes.

This work investigated the nanoconfinement effect on the molecular dynamics and phase transition of confined benzene inside titanate nanotubes with a uniform inner diameter of approximately 5.3 nm. For 13C-enriched organics, the 13C nuclear spin-spin relaxation was demonstrated as a sensitive tool to differentiate molecular translational motion and reorientation and, thus, was shown to be advantageous over the commonly employed 1H and 2H NMR for studying complex phase diagram, specifically, for separating the phase behavior of translational motion and the phase behavior of molecular reorientation. In such an approach, the melting of translational motion of confined benzene was explicitly observed to take place in a broad temperature range below the bulk melting temperature. The abrupt change of the 13C nuclear spin-spin relaxation time of the confined liquid benzene at about 260 K suggested that nanoconfinement induced two topologically distinct liquid phases.

Benzene↗
PubMed2006

Phase-transition and aggregation characteristics of a thermoresponsive dextran derivative in aqueous solutions.

Grafting of poly(N-vinylcaprolactam) side chains onto a hydrophilic dextran backbone was found to provide the dextran with new, thermoresponsive properties in aqueous solutions. Depending on its solution concentration, the resulting dextran derivative could exhibit a temperature-induced phase-transition and critical transition temperature (T(c)). Different anions and cations of added salts, including five potassium salts and five alkali-metal chlorides, were observed to influence the T(c) value of its aqueous solution. Except for potassium iodide, all added salts were found to lower the T(c) value. The addition of the surfactant, cationic cetyltrimethylammonium bromide or anionic sodium dodecyl sulfate, resulted in an increase of the T(c) value. With the help of the Coomassie Brilliant Blue dye as a polarity probe, the formation of hydrophobic aggregates above the T(c) was revealed for this new dextran derivative in aqueous solution.

Caprolactam↗
PubMed2003

Structures and phase transitions in polar smectic liquid crystals.

A discrete phenomenological model of antiferroelectric liquid crystals is used to study the structures and phase transitions in bulk samples and thin films. An important ingredient of our investigations is minimization of the free energy with respect to the phase and modulus of the order parameter. A simple version of the free energy, which contains only the nearest-neighbor and the next-nearest-neighbor layer interactions gives a complete phase diagram with all the observed smectic-C* (SmC*) variant phases. In thin free-standing films, surface ordering may lead to suppression of the bulk SmC(*)(alpha) helix and to formation of planar structures. Transitions between these structures are accompanied by the 90 degrees reorientation of the polarization direction. We also discuss the influence of chirality on subphase structures.

Journal Article↗