Showing papers on "Phase transition published in 1991"

Thermal fluctuations, quenched disorder, phase transitions, and transport in type-II superconductors.

Abstract: The effects of thermal fluctuations, quenched disorder, and anisotropy on the phases and phase transitions in type-II superconductors are examined, focusing on linear and nonlinear transport properties. In zero magnetic field there are two crossovers upon approaching ${\mathit{T}}_{\mathit{c}}$, first the ``Ginzburg'' crossover from mean-field behavior to the universality class of an uncharged superfluid, and then, much closer to ${\mathit{T}}_{\mathit{c}}$ for strongly type-II systems, a crossover to the universality class of a charged superfluid. The primary focus of this paper is on the behavior in the presence of a penetrating magnetic field. In a clean system the vortex-lattice phase can melt due to thermal fluctuations; we estimate the phase boundary in a variety of regimes. Pinning of vortices due to impurities or other defects destroys the long-range correlations of the vortex lattice, probably replacing it with a new vortex-glass phase that has spin-glasslike off-diagonal long-range order and is truly superconducting, in contrast to conventional theories of ``flux creep.'' The properties of this vortex-glass phase are examined, as well as the critical behavior near the transition from the vortex-glass to the vortex-fluid phase. The crossover from lattice to vortex-glass behavior for weak pinning is also examined. Linear and nonlinear conductivity measurements and other experiments on the high-${\mathit{T}}_{\mathit{c}}$ superconductors Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O are discussed in light of the results. The latter is found to exhibit strongly two-dimensional behavior over large portions of its phase diagram.

Rapid‐phase transitions of GeTe‐Sb2Te3 pseudobinary amorphous thin films for an optical disk memory

Abstract: Amorphous films having a component of the stoichiometric GeTe‐Sb2Te3 pseudobinary alloy system, GeSb2Te4 or Ge2Sb2Te5 representatively, were found to have featuring characteristics for optical memory material presenting a large optical change and enabling high‐speed one‐beam data rewriting. The material films being sandwiched by heat‐conductive ZnS layers can be crystallized (low power) or reamorphized (high power) by laser irradiation of very short duration, less than 50 ns. The cooling speed of the sandwiched film is extremely high: more than 1010 deg/s, which permits the molten material to convert to the amorphous state spontaneously; whereas, a low‐power pulse irradiation of the same duration changed the exposed portion into the crystalline state. The optical constant changes between the amorphous state and the crystalline state of them were measured to be large: from 4.7+i1.3 to 6.9+i2.6 and from 5.0+i1.3 to 6.5+i3.5, respectively. The crystallized portion was known to have a GeTe‐like fcc structure ...

Where the really hard problems are

Abstract: It is well known that for many NP-complete problems, such as K-Sat, etc., typical cases are easy to solve; so that computationally hard cases must be rare (assuming P = NP). This paper shows that NP-complete problems can be summarized by at least one "order parameter", and that the hard problems occur at a critical value of such a parameter. This critical value separates two regions of characteristically different properties. For example, for K-colorability, the critical value separates overconstrained from underconstrained random graphs, and it marks the value at which the probability of a solution changes abruptly from near 0 to near 1. It is the high density of well-separated almost solutions (local minima) at this boundary that cause search algorithms to "thrash". This boundary is a type of phase transition and we show that it is preserved under mappings between problems. We show that for some P problems either there is no phase transition or it occurs for bounded N (and so bounds the cost). These results suggest a way of deciding if a problem is in P or NP and why they are different.

Orientational ordering transition in solid C60.

Abstract: Synchrotron-x-ray powder-diffraction and differential-scanning-calorimetry measurements on solid ${\mathrm{C}}_{60}$ reveal a first-order phase transition from a low-temperature simple-cubic structure with a four-molecule basis to a face-centered-cubic structure at 249 K. The free-energy change at the transition is approximately 6.7 J/g. Model fits to the diffraction intensities are consistent with complete orientational disorder at room temperature, and with the development of orientational order rather than molecular displacements or distortions at low temperature.

Phase-field model for isothermal phase transitions in binary alloys

Abstract: In this paper we present a phase-field model to describe isothermal phase transitions between ideal binary-alloy liquid and solid phases. Governing equations are developed for the temporal and spatial variation of the phase field, which identifies the local state or phase, and for the composition. An asymptotic analysis as the gradient energy coefficient of the phase field becomes small shows that our model recovers classical sharp-interface models of alloy solidification when the interfacial layers are thin, and we relate the parameters appearing in the phase-field model to material and growth parameters in real systems. We identify three stages of temporal evolution for the governing equations: the first corresponds to interfacial genesis, which occurs very rapidly; the second to interfacial motion controlled by diffusion and the local energy difference across the interface; the last takes place on a long time scale in which curvature effects are important, and corresponds to Ostwald ripening. We also present results of numerical calculations.

Surface effects and anchoring in liquid crystals

Abstract: As their name indicates, liquid crystals simultaneously exhibit some characteristics common to both ordinary isotropic liquids and solid crystals. This ambivalence is also found in the effects of surfaces on these systems which lead to a great diversity of phenomena. These phenomena are reviewed focusing on nematic liquid crystals which have the simplest structure among the many existing types and which have been the most extensively studied. Three main kinds of effects can be distinguished. The first concerns the perturbation of the liquid crystalline structure close to the surface. Beyond this transition region, the bulk liquid crystalline structure is recovered with an orientation which is fixed by the surface: this phenomenon of orientation of liquid crystals by surfaces is the so-called anchoring. Finally, close to bulk phase transitions, critical adsorption or wetting can occur at surfaces as is also seen in isotropic systems.

Phase Diagrams of the Elements

Abstract: A summary of the pressure-temperature phase diagrams of the elements is presented, with graphs of the experimentally determined solid-solid phase boundaries and melting curves. Comments, including theoretical discussion, are provided for each diagram. The crystal structure of each solid phase is identified and discussed. This work is aimed at encouraging further experimental and theoretical research on phase transitions in the elements. (auth)

Boundary-induced phase transitions in driven diffusive systems.

Abstract: Steady states of driven lattice gases with open boundaries are investigated. Particles are red into the system at one edge, travel under the action of an external field, and leave the system at the opposite edge. Two types of phase transitions involving nonanalytic changes in the density profiles and the particle number fluctuation spectra are encountered upon varying the feeding rate and the particle interactions, and associated diverging length scales are identified. The principle governing the transitions is the tendency of the system to maximize the transported current.

Volume transition in a gel driven by hydrogen bonding

Abstract: INTERACTIONS between macromolecules fall into four categories: ionic, hydrophobic, van der Waals and hydrogen bonding. Phase transitions in polymer gels provide a means of studying these interactions. Many gels will undergo reversible, discontinuous volume changes in response to changes in, for example, temperature, gel composition or light irradiation1–5. These transitions result from the competition between repulsive intermolecular forces, usually electrostatic in nature, that act to expand the polymer network, and an attractive force that acts to shrink it. Volume transitions in gels have been observed that are driven by all of the above-mentioned forces except hydrogen bonding (ref 6–10; T.T. et al, unpublished data; H. Inomata et al., personal communication). Here we report on a phase transition in an interpenetrating polymer network of poly(acrylamide) and poly(acrylic acid) that completes this picture—it is controlled by cooperative 'zipping' interactions between the molecules which result from hydrogen bonding. Cooperativity is an essential feature of the interactions, in that independent hydrogen bonds would not provide a sufficient driving force for the transition. A further novel characteristic of this phase transition is that the swelling (in water) is induced by an increase rather than a decrease in temperature.

Kinetic relations and the propagation of phase boundaries in solids

Abstract: This paper treats the dynamics of phase transformations in elastic bars The specific issue studied is the compatibility of the field equations and jump conditions of the one-dimensional theory of such bars with two additional constitutive requirements: a kinetic relation controlling the rate at which the phase transition takes place and a nucleation criterion for the initiation of the phase transition A special elastic material with a piecewise-linear, non-monotonic stress-strain relation is considered, and the Riemann problem for this material is analyzed For a large class of initial data, it is found that the kinetic relation and the nucleation criterion together single out a unique solution to this problem from among the infinitely many solutions that satisfy the entropy jump condition at all strain discontinuities

Molecular dynamics and the phase transition in solid C 60

Abstract: We characterize the molecular reorientational dynamics in two phases of solid ${\mathrm{C}}_{60}$ with $^{13}\mathrm{C}$ NMR measurements. A change in the nature of the dynamics, indicated by a change in kinetic parameters extracted from spin-lattice relaxation data, occurs at the phase transition at 260 K. Above the transition, the molecules appear to execute continuous rotational diffusion; below the transition, they appear to jump between symmetry-equivalent orientations. This interpretation is consistent with the x-ray-diffraction results of Heiney et al. as well as our NMR relaxation and spectral data.

Neutron diffraction study on structural and magnetic properties of La2NiO4

Abstract: An overall survey of the structural and magnetic features of the La2NiO4+ delta system is presented as a result of neutron diffraction experiments. The stoichiometric compound ( delta =0) presents two structural phase transitions. At T0 approximately=770 K, La2NiO4 transforms from tetragonal (I4/mmm) to orthorhombic (Bmab); at T1 approximately=80 K, from orthorhombic to a new tetragonal (P42/ncm) phase. Associated with this second phase transition a strong microstrain produces anisotropic broadening of Bragg reflections. La2NiO4 is three-dimensional (3D) antiferromagnetically ordered at room temperature (TN=330 K). A weak ferromagnetic component appears below T1. Oxygen excess suppress the 3D magnetic ordering and the structural phase transformations, giving rise to a non-stoichiometric compound with interstitial oxygens. A tentative phase diagram is proposed.

On the pseudo-elastic hysteresis

Abstract: It is often considered that in solids a non-monotone load-deformation characteristic of the “van der Waals type” may give rise to hysteretic behaviour in a phase transition. Here we consider a pseudo-elastic hysteresis that occurs in the austenitic-martensitic phase transition of shape memory alloys. For demonstrative purposes we let the non-monotone characteristic first be defined by the Landau-Devonshire model, later we even use straight lines. We ask the following questions: (i) How wide is the hysteresis loop? (ii) How does the width of the hysteresis loop change with temperature? (iii) What are the possible processes inside the hysteresis loop? Answers are provided for all three questions, viz. On (i) The width is determined by the interfacial energy of the phases. On (ii) There is no significant dependence on temperature. On (iii) The hysteresis loop contains metastable states that loose their (meta-)stability on a line defining phase equilibrium. The theoretical predictions are qualitative results. They are supported by tensile experiments on a CuZnAl single crystal.

Softening of bulk modulus and negative Poisson's ratio near the volume phase transition of polymer gels

Abstract: The static bulk and shear modulus of poly (N‐isopropylacrylamide)/water gel (NIPA gel) have been obtained as a function of temperature by measuring the change of sample size induced by an applied tension. It is deduced that the method gives the static elastic moduli of gels to a good approximation except just above the discontinuous transition temperature T0, where the stress‐induced shift of T0 made the estimation of elastic strain impossible. Two kinds of NIPA gels with different polymer concentration and crosslinking density were used as samples, of which one undergoes a discontinuous volume phase transition and the other a continuous one. In both gels, the bulk modulus softens drastically towards the transition showing that the soft mode of this transition is a uniform and isotropic volume‐deformation mode. The shear modulus, on the other hand, shows only a small anomaly at the transition. The anomaly exhibited by the Poisson’s ratio σ is quite peculiar, i.e., it becomes negative and approaches −1 towards the critical point. It is shown that the results can be explained fairly well on the basis of the Flory‐type phenomenological theory extended to take into account the concentration dependence of the interaction parameter.

Specific heat in some gadolinium compounds. I. Experimental

Abstract: Specific-heat measurements have been performed on the gadolinium compounds ${\mathrm{GdCu}}_{2}$${\mathrm{Si}}_{2}$, ${\mathrm{GdNi}}_{2}$${\mathrm{Si}}_{2}$, ${\mathrm{GdGa}}_{2}$, and ${\mathrm{GdCu}}_{5}$ in order to investigate the phase transitions between paramagnetic and commensurate or incommensurate antiferromagnetic states. Various shapes of specific-heat curves are observed at and below the ordering temperature ${\mathit{T}}_{\mathit{N}}$. In particular, the specific-heat discontinuity at ${\mathit{T}}_{\mathit{N}}$ is strongly reduced in the incommensurate systems. This behavior is related to the exact type of magnetic arrangement involved.

Thermal fluctuations and phase transitions in the vortex state of a layered superconductor.

Abstract: We study thermal fluctuations in a layered superconductor in the presence of a magnetic field applied orthogonal to the layers. A phase diagram for this case is proposed. In the weak-field region, fluctuations of a vortex lattice are of three-dimensional (3D) nature. This leads to a two-stage melting: When the temperature is raised, a phase transition to the vortex-line liquid occurs, then independent liquid systems of 2D vortices in different layers are formed. For fields larger than the crossover value, both fluctuations and melting of the vortex lattice become of 2D type. We study the effect of vortex-lattice fluctuations on the long-range superconducting order. We demonstrate the existence of a phase transition within the vortex-lattice state: The superconducting coherence between distant layers vanishes at a temperature which is substantially lower than the melting temperature.

SQUID picovoltometry of YBa2Cu3O7 single crystals: Evidence for a finite-temperature phase transition in the high-field vortex state.

Abstract: Using a SQUID picovoltmeter. we have measured current-voltage curves of YBa2Cu3O7 microtwinned crystals as a function of temperature in fields from I to 6 T. The data constitute evidence for a finite-temperature phase transition in the vortex state. Exponents derived within the framework of the vortex-glass model are found to be similar to thin-film values. Strongly temperature-dependent correlations in-volving up to 105 vortices near the transition. combined with the qualitative failure of thermal-activation models to fit our data. support the existence of a phase transition.

Order-parameter saturation and low-temperature extension of Landau theory

Abstract: The temperature evolution of the structural order parameter in displacive phase transitions with high transition temperatures (e.g. quartz, As2O5, LaAlO3, CaCO3, NaNO3, Pb3(PO4)2 etc.) follows a Landau-type behaviour over large temperature intevals. Below a saturation temperatureT s , however, the order parameter tends to become temperature independent. A quantitative analysis shows the correlationk B T s =ħΩ/4, where Ω is the frequency of the relevant local excitation atT s . A general Landau-type expression for the Gibbs energy is given which includes the effects of order-parameter saturation.

Premelting crystalline relaxations and phase transitions in nylon 6 and 6,6

Abstract: Variable-temperature XRD and NMR measurements show that nylon 6 (N6) undergoes crystalline relaxations between the glass transition temperature and the melting point. These relaxations bring about a crystalline transition between 80 and 170 o C from a monoclinic structure to a new crystalline structure, which is also most likely monoclinic. Thus, the presence of a Brill transition in N 6 is demonstrated for the first time; this phenomena has been extensively studied in nylon 6,6 (N6,6)

Saccharide-sensitive phase transition of a lectin-loaded gel

Abstract: A GEL system that swells and shrinks in response to specific molecules could serve as the basis for technological applications of polymer gels, for example as sensors, drug-delivery devices and actuators. Here we describe a lectin-loaded polymer gel that undergoes distinct swelling behaviour in response to different saccharides. The gel consists of a covalently cross-linked polymer network of N-isopropylacrylamide in which the lectin, concanavalin A, is immobilized. Concanavalin A displays selective binding affinities for certain saccharides. The gel undergoes a volume phase transition at ∼34 °C. When the saccharide dextran sulphate is added (as the sodium salt DSS) to the gel, it swells to a volume up to five times greater at temperatures close to this transition, and the transition itself changes from discontinuous to continuous. Replacing DSS with the non-ionic saccharide α-methyl-D-mannopyranoside brings about collapse of the gel back to almost its native volume. This process is reversible and repeatable. These results point to a general principle for the design of such molecule-specific systems.

Equivalence and solution of anisotropic spin-1 models and generalized t-J fermion models in one dimension

Abstract: The authors study the relationship of two 'q-deformed' spin-1 chains-both of them are solvable models-with a generalized supersymmetric t-J fermion model in one dimension. One of the spin-1 chains is an anisotropic VBS model for which they calculate ground state and ground-state properties. The other spin-1 chain corresponds to the Zamolodchikov-Fateev model which is solvable by Bethe ansatz and is equivalent to a certain t-J model. The two spin-1 models intersect for a certain value of the 'deformation' parameter q in a second-order phase transition.

Decay of density fluctuations in gels.

Abstract: Near the sol-gel transition, gelling systems exhibit an extremely slow relaxation of thermally driven density fluctuations. We have made a detailed quasielastic light scattering study of the decay of density fluctuations in reacting silica sol-gels in the pre- and post-gel regimes, and at the gel point. In the pre-gel regime the dynamic structure factor {ital S}({ital q},{ital t}) for the branched polymer melt has a stretched exponential tail whose characteristic time diverges at the gel point. This {ital critical} {ital slowing} {ital down} is due to the divergence of the average cluster size and is distinct from the usual critical slowing down observed in second-order thermodynamic phase transitions, since the initial decay rate of {ital S}({ital q},{ital t}) is nondivergent at the gel point. In fact, at the gel point, {ital S}({ital q},{ital t}) becomes a power law, indicating a fractal time set in the scattered field. These observations are accounted for by considering the dynamics of percolation clusters, and in this connection the analogy to viscoelasticity is described. Beyond the gel point {ital S}({ital q},{ital t}) remains a power law, but the amplitude of the relaxing part of the intensity autocorrelation function diminishes. Finally, the dynamics of clusters dilutedmore » from the reaction bath is studied, and a crossover from power law to stretched exponential decay of {ital S}({ital q},{ital t}) is observed. It is shown that at infinite dilution the long-time tail of the correlation function describes the internal modes of a single percolation cluster.« less

NMR study of poly(N-isopropylacrylamide) gels near phase transition

Abstract: Nonionic and ionic poly(N-isopropylacrylamide) gels undergo continuous second-order volume phase transition or discontinuous first-order transitions in water in response to temperature changes. The results are compared with the NMR data on the nonionic and ionic poly(N-isopropylacrylamide) polymers, which undergo a phase separation in water

Analytical properties of polaron systems or: Do polaronic phase transitions exist or not?

Abstract: For more than 40 years it was thought that polaron- and exciton-phonon systems exhibited unexpected localization properties. Particular attention was paid to the so-called phonon-induced self-trapping transition, which, it was believed, should manifest itself as a point of nonanalyticity in the ground-state energy as a function of the electron-phonon coupling parameter. It will be demonstrated for a large class of (generalized Fr\"ohlich) models that no such transition exists. The dimensionality of space has no qualitative influence; insofar, an application of the authors' results to problems in lower dimensions (e.g., polarons in quantum wells) is straightforward. The same holds true if homogeneous external fields are involved; for example, a discontinuous mass stripping for magnetopolarons can be excluded. On the other hand, a phase-transition-like behavior will be found, if a polaron or exciton is exposed to a short-range potential, allowing a so-called pinning transition. The authors emphasize, however, that even in this case the transition is only modified, and not induced, by phonons.