Showing papers on "Phase transition published in 1992"

Electrochemical and In Situ X‐Ray Diffraction Studies of Lithium Intercalation in Li x CoO2

Abstract: Electrochemical properties of are studied as Li is deintercalated from . High precision voltage measurements and in situ x‐ray diffraction indicate a sequence of three distinct phase transitions as varies from 1 to 0.4. Two of the transitions are situated slightly above and below and are caused by an order/disorder transition of the lithium ions. The order/disorder transition is studied as a function of temperature allowing the determination of an order/disorder phase diagram. In situ x‐ray diffraction measurements facilitate a direct observation of the effects of deintercalation on the host lattice crystal structure. The other phase transition is shown to be first order (coexisting phases are observed for ) involving a significant expansion of the parameter of the hexagonal unit cell. We report the variation of the lattice constants of with and show that the phase transition to the lithium ordered phase near is accompanied by a lattice distortion to a monoclinic unit cell with , , and . Finally we report an overall phase diagram for and .

Systematic study of insulator-metal transitions in perovskites RNiO3 (R=Pr,Nd,Sm,Eu) due to closing of charge-transfer gap.

Abstract: The detailed behavior of the phase transitions was mapped out for the series R${\mathrm{NiO}}_{3}$ as a function of the rare earth (R). A sharp insulator-metal transition is observed, which depends strongly on R.Forsmall$R it occurs at a higher temperature than the antiferromagnetic ordering (measured by muon-spin relaxation). By increasing either the temperature or the size of R, an insulator-metal transition is observed, most probably caused by the closing of the charge-transfer gap, induced by increase in the electronic bandwidth.

Phase transitions in lyotropic colloidal and polymer liquid crystals

Abstract: An overview is given of theory and experiments on liquid crystal phases which appear in solutions of elongated colloidal particles or stiff polymers. The Onsager (1949) virial thecry for the isotropionematic transition of thin rodlike particles is treated comprehensively along with extensions to polydisperse solutions and soft interactions. Computer simulations of liquid crystal phases in hard particle fluids are summarized and used to assess the quality of statistical mechanical thwries for stiff panicles at higher dume haion-like the inclusion of higher Virial coefficients, yexpansion, scaled particle theory and density functional theory. Both computer simulations and density functional theory indicate formation of more highly ordered smectic phases. The range of experimental applicability h strongly widened by the extension of the viriai theory to wormlike chains by Khokhlov and Semenov (1981, 1982). Fmally, experimental results for a number of carefully studied, charged and uncharged colloids and polymers are summarized and wmpared to theoretical results. IE many cases the agreement is semi-quantitative.

First-order phase transitions with more than one conserved charge: Consequences for neutron stars.

Abstract: We consider how first-order phase transitions in systems having more than one conserved charge (multicomponent systems) differ from those in systems having only one. In general, the properties of the transition are quite different in the two cases. Perhaps most importantly the pressure varies continuously with the proportion of phases in equilibrium, and is not a constant in the mixed phase as in the example of the gas-liquid transition in familiar one-component systems. We identify the microphysics responsible for the difference. In the case that one of the conserved charges is the electric charge, a geometrical structure in the mixed phase is expected. As an example, possible consequences are developed for the structure of a neutron star in which the transition to quark matter in the core occurs. It is also pointed out that the general results pertain to relativistic nuclear collisions in the so-called stopping or baryon-rich domain where there are three conserved charges (baryon, electric, and strangeness), and impact the expected phase transition from confined hadronic matter to quark matter as regards signals that are supposedly driven by pressure. The physics discussed here is also relevant to the subunclear gas-liquid transition that is under study in lower-energy nuclear collisions.

Structural Phase Transitions in the Fullerene C60

Abstract: High-resolution powder neutron diffraction has been used to study the crystal structure of the fullerence C60 in the temperature range 5 K to 320 K. Solid C60 adopts a cubic structure at all temperatures. The experimental data provide clear evidence of a continuous phase transition at ca. 90 K and confirm the existence of a first-order phase transition at 260 K. In the high-temperature face-centred-cubic phase (T > 260 K), the C60 molecules are completely orientationally disordered, undergoing continuous reorientation. Below 260 K, interpretation of the diffraction data is consistent with uniaxial jump reorientation principally about a single 111 direction. In the lowest-temperature phase (T < 90 K), rotational motion is frozen although a small amount of static disorder still persists.

Effective potential at finite temperature in the standard model.

Abstract: There has been much recent interest in the nature of the electroweak phase transition. This information is of importance in the context of the sphaleron models that have recently been proposed to explain the observed net baryon number in the Universe. The presence of a term that is cubic in the Higgs condensate in the one-loop effective potential appears to indicate a first-order phase transition. However, the infrared singularities inherent in massless models produce cubic terms that are of the same order in the coupling. In this paper, we include these terms and show that the standard model has a first-order phase transition.

Gravitational waves from first-order cosmological phase transitions.

Abstract: A first-order cosmological phase transition that proceeds through the nucleation and collision of true-vacuum bubbles is a potent source of gravitational radiation. Possibilities for such include first-order inflation, grand-unified-theory-symmetry breaking, and electroweak-symmetry breaking. We have calculated gravity-wave production from the collision of two scalar-field vacuum bubbles, and, using an approximation based upon these results, from the collision of 20 to 30 vacuum bubbles. We present estimates of the relic background of gravitational waves produced by a first-order phase transition.

Towards the theory of the electroweak phase transition

Abstract: We investigate various problems related to the theory of the electroweak phase transition. This includes determination of the nature of the phase transition, discussion of the possible role of the higher-order radiative corrections, and the theory of the formation and evolution of bubbles of the new phase. We show, in particular, that no dangerous linear terms in the scalar field $\ensuremath{\varphi}$ appear in the expression for the effective potential. We have found that, for the Higgs-boson mass smaller than the masses of $W$ and $Z$ bosons, the phase transition is of the first order. However, its strength is approximately ⅔ times less than what follows from the one-loop approximation. The phase transition occurs due to production and expansion of critical bubbles. Subcritical bubbles may be important only if the phase transition is very weakly first order. A general analytic expression for the probability of the bubble formation is obtained, which may be used for study of tunneling in a wide class of theories. The bubble-wall velocity depends on many factors, including the ratio of the mean free path of the particles to the thickness of the wall. Thin walls in the electroweak theory have a nonrelativistic velocity, whereas thick walls may be relativistic. A decrease of the cubic term by the factor ⅔ rules out baryogenesis in the minimal version of the electroweak theory. Even though we concentrate in this paper on the phase transition in this theory, most of our results can be applied to more general models as well, where baryogenesis is possible.

Phase behavior of disklike hard-core mesogens

Abstract: We report a computer-simulation study of a system of ‘‘hard cut spheres,’’ oblate particles that are obtained by slicing off the top and bottom caps of a sphere at a distance L/2 from the equatorial plane. This system serves as a model for a disklike mesogen. The phase behavior of this system is found to be strongly dependent on the length-to-width ratio (L/D) of the particles. In addition to isotropic and solid phases, we find nematic and columnar phases for L/D=0.1. For L/D=0.2 we find a columnar phase, and a phase with cubic orientational order but no translational order. For L/D=0.3, only the isotropic fluid and the solid are stable. Where possible, we have located the phase transitions.

Simulation of polyethylene above and below the melting point

Abstract: Polyethylene at equilibrium is studied by computer simulation. Configuration space is sampled efficiently by a novel Monte Carlo simulation scheme developed for the study of long molecules at high densities. Simulations are carried out in an isobaric‐isothermal statistical‐mechanical ensemble which permits calculation of the density of the polymer matrix at specified conditions of pressure and temperature. A systematic study of the polymer at different temperatures indicates a phase transition; in agreement with experiment, at low temperatures, the polyethylene model studied here crystallizes spontaneously. At temperatures above the melting point, the simulated melt is described accurately by the model.

Experimental evidence for a first-order vortex-lattice-melting transition in untwinned, single crystal YBa2Cu3O7.

Abstract: We report on current-voltage measurements in clean, untwinned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ single crystals with picovolt voltage sensitivity and millikelvin temperature resolution in magnetic fields ranging up to 7 T. We find evidence for a melting transition in the vortex lattice which is hysteretic in both temperature and magnetic field. The measured thermal and magnetic hysteresis widths are related by the local slope of the phase boundary. This strongly supports the picture that, in the clean limit, the melting transition of the Abrikosov vortex lattice is a first-order phase transition.

Phase transitions and universal dynamics in confined films.

Abstract: We describe molecular dynamics simulations of fluid films confined between two solid walls. The films consist of spherical molecules, or flexible linear chains with up to twenty monomers. When the wall separation is only a few molecular diameters, crystalline or glassy order is induced across the film. The onset of the glassy phase is characterized by rapidly increasing relaxation times. These manifest themselves through changes in the diffusion constant and in the response to shear. The viscosity exhibits the same power-law scaling with shear rate that was observed in recent experiments. Our study suggests that this response is a universal property of lubricants near a glass transition.

Magnetization direction switching in Fe/Cu(100) epitaxial films: Temperature and thickness dependence.

Abstract: The magnetization direction switching is investigated in epitaxial Fe/Cu(100) films by spin-polarized scanning electron microscopy. We follow the transition from perpendicular to in-plane magnetization with both increasing film thickness and varying temperature. No variation of magnetic moment with magnetization direction change is found. A perpendicular stripe domain configuration is identified which evolves from the low-temperature single-domain state during the reorientation phase transition.

Static and Dynamic Critical Phenomena at a Second Order QCD Phase Transition

Abstract: In QCD with two flavors of massless quarks, the chiral phase transition is plausibly in the same universality class as the classical four component Heisenberg antiferromagnet. Therefore, renormalization group techniques developed in the study of phase transitions can be applied to calculate the critical exponents which characterize the scaling behaviour of universal quantities near the critical point. This approach to the QCD phase transition has implications both for lattice gauge theory and for heavy ion collisions. Future lattice simulations with longer correlation lengths will be able to measure the various exponents and the equation of state for the order parameter as a function of temperature and quark mass which we describe. In a heavy ion collision, the consequence of a long correlation length would be large fluctuations in the number ratio of neutral to charged pions. Unfortunately, we show that this phenomenon will not occur if the plasma stays close to equilibrium as it cools. If the transition is far out of equilibrium and can be modelled as a quench, it is possible that large volumes of the plasma with the pion field correlated will develop, with dramatic phenomenological consequences. }

Inverse melting transition and evidence of three-dimensional cubatic structure in a block-copolymer micellar system.

Abstract: Structural studies on aqueous solutions of PEO-PPO-PEO block copolymers show three phases as the temperature is varied. At low T, the tri-block units are dissolved Gaussian chains. As T is increased still more unimers aggregate in micelles, until a volume fraction of 0.52 is reached. The micelles then crystallize in a body-centered-cubic lattice. Upon the application of a small shear, the high-T polycrystalline phase abruptly transforms into a single crystal, characterized by only quasi-long-range order in bond length, but true long-range correlations in the bond angle.

C60 rotation in the solid state: dynamics of a faceted spherical top.

Abstract: The rotational dynamics of C60 in the solid state have been investigated with carbon-13 nuclear magnetic resonance (13C NMR). The relaxation rate due to chemical shift anisotropy (1/9T1CSA1) was precisely measured from the magnetic field dependence of T1, allowing the molecular reorientational correlation time, τ, to be determined. At 283 kelvin, τ = 9.1 picoseconds; with the assumption of diffusional reorientation this implies a rotational diffusion constant D = 1.8 x 1010 per second. This reorientation time is only three times as long as the calculated τ for free rotation and is shorter than the value measured for C60 in solution (15.5 picoseconds). Below 260 kelvin a second phase with a much longer reorientation time was observed, consistent with recent reports of an orientational phase transition in solid C60. In both phases τ showed Arrhenius behavior, with apparent activation energies of 1.4 and 4.2 kilocalories per mole for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The results parallel those found for adamantane.

Phase Transitions of Gels

Abstract: Polymer gels are known to exist in two distinct phases, swollen and collapsed. Volume transition occurs between the phases either continuously or discontinuously in response to chemical and physical stimuli such as temperature, solvent composition, pH, ionic composition, electric field, light, and particular molecules. For a gel to undergo the phase transition, it is necessary that polymers interact with each other through both repulsive and attractive interactions and the balance of competing interactions has to be modified by various stimuli. The phase behavior of a gel, therefore, crucially depends on the nature of interactions between polymers. Recently new phases and volume transitions between them have been discovered in some gels. Detailed examination of the gel phase behavior provides a deep insight into the polymer-polymer interactions and configurations of polymers. The knowledge on physical and chemical fundamentals of gel phase transition will play a role as guiding principles for a wide variety of technological applications of gels as functional elements.

Patterns in shrinking gels

Abstract: POLYMER gels can undergo a volume phase transition (either continuous or discontinuous) when an external condition, such as temperature or solvent composition, is altered1–3. During this transition, the volume may change by a factor of several thousand, and various patterns develop in the gel. The patterns arising from swelling and shrinking differ in both their appearance and their physical mechanisms. The mechanism for the formation and evolution of patterns on swelling gels has been established as being due to a single kind of mechanical instability4–7; in contrast, the shrinking patterns seem to be sensitive to both the initial and final states of the transition. Here we classify the various shrinking patterns in the form of a phase diagram, and explain the poly-morphism in terms of macroscopic phase separation.

Error thresholds for molecular quasispecies as phase transitions: From simple landscapes to spin-glass models.

Abstract: The correspondence between Eigen's model [Naturwissenschaften 58, 465 (1971)] for molecular quasispecies and the equilibrium properties of a lattice system proposed by Leuthausser [J. Chem. Phys. 84, 1884 (1986); J. Stat. Phys. 48, 343 (1987)] is used to characterize the error thresholds for the existence of quasispecies as phase transitions. For simple replication landscapes the error threshold is related to a first-order phase transition smoothed by the complete wetting of the time surface

Reversible photoinduced phase transitions in single crystals of polydiacetylenes.

Abstract: It was observed that bi-directional phase changes can be triggered by genuine photoexcitation between two phases with different bond structures in polydiacetylenes The observed photoinduced phase transition was found to be mediated by photogeneration of charge carriers (and not of excitons) The extremely high efficiency is perhaps due to cooperative interaction between the locally photoconverted domains

Ordering of thin diblock copolymer films.

Abstract: The ordering in thin films of symmetric diblock copolymers has been studied by neutron reflectivity as a function of temperature and film thickness. Exponentially damped oscillatory variations in the concentration are shown to propagate into the specimens from both surfaces with decay lengths increasing with decreasing temperature. No distinguishable order-to-disorder transition is observed; only a transition from a partially to fully ordered state is found. The temperature of this transition is found to depend in a power-law manner on the film thickness.

The relationship between deposition conditions, the beta to alpha phase transformation, and stress relaxation in tantalum thin films

Abstract: We demonstrate that the high temperature polymorphic tantalum phase transition from the tetragonal beta phase to the cubic alpha phase causes a large decrease in the resistance of thin films and a complete stress relaxation in films that were intrinsically compressively stressed. 100 nm beta tantalum thin films with intrinsic stresses of 2.0×1010 dynes/cm2 (tensile) to −2.3×1010 dynes/cm2 (compressive) were deposited onto thermally oxidized (100) silicon wafers by evaporation or dc magnetron sputtering with argon. In situ stress and resistance at temperature were measured at 10 °C/min up to 850 °C in purified helium. Upon heating, the main stress mechanisms were elastic deformation at low temperature, plastic deformation at moderate temperatures and stress relief because of the beta‐to‐alpha phase transition at high temperatures. The temperature ranges over which the elastic and plastic deformation and the beta‐to‐alpha phase transition occurred varied with deposition pressure and substrate biasing. Incomplete compressive stress relaxation at high temperatures was observed if the film was initially deposited in the alpha phase or if the beta phase did not completely transform into alpha by 800 °C due to substrate biasing during the deposition. We conclude that the main stress relief mechanism for tantalum films with intrinsic compressive stresses to completely relax their stress is the beta‐to‐alpha phase transition, while for intrinsically tensile films, this transformation has a much smaller effect on the stress.

Universal quantum-critical dynamics of two-dimensional antiferromagnets.

Abstract: The universal dynamic and static properties of two-dimensional antiferromagnets in the vicinity of a zero-temperature phase transition from long-range magnetic order to a quantum-disordered phase are studied. Random antiferromagnets with both N\'eel and spin-glass long-range magnetic order are considered. Explicit quantum-critical dynamic scaling functions are computed in a 1/N expansion to two-loop level for certain nonrandom, frustrated square-lattice antiferromagnets. Implications for neutron scattering experiments on the doped cuprates are noted.

The Molecular-Metallic Transition of Hydrogen and the Structure of Jupiter and Saturn

Abstract: Recently, a new equation of state for hydrogen which predicts a molecular-metallic phase transition at finite temperature has become available. It is combined with a helium equation of state, and the resulting thermodynamic description of H/He mixtures is used to compute interior models of Jupiter and Saturn, subject to the constraints of the measured gravitational harmonics of both planets. The inferred heavy element abundance distribution in their interiors and the possible consequences on their formation are discussed

Phase transitions of liquid-crystal films on an air-water interface.

Abstract: Surface-balance, ellipsometry, and optical second-harmonic-generation techniques are employed to investigate the phase transitions of liquid-crystal 4'-n-octyl-4-cyanobiphenyl films on an air-water interface. We have found, for the first time, a distinctive first-order phase transition between a stable monolayer phase and a stable 3-layer phase of the liquid-crystal film