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At least 757 records · Page 42Linked to original sources
PubMed1982

Electron microscopic observation on phase transition of purple membrane.

Structural changes during the thermal phase transition of purple membrane were observed by freeze-fracture electron microscopy. Native Halobacterium halobium cells contain broad purple membrane areas about 1 micron in diameter. The boundary separating purple and red membranes is obvious. On warming at 80 degrees C, particles of red membrane spread out beyond the boundary. Then a purple membrane area is eventually divided into small areolae of similar size, about 0.1 micron in diameter. By cooling down slowly, the purple membrane area is reformed and the crystalline arrangement also restored.

Bacteriorhodopsins↗
PubMed2002

Failure of thermodynamics near a phase transition.

In the vicinity of a first-order phase transition, the equation of state might be different when the extensive variable is controlled instead of the intensive one, violating the uniqueness of thermodynamics. A sufficient condition for this nonequivalence to survive at the thermodynamical limit is worked out for classical systems. If energy consists of a kinetic and a potential part, the microcanonical ensemble does not converge towards the canonical ensemble when the kinetic heat capacity is larger than the modulus of the negative interaction heat capacity.

Journal Article↗
PubMed2003

Universal behavior of crossover scaling functions for continuous phase transitions.

We consider two different systems exhibiting a continuous phase transition into an absorbing state. Both models belong to the same universality class; i.e., they are characterized by the same scaling functions and the same critical exponents. Varying the range of interactions, we examine the crossover from the mean-field-like to the non-mean-field scaling behavior. A phenomenological scaling form is applied in order to describe the full crossover region, which spans several decades. Our results strongly support the hypothesis that the crossover function is universal.

Journal Article↗
PubMed2000

Fracture and second-order phase transitions.

Using the global fiber bundle model as a tractable scheme of progressive fracture in heterogeneous materials, we define the branching ratio in avalanches as a suitable order parameter to clarify the order of the phase transition occurring at the collapse of the system. The model is analyzed using a probabilistic approach suited to smooth fluctuations. The branching ratio shows a behavior analogous to the magnetization in known magnetic systems with second-order phase transitions. We obtain a universal critical exponent beta approximately = 0.5 independent of the probability distribution used to assign the strengths of individual fibers.

Journal Article↗
PubMed2006

Distinguishing individual lipid headgroup mobility and phase transitions in raft-forming lipid mixtures with 31P MAS NMR.

A model membrane system composed of egg sphingomyelin (SM), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and cholesterol was studied with static and magic angle spinning (31)P NMR spectroscopy. This model membrane system is of significant biological relevance since it is known to form lipid rafts. (31)P NMR under magic angle spinning conditions resolves the SM and DOPC headgroup resonances allowing for extraction of the (31)P NMR parameters for the individual lipid components. The isotropic chemical shift, chemical shift anisotropy, and asymmetry parameter can be extracted from the spinning side band manifold of the individual components that form liquid-ordered and liquid-disordered domains. The magnitude of the (31)P chemical shift anisotropy and the line width is used to determine headgroup mobility and monitor the gel-to-gel and gel-to-liquid crystalline phase transitions of SM as a function of temperature in these mixtures. Spin-spin relaxation measurements are in agreement with the line width results, reflecting mobility differences and some heterogeneities. It will be shown that the presence of DOPC and/or cholesterol greatly impacts the headgroup mobility of SM both above and below the liquid crystalline phase transition temperature, whereas DOPC displays only minor variations in these lipid mixtures.

Cholesterol↗
PubMed2004

Photocontrolled gel-to-sol-to-gel phase transitioning of meta-substituted azobenzene bisurethanes through the breaking and reforming of hydrogen bonds.

An azobenzene derivative substituted at its meta positions with two urethane moieties linked to two cholesteryl ester units forms a gel that exhibits sol-gel phase transitions upon photoirradiation as a result of trans-cis isomerization of the azobenzene unit. During the sol-gel phase transitions, hydrogen bonds, which are partly responsible for stabilizing the gels, are broken or reformed. Using circular dichroism spectroscopy, we detected asymmetrical fibrous networks in the aggregates of these gelators.

Journal Article↗
PubMed2004

Static and dynamic dielectric behavior of mesogenic compounds of different polarity in the vicinity of the isotropic to nematic phase transition.

Mesogenic compounds belonging to the two well-known -cyanophenyl, and -isothiocyanatophenyl homologous series, which distinctly differ in the molecular polarity (-C identical with N, 5D; -N=C=S, 2.5D), show an essential difference in the pretransitional dielectric behavior in the vicinity of the isotropic to nematic (I-N) phase transition. Taking into account the results presented in Phys. Rev. E 67, 041705 (2003), the features of the I-N transition observed for the less polar mesogens are characteristic for the first order phase transition, whereas in the case of the strongly polar ones, the I-N transition is undoubtedly close to the second order.

Journal Article↗
PubMed2003

Surface phase transitions induced by electron mediated adatom-adatom interaction.

We propose that the indirect adatom-adatom interaction mediated by the conduction electrons of a metallic surface is responsible for the sqrt[3]xsqrt[3]<==>3x3 structural phase transitions observed in Sn/Ge (111) and Pb/Ge (111). When the indirect interaction overwhelms the local stress field imposed by the substrate registry, the system suffers a phonon instability, resulting in a structural phase transition in the adlayer. Our theory is capable of explaining all the salient features of the sqrt[3]xsqrt[3]<==>3x3 transitions observed in Sn/Ge (111) and Pb/Ge (111), and is in principle applicable to a wide class of systems whose surfaces are metallic before the transition.

Journal Article↗
PubMedDate not supplied

Modelling general anaesthesia as a first-order phase transition in the cortex.

Since 1997 we have been developing a theoretical foundation for general anaesthesia. We have been able to demonstrate that the abrupt change in brain state brought on by anaesthetic drugs can be characterized as a first-order phase transition in the population-average membrane voltage of the cortical neurons. The theory predicts that, as the critical point of phase change into unconsciousness is approached, the electrical fluctuations in cortical activity will grow strongly in amplitude while slowing in frequency, becoming more correlated both in time and in space. Thus the bio-electrical change of brain-state has deep similarities with thermodynamic phase changes of classical physics. The theory further predicts the existence of a second critical point, hysteretically separated from the first, corresponding to the return path from comatose unconsciousness back to normal responsiveness. There is a steadily accumulating body of clinical evidence in support of all of the phase-transition predictions: low-frequency power surge in EEG activity; increased correlation time and correlation length in EEG fluctuations; hysteresis separation, with respect to drug concentration, between the point of induction and the point of emergence.

Action Potentials↗
PubMed2006

Charge-transfer phase transition and zero thermal expansion in caesium manganese hexacyanoferrates.

A series of caesium manganese hexacyanoferrates is prepared; Cs(I)(1.78)Mn(II)[Fe(II)(CN)6]0.78[Fe(III)(CN)6](0.22) (1), Cs(I)(1.57)Mn(II)[Fe(II)(CN)6]0.57[Fe(III)(CN)6](0.43) (2), Cs(I)(1.51)Mn(II)[Fe(II)(CN)6]0.51[Fe(III)(CN)6](0.49) (3), and Cs(I)(0.94)Mn(II)[Fe(II)(CN)6]0.21[Fe(III)(CN)6](0.70).0.8H2O (4). 1-3 show charge-transfer phase transitions between the high-temperature (HT) and low-temperature (LT) phases with transition temperatures (T(1/2 downward arrow), T(1/2 upward arrow)) of (207 K, 225 K) (1), (190 K, 231 K) (2), and (175 K, 233 K) (3) at a cooling and warming rates of 0.5 K min(-1). Variable temperature IR spectra indicate that the valence states of the LT phases of 1-3 are Cs(I)(1.78)Mn(II)(0.78)Mn(III)(0.22)[Fe(II)(CN)6], Cs(I)(1.57)Mn(II)(0.57)Mn(III)(0.43)[Fe(II)(CN)6], and Cs(I)(1.51)Mn(II)(0.51)Mn(III)(0.49) [Fe(II)(CN)6], respectively. The XRD measurements for 1-3 show that crystal structures of the HT and LT phases are cubic structures (Fm3[combining macron]m), but the lattice constants decrease from the HT phase to the LT phase; a = 10.5446(17) --> 10.4280(7) A (1), 10.5589(17) --> 10.3421(24) A (2), and 10.5627(11) --> 10.3268(23) A (3). The magnetization vs. temperature curves and the magnetization vs. external magnetic field curves show that the LT phases are ferromagnetic with Curie temperatures of 4.3 (1), 5.0 (2), and 5.6 K (3). At a cooling rate of -0.5 K min(-1), 4 does not show the charge-transfer phase transition, but does show a behavior of zero thermal expansion with a thermal expansivity of +0.2 x 10(-6) K(-1) throughout the temperature range 300 and 20 K.

Journal Article↗
PubMed2004

Self-consistent model of blue phase III to isotropic phase transition.

In previous publications [Phys. Rev. Lett. 81, 1457 (1998)]; Phys. Rev. E 61, 2759 (2000)]], a simplified model with the scalar order parameter and without the cubic term in the Hamiltonian has been used to account for the phase transition between the two isotropic chiral liquids. The present approach is a step towards full analysis of this transition using de Gennes tensor order parameter and the higher-order self-consistent approach. The importance of the cubic term for a proper description of this phase transition is indicated.

Journal Article↗
PubMed2001

Phase transitions for flat anti-de Sitter black holes.

We reexamine the thermodynamics of anti-de Sitter (adS) black holes with Ricci flat horizons using the adS soliton as the thermal background. We find that there is a phase transition which is dependent not only on the temperature but also on the black hole area, which is an independent parameter. As in the spherical adS black hole, this phase transition is related via the adS/conformal-field-theory correspondence to a confinement-deconfinement transition in the large- N gauge theory on the conformal boundary at infinity.

Journal Article↗
PubMed1976

A calorimetric and fluorescent probe study of the gel-liquid crystalline phase transition in small, single-lamellar dipalmitoylphosphatidylcholine vesicles.

The results of a calorimetric and fluorescent probe study of the thermotropic behavior of various types of dispersions of dipalmitolphosphatidylcholine bilayer vesicles are reported. Bangham-type, multilamellar vesicles exhibit tow distinct phase transitions at 34.6 and 41.2 degrees C. On the other hand, single-lamellar spherical vesicles appear to exhibit a single transition at 37 degrees C. The single-lamellar vesicles are thermodynamically unstable below 27 degrees C and slowly transform into a multilamellar structure with a single phase transition of 41.2 degrees C. These transformed structures resemble, but are not identical with, Bangham-type vesicles. An experimentally testable thermodynamic and kinetic model based upon these results is developed.

Calorimetry↗
PubMed1990

Surface phase transition of hydrated calcium oxalate crystal in the presence of normal and stone-formers' urine.

Crystals of calcium oxalate monohydrate (COM), dihydrate (COD) and trihydrate (COT) were grown by slow diffusion of reacting ions from solutions using interfacially controlled crystallization. Phase transition of COT to COD and COM, and COD to COM were studied on single crystal by X-ray diffraction analysis of the same crystal before and after exposure to normal and stone formers' urine. Phase transition on the surface of single crystals has been demonstrated by SEM energy dispersive X-ray microanalysis using windowless detector, and scanning Auger electron microprobe. Data obtained in this study offer direct experimental evidence for phase transformation on the surface of the hydrated calcium oxalate single crystal. In presence of normal urine the surface of COT single crystal undergoes transformation into COD and in presence of recurrent calcium oxalate stone former's urine surface transformation to COM takes place.

Calcium Oxalate↗
PubMed2003

Phase transition between hidden and antiferromagnetic order in URu2Si2.

We have carried out thermal expansion measurements on the heavy fermion compound URu2Si2. We have discovered a new anomaly that appears only under pressure for both the tetragonal a and c axes. Plotting these temperatures on a temperature-pressure plane, we have constructed a phase diagram, in which there is a first order phase transition boundary separating a hidden and antiferromagnetic order. When the system enters the antiferromagnetic phase from the hidden order, the lattice constant shrinks along the a axis and elongates along the c axis, leading to an increase in the lattice constant ratio of c/a. We conclude that the system lies close to a bicritical point from which the first order phase transition line emanates.

Journal Article↗
PubMed2003

Pressure-induced crossover between diffusive and displacive mechanisms of phase transitions in single-crystalline alpha-GeO2.

We present the first example of the phase transition occurring via the different kinetic mechanisms, displacive or diffusive, competing with each other in quartz-like alpha-GeO2 single crystals. Upon room-pressure heating, alpha-GeO2 transforms to the rutile-type phase (the alpha-->r transition) via the diffusive mechanism. With increase of the treatment pressure the diffusive mode of the temperature-induced alpha-->r transition is substituted at approximately 4 GPa by a displacive-like mode, and then at approximately 6 GPa the transition type changes from the alpha-->r sequence to a displacive martensitic-like transition to a distorted rutile-like phase (alpha-->r'. A crossover between diffusive and displacive transition modes suggests a new way to control the meso- and nanometer-scale morphology of high-pressure phases.

Journal Article↗
PubMed2003

Phase transitions in sexual populations subject to stabilizing selection.

We show that a simple model of an evolving sexual population, which dates back to some of the earliest work in theoretical population genetics, exhibits an unexpected and previously unobserved phase transition between ordered and disordered states. This behavior is not present in populations evolving asexually without recombination and is thus important in any comparison of sexual and asexual populations. In order to calculate the details of the phase transition, we use techniques from statistical physics. We introduce the correlation of the population as the order parameter of the system and use maximum entropy inference to find the state of the population at any time.

Biological Evolution↗
PubMed2002

Angular-momentum-induced phase transition in spherical gravitational systems: N-body simulations.

The role of thermodynamics in the evolution of systems evolving under purely gravitational forces is not completely established. Both the infinite range and singularity in the Newtonian force law preclude the use of standard techniques. However, astronomical observations of globular clusters suggest that they may exist in distinct thermodynamic phases. Here, using dynamical simulation, we investigate a model gravitational system that exhibits a phase transition in the mean-field limit. The system consists of rotating, concentric, mass shells of fixed angular-momentum magnitude and shares identical equilibrium properties with a three-dimensional point mass system satisfying the same condition. The mean-field results show that a global entropy maximum exists for the model, and a first order phase transition takes place between "quasi-uniform" and "core-halo" states, in both the microcanonical and canonical ensembles. Here we investigate the evolution and, with time averaging, the equilibrium properties of the isolated system. Simulations were carried out in the transition region, at the critical point, and in each clearly defined thermodynamic phase, and striking differences were found in each case. We find full agreement with mean-field theory when finite-size scaling is accounted for. In addition, we find that (1) equilibration obeys power-law behavior, (2) virialization, equilibration, and the decay of correlations in both position and time, are very slow in the transition region, suggesting that the system is also spending time in the metastable phase, and (3) there is a strong evidence of long-lived, collective oscillations in the supercritical region.

Journal Article↗