Showing papers on "Phase transition published in 1988"

Particle Physics and Inflationary Cosmology

Abstract: A monograph on inflationary cosmology and cosmological phase transitions, investigating modern cosmology's relationship to elementary particle physics. This work also includes a non-technical discussion of inflationary cosmology for those unfamiliar with the theory.

Phase diagram and dynamics of Yukawa systems

Abstract: The phase diagram and dynamical properties of systems of particles interacting through a repulsive screened Coulomb (Yukawa) potential have been calculated using molecular and lattice dynamics techniques. The phase diagram contains both a melting transition and a transition from fcc to bcc crystalline phases. These phase transitions have been studied as a function of potential shape (screening length) and compared to phenomenological criteria for transition temperatures such as those of Lindemann and of Hansen and Verlet. The transition from fcc to bcc with increasing temperature is shown to result from a higher entropy in the bcc phase because of its softer shear modes. Even when the stable solid phase below the melting temperature is fcc, bcc‐like local order is found in the liquid phase. This may substantially slow crystallization. The calculated phase diagram and shear modulus are in good agreement with experiments on colloidal suspensions of polystyrene spheres. The single particle dynamics of Yukawa systems show several unusual features. There is a pronounced subdiffusive regime in liquids near and below the melting temperature. This regime reflects the existence of two time scales: a typical phonon period, and the time for a particle to feel a new environment. The second time scale becomes longer as the temperature is lowered or the range of interaction (screening length) increases.

Kinetics of discontinuous volume-phase transition of gels

Abstract: Kinetics of the first order volume–phase transition of submillimeter gels is investigated for various initial and final states of the transition. Characteristic times for swelling and shrinking critically depend on the final state, but are much less influenced by the intial state. The transition becomes infinitely slow when the final temperature is near the transition threshold. The study of the dependence of the transition time on the gel size reveals that the overall volume change is described approximately as a collective diffusion process, but not precisely. The exponent for time–radius relation is smaller than 2. The transitions having large volume change are accompanied by formation and evolution of transient patterns which appear on the surface of a gel. The patterns for swelling and shrinking are quite different, but both play an important role in the kinetic processes.

Polymers on disordered trees, spin glasses, and traveling waves

Abstract: We show that the problem of a directed polymer on a tree with disorder can be reduced to the study of nonlinear equations of reaction-diffusion type. These equations admit traveling wave solutions that move at all possible speeds above a certain minimal speed. The speed of the wavefront is the free energy of the polymer problem and the minimal speed corresponds to a phase transition to a glassy phase similar to the spin-glass phase. Several properties of the polymer problem can be extracted from the correspondence with the traveling wave: probability distribution of the free energy, overlaps, etc.

Nonclassical nucleation theory for the gas-liquid transition

Abstract: We use density functional methods to develop a new nonclassical theory for the homogeneous nucleation of the gas to liquid phase transition. The extent of agreement between our results and the classical prediction of Becker, Doring, and Zeldovich is strongly dependent on the range of the attractive potential which we employ. We show that our predictions are consistent with experimental data using cloud chambers, and we suggest several directions in which experimentalists might look in order to find nonclassical effects. In particular, we suggest that cavitation (gas bubble formation in a liquid subjected to tensile stress) should nucleate at a significantly greater rate than that predicted by classical theory.

Nucleation of a new phase from the interaction of two adjacent phases: Some silicides

Abstract: The reactions of metal layers with their silicon substrates resulting in the formation of various silicides are considered generally not only as phenomena common to all diffusion couples where new phases are formed, but also as typical of all transitions from two to three phases. The conditions under which such transitions will display the same characteristics as encountered in the usual one-to-two phase transitions (condensation, crystallization, boiling) are analyzed by comparison to the classical theory of nucleation. Because of the lack of knowledge about the exact values of the relevant parameters, the discussion is carried out mostly in descriptive thermodynamic terms. Although nucleation effects are analyzed in general terms, the main focus of attention is a class of reactions where nucleation dominates the formation of a new phase; a salient feature of these reactions is the absence of any equilibrium temperature, although the nucleation temperatures are relatively well defined within narrow limits. Nucleation effects are correlated to such material characteristics as the stability of the nucleated phases, and to such kinetic characteristics as the sequence of phase formation. The modification of the energy levels of the different phases brought about by stress, ion bombardment, or the replacement of usual phases by metastable ones, are considered with respect to their effect on nucleation processes. The nearly total absence of literature references to nucleation in metal-metal diffusion couples is discussed with respect to some specific aspects of the metal-silicon reactions.

Molecular tribometry of ultrathin liquid films.

Abstract: We study the resistance to sliding of liquid films 1-6 molecules thick. The apparent dynamic viscosity at 1 Hz of nonpolar liquids confined between parallel plates of atomically smooth mica is considerably enhanced over that of the isotropic fluids and shows extreme dependence on pressure. Rapid, reversible switching between liquidlike and noncompliant responses as a function of small changes in normal pressure is also found, suggesting a phase transition to a solidlike structure.

Structure and mechanical properties of giant lipid (DMPC) vesicle bilayers from 20 degrees C below to 10 degrees C above the liquid crystal-crystalline phase transition at 24 degrees C.

Abstract: We have used micromechanical tests to measure the thermoelastic properties of the liquid and gel phases of dimyristoylphosphatidylcholine (DMPC). We have found that the rippled P beta' phase is only formed when a vesicle is cooled to temperatures below the main acyl chain crystallization transition, Tc, under zero or very low membrane tension. We also found that the P beta' surface ripple or superlattice can be pulled flat under high membrane tension into a planar structure. For a ripple structure formed by acyl chains perpendicular to the projected plane, the projected area change that results from a flattening process is a direct measure of the molecular crystal angle. As such, the crystal angle was found to increase from about 24 degrees just below Tc to about 33 degrees below the pretransition. It was also observed that the P beta' superlattice did not form when annealed L beta' phase vesicles were heated from 5 degrees C to Tc; likewise, ripples did not form when the membrane was held under large tension during freezing from the L alpha phase. Each of these three procedures could be used to create a metastable planar structure which we have termed L*beta' since it is lamellar and plane-crystalline with acyl chains tilted to the bilayer plane. However, we show that this structure is not as condensed as the L beta' phase below 10 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Metastability effects in bootstrap percolation

Abstract: Bootstrap percolation models, or equivalently certain types of cellular automata, exhibit interesting finite-volume effects. These are studied at a rigorous level. The authors find that for an initial configuration obtained by placing particles independently with probability p or=2), the density of the 'bootstrapped' (final) configurations in the sequence of cubes (-L/2, L/2)d typically undergoes an abrupt transition, as L is increased, from being close to 0 to the value 1. With L fixed at a large value, the mean final density as a function of p changes from 0 to 1 around a value which varies only slowly with L-the pertinent parameter being lambda =p1(d-1)/ln L. The driving mechanism is the capture of a 'critical droplet'. This behaviour is analogous to the decay of a metastable state near a first-order phase transition, for which the analysis offers some suggestive ideas.

Multiple liquid crystal phases of DNA at high concentrations.

Abstract: DNA packaging in vivo is very tight, with volume concentrations approaching 70% w/v in sperm heads, virus capsids and bacterial nucleoids1–3. The packaging mechanisms adopted may be related to the natural tendency of semi-rigid polymers to form liquid crystalline phases in concentrated solutions4–8. We find that DNA forms at least three distinct liquid crystalline phases at concentrations comparable to those in vivo, with phase transitions occurring over relatively narrow ranges of DNA concentration. A weakly birefringent, dynamic, 'precholesteric' mesophase with microscopic textures intermediate between those of a nematic and a true cholesteric phase forms at the lowest concentrations required for phase separation. At slightly higher DNA concentrations, a second mesophase forms which is a strongly birefringent, well-ordered cholesteric phase with a concentration-dependent pitch varying from 2 to lOμm. At the highest DNA concentrations, a phase forms which is two-dimensionally ordered and resembles smectic phases of thermotropic liquid crystals observed with small molecules.

Phase transitions in magnetic superlattices

Abstract: We investigate the magnetic-field- and temperature-dependent equilibrium structure of magnetic superlattices formed from two ferromagnetic materials which couple antiferromagnetically at the interfaces. Both a macroscopic, Landau-Ginzburg, and a microscopic approach are used. Due to competing exchange and Zeeman interactions, a variety of phases exist in the superlattice. There are aligned phases where all the spins are either parallel or antiparallel to the applied field, and there is a twisted phase where the spins in each layer lie at a different angle with respect to the applied field. We show that small changes in the layering structure can lead to dramatic changes in the phase diagram.

Critical points of the volume phase transition in N‐isopropylacrylamide gels

Abstract: Two critical points of the gel–gel transition in N‐isopropylacrylamide gels were determined on the basis of the measurement of the coexistence curve. One of them was realized by changing the composition of solvent (water–methanol mixture) and the other by applying the osmotic pressure to a gel network, which was generated by adding large molecules to the outer solution. From the result the phase diagram in the xm(solvent composition)–T(temperature)–π(osmotic pressure) space was constructed. The order parameter of the gel–gel transition was determined for the first time as functions of temperature and solvent composition. The existence of the critical points was discussed in terms of recent theoretical studies.

Entropy and enthalpy catastrophe as a stability limit for crystalline material

Abstract: The transition from a crystalline material with long-range order to a glass-like disordered structure has been observed in several metal alloys and minerals using experimental techniques such as solid-state reaction, mechanical alloying, pressure application, ion-beam mixing and hydriding1–3. In all these examples the vitrification occurs below the glass transition temperature, and because the glass can be thought of as a highly undercooled liquid, one may draw an analogy between the vitrification process and melting. The melting process is often viewed as a catastrophic instability of the crystal lattice4–6, although none of these theories predicts the melting temperature correctly. The transition from crystal to glass could also be triggered by some type of instability7; indeed, Kauzmann8 has argued that an undercooled liquid whose entropy falls below that of the crystalline phase must undergo massive freezing to a glass. Applying an 'inverse' Kauzmann argument to the problem of melting, an entropy catastrophe is predicted when the entropy of a superheated crystal exceeds that of the liquid phase. We propose that this temperature represents an ultimate stability limit for superheated or supersaturated crystals.

Size-induced structural phase transitions and hyperfine properties of microcrystalline Fe2O3

Abstract: Microcrystalline particles of Fe2O3 having different sizes (varying between 70 and 5 nm) have been synthesised using a novel three-component micro-emulsion technique. A succession of crystal-size-induced structural transitions was observed. While alpha -Fe2O3 was found to nucleate for a particle size above 30 nm, gamma -Fe2O3 was preferentially formed for a size below 30 nm, whereas amorphous Fe2O3 was formed at a particle size of 5 nm. These structural transformations have been related to the increase in the unit-cell volume that occurs as the particle size is decreased. The size dependence of the lattice parameter is shown to arise from a coupling of the surface energy to the dilatational lattice mode. A model Hamiltonian which incorporates this interaction and displays size-induced phase transitions is defined. The Mossbauer hyperfine field in the microcrystalline samples at 4.2 K was found to be substantially smaller than in the 'bulk'. The hyperfine parameters of the amorphous sample were found to be similar to those pertaining to samples prepared by conventional techniques such as melt quenching. A large anisotropy in the ionic vibrational amplitudes was detected in samples with particles smaller than about 10 nm.

Anomalous spontaneous-stimulated-decay phase transition and zero-threshold laser action in a microscopic cavity.

Abstract: In an active ``microcavity'' the condition of maximum ``enhancement'' of spontaneous emission corresponds to resonant coupling of atoms with a single field mode. We demonstrate that this exceptional condition results in the effective cancellation of spontaneous emission, in the onset of stimulated emission at exceedingly low excitation levels, and in an anomalously high stimulated-emission gain. In the context of phase-transition theory the active microcavity behaves as a statistical ensemble undergoing an order-disorder transition at an extremely high value of the critical temperature.

Critical dynamics of the sol-gel transition.

Abstract: The dynamics of the sol-gel transition is probed by use of quasielastic light scattering. A type of critical dynamics is observed that is associated with a divergent friction, rather than a singularity in a thermodynamic quantity. Several novel effects are reported, including power-law time decay of the intensity autocorrelation function, critical slowing down of the average relaxation time, and observation of a fractal time set in the scattered field.

Heat Transfer During Melting and Solidification of Metals

Abstract: Discussion relative aux processus de transfert de chaleur fondamentaux lors de la transformation de phase solide-liquide et comparaison entre donnees experimentales et previsions de modeles mathematiques numeriques. Role important joue par l'ecoulement du liquide induit par la poussee. Identification des problemes necessitant des recherches

Phase transition in the 3d random field Ising model

Abstract: We show that the three-dimensional Ising model coupled to a small random magnetic field is ordered at low temperatures. This means that the lower critical dimension,dl for the theory isdl≦2, settling a long controversy on the subject. Our proof is based on an exact Renormalization Group (RG) analysis of the system. This analysis is carried out in the domain wall representation of the system and it is inspired by the scaling arguments of Imry and Ma. The RG acts in the space of Ising models and in the space of random field distributions, driving the former to zero temperature and the latter to zero variance.

Phase Transition in Solid Molecular Hydrogen at Ultrahigh Pressures

Abstract: Solid normal hydrogen was compressed in a diamond-anvil cell to pressures above 200 GPa. Spontaneous Raman spectra demonstrate that the solid undergoes a structural phase transformation beginning at 145 GPa at 77 K, as evidenced by an abrupt discontinuity in the intramolecular vibron frequency as a function of pressure. The magnitude of the vibron-frequency shift and the pressure-temperature conditions of the phase transformation are consistent with its identification as the theoretically predicted pressure-induced orientational ordering of the molecular solid.

Theory of nematic networks

Abstract: Using classical elasticity theory, the rise in free energy upon crosslinking nematogenic polymers into a network is calculated for the isotropic and nematic phases. Spontaneous strains are allowed for in the network. The consequence of network formation upon nematic–isotropic equilibria is calculated by adding these elastic contributions to a conventional Landau theory. Memory of the crosslinking conditions yields quartic and quadratic additions to the standard Landau theory. We find that crosslinking in the isotropic state lowers the nematic–isotropic phase transition temperature compared with the unlinked case and the application of suitable stress raises it again. Crosslinking in the nematic state raises the transition temperature. We recover the mechanical critical point proposed long ago by de Gennes. Our Gaussian theory encompasses both main‐ and side‐chain polymers. The hairpin limit for main chain networks yields a modulus varying exponentially with temperature. The Landau–de Gennes free energy for comb polymers is presented for the first time.

Defects and Structural Phase Transitions

Abstract: It is widely understood that many important properties of solids are conditioned or strongly influenced by lattice defects. This book covers the following subjects: elements of the theory of phase transitions in perfect crystals; phase transitions in imperfect crystals; space distribution of the order parameter induced by a point defect near the phase transition; phase transition anomalies in imperfect crystals: thermodynamic quantities and kinetic coefficients; scattering phenomena; polarized defects: bias field; model theories; and miscellaneous topics.

Fluids in micropores. II. Self‐diffusion in a simple classical fluid in a slit pore

Abstract: Self‐diffusion coefficients D are computed for a model slit pore consisting of a rare‐gas fluid confined between two parallel face‐centered cubic (100) planes (walls) of rigidly fixed rare‐gas atoms. By means of an optimally vectorized molecular‐dynamics program for the CYBER 205, the dependence of D on the thermodynamic state (specified by the chemical potential μ, temperature T, and the pore width h) of the pore fluid has been explored. Diffusion is governed by Fick’s law, even in pores as narrow as 2 or 3 atomic diameters. The diffusion coefficient oscillates as a function of h with fixed μ and T, vanishing at critical values of h, where fluid–solid phase transitions occur. A shift of the pore walls relative to one another in directions parallel with the walls can radically alter the structure of the pore fluid and consequently the magnitude of D. Since the pore fluid forms distinct layers parallel to the walls, a local diffusion coefficient D(i)∥ associated with a given layer i can be defined. D(i)∥ is least for the contact layer, even for pores as wide as 30 atomic diameters (∼100 A). Moreover, D(i)∥ increases with increasing distance of the fluid layer from the wall and, for pore widths between 16 and 30 atomic diameters, D(i)∥ is larger in the center of the pore than in the bulk fluid that is in equilibrium with the pore fluid. The opposite behavior is observed in corresponding smooth‐wall pores, in which the discrete fluid–wall interactions have been averaged by smearing the wall atoms over the plane of the wall. The temperature dependence of D for fixed h is determined and the nature of melting of a pore solid is examined. It is found that the solid tends to melt first in the middle of the pore. All of the various results are related to the structural properties of the pore fluid, as manifested by the local density and pair correlation functions.

High pressure solid phases of benzene. I. Raman and x‐ray studies of C6H6 at 294 K up to 25 GPa

Abstract: Crystalline benzene has been investigated at room temperature as a function of pressure up to 25 GPa in diamond anvil cells by Raman scattering and powder x‐ray diffraction techniques. The concomitant spectroscopic and crystallographic results show the existence of numerous pressure‐induced phases. Changes in the profiles of the Raman spectra and in the x‐ray diffraction patterns, as well as changes in the variations of the Raman frequencies and the cell parameters with pressure indicate two first‐order phase transitions at 1.4±0.1 and 4±1 GPa and a second‐order one at 11±1 GPa. At 24 GPa the x‐ray diffraction pattern seems to indicate the existence of a new phase. Two monoclinic structures are proposed for the phases above 1.5 GPa, in addition to the already known one. From these data, molar volume has been determined as a function of pressure and the Gruneisen parameters have been inferred in the different phases. Their pressure dependences are analyzed in the light of theoretical predictions. Arguments are given for a phase transformation at normal pressure and below 140 K or at room temperature below 1 GPa. A schematic P–T phase diagram is suggested and a controversy on the nature of the triple points located on the melting curve is clarified.

Random Surface Model for the L3-Phase of Dilute Surfactant Solutions

Abstract: We present a simple model for the anomalous (flow-birefringent) isotropic phase, known as L3, that is seen in certain surfactant solutions at volume fractions of a few percent. The proposed structure consists of locally sheetlike sections of semi-flexible surfactant bilayer, connected up at larger distances into a multiply connected random surface, having a preferred structural length scale of order the persistence length of the bilayer. A first-order transition between this isotropic sheetlike phase and the nearby swollen lamellar phase is described.

Ferromagnetism of ultrathin films.

Abstract: We discuss the nature of ferromagnetism in ultrathin films of magnetic ions, here regarded as two-dimensional Heisenberg ferromagnets subject to uniaxial anisotropy with the easy axis normal to the film We show that a phase transition to ferromagnetism occurs always for arbitrarily small anisotropy Renormalization-group scaling relations for the transition temperature and the temperature variation of the correlation length are obtained Implications of these results are discussed