Showing papers on "Phase transition published in 1996"

Bose-Einstein condensation in a gas of sodium atoms

Abstract: Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

The Self-Assembly Mechanism of Alkanethiols on Au(111)

Abstract: The self-assembly mechanism of alkanethiol monolayers on the (111) surface of gold was discovered with the use of an ultrahigh-vacuum scanning tunneling microscope. Monolayer formation follows a two-step process that begins with condensation of low-density crystalline islands, characterized by surface-aligned molecular axes, from a lower density lattice-gas phase. At saturation coverage of this phase, the monolayer undergoes a phase transition to a denser phase by realignment of the molecular axes with the surface normal. These studies reveal the important role of molecule-substrate and molecule-molecule interactions in the self-assembly of these technologically important material systems.

Origins of Complex Self-Assembly in Block Copolymers

Abstract: Amphiphilic molecules are renowned for their ability to partition chemically immiscible components into nanoscale domains. Often these domains exhibit intriguing complex periodic geometries with long-range order. Surprisingly, the diverse systems that selfassemble in this manner, surfactants, lipids, soaps, and block copolymers, exhibit topologically identical geometries, suggesting to researchers that a common set of principles govern amphiphilic phase selection. From this association has emerged the belief that constant mean curvature (CMC) interfaces are generally good models for block copolymer microdomain geometries. By taking advantage of new developments in polymer theory, we accurately examine this hypothesis for the first time, and find it to be wrong. Furthermore, our study reveals new explanations for complex phase selection that are relevant to numerous block copolymer systems. A block copolymer consists of chemically distinct polymer chains (i.e., blocks) joined together to form a single macromolecule. As a consequence of a general tendency for the blocks to separate, tempered by the restriction imposed by the covalent bonds that connect them, these molecules exhibit amphiphilic behavior. Even in the simplest case, AB diblock copolymers, a rich assortment of ordered phases has been documented.1-8 The composition of the AB diblock (i.e., the volume fraction f of block A) controls the geometry of the structure (see Figure 1). For nearly symmetric diblocks (f ∼ /2), a lamellar (L) phase occurs. For moderate asymmetries, a complex bicontinuous state, known as the gyroid (G) phase, has been observed in which the minority blocks form domains consisting of two interweaving threefold-coordinated lattices.1,2 (Prior to the discovery of the G phase, a double-diamond (D) structure formed from two fourfold-coordinated lattices3 was erroneously associated with the bicontinuous state in these materials.4) Another complex structure, the perforated lamellar (PL) phase, occurs when the minority-component layers of the L phase develop a hexagonal arrangement of passages.5 At yet higher asymmetries, the minority component forms hexagonally packed cylinders (C) and then spheres (S) arranged on a bodycentered cubic lattice. Eventually, as f f 0 or 1, a disordered phase results. The complete mean field or rather self-consistent field theory (SCFT) for block copolymers was developed by Helfand and co-workers.9 However, at the time of its development, it had to be supplemented with approximations limiting its effectiveness. Nevertheless, important advances were made by examining this theory in the limits of weak10 and strong11 segregation. (The degree to which the A and B blocks segregate is determined by the product oN, where o is the FloryHuggins A/B interaction parameter and N is the total degree of polymerization.) The combination of these works established that the underlying physics controlling block copolymer phase behavior involves a competition between interfacial tension and the entropic penalty for stretching polymer coils so as to fill space uniformly. The balance determines the equilibrium size of the microdomains and dictates the geometry of the structure. Although these earlier approaches correctly predicted the classical phases (i.e., L, C, and S),10,11 they failed to account for the more recently discovered complex phases (i.e., G and PL).12,13 With new advances,14 it is now possible to implement the full SCFT. The first calculations14,15 to do so evaluated the free energies of the structures described above and established the phase diagram. This demonstrated that complex phase behavior occurs in the intermediatesegregation regime as opposed to the weakand strongsegregation regimes treated by Leibler10 and Semenov,11 respectively. For intermediate segregation (e.g., oN ) 20), the new calculations predict the sequence L f G f C f S f disordered as f progresses from /2 to either 0 or 1. Although PL is absent from this sequence, it is nearly stable at the L/G phase boundary, consistent with where it is observed experimentally.6 This supports very recent experiments indicating that the PL structure is only a long-lived metastable state. The D phase is clearly unstable, in agreement with current experiments.4,6 Given this theoretical accomplishment, we now probe deeper into the theory14 to examine the physical factors responsible for complex phase behavior. As described below, the explanation lies in the detailed shape of the dividing interface between the A and B microdomains. Earlier works, such as that of Semenov,11 illustrate that the phase transitions are driven by a tendency to curve the interface as the diblocks become asymmetric in composition. The curvature allows the molecules to balance the degree of stretching between the A and B blocks. We demonstrate this quantitatively in Figure * Present address: Polymer Science Centre, University of Reading, Whiteknights, Reading RG6 6AF, UK. Figure 1. Area-averaged mean curvature 〈H〉 as a function of the A-block volume fraction f for each of the structures shown schematically calculated using self-consistent meanfield theory.14,15 The stable and metastable states are shown with solid and dashed lines, respectively, and transitions are denoted by dots. As the molecules become asymmetric, structures with more curvature are preferred. 7641 Macromolecules 1996, 29, 7641-7644

Giant oxygen isotope shift in the magnetoresistive perovskite La1-xCaxMnO3+y

Abstract: FERROMAGNETIC perovskites of the form La1–XMexMnO3–Y (where Me is Ca or Sr) have been known1 since 1950, but there has been a recent resurgence of interest following the discovery of giant magnetoresistance in this class of compounds2,3. The compounds contain both Mn3+ and Mn4+ ions; as the electronic ground state of the Mn3+ ions is degenerate, their energy is lowered by a spontaneous distortion of the surrounding lattice—the Jahn–Teller effect4. The charge carriers in these materials are strongly coupled to (and mediate the ferromagnetic interaction between) the manganese ions5, suggesting that localized lattice distortions could also play an important role in determining the electronic and magnetic properties of these compounds. Here we investigate this possibility by examining the effect on the ferromagnetic transition temperature of varying the oxygen isotope mass (replacing 16O with 18O). For La0.8Ca0.2MnO3+y, we measure an isotope shift of >20 K, significantly larger than that found for any magnetic or electronic phase transition in other oxides. In contrast, we observe no significant isotope shift for the structurally related ferromagnet SrRuO3, in which the Jahn–Teller effect is negligible. These results imply that the large isotope shift arises from coupling of the charge carriers to Jahn–Teller lattice distortions, and we suggest that such Jahn–Teller 'polarons' may also be responsible for the magnetoresistive properties of these materials.

Melting dynamics of a plasma crystal

Abstract: PLASMAS have long been regarded as the most disordered state of matter; nevertheless, a set of colloidal particles introduced into a charge-neutral plasma can spontaneously exhibit ordered crystalline structures1,2—so-called 'plasma crystals'. Such systems, which reach equilibrium very rapidly and can be easily tuned between their ordered and disordered states, are ideally suited for investigating the processes underlying the solid-to-liquid phase transition. Here we report the results of experiments on 'flat' plasma crystals (with thicknesses of only a few lattice planes) which suggest that the melting transition occurs through two fundamental intermediate stages. On melting, the crystal first enters a state characterized by islands of crystalline order, about which streams of particles flow. The crystalline regions then dissolve as the vibrational energy of the system increases, but this is accompanied by a temporary increase in orientational order before the system finally enters a disordered, liquid state. The unexpected 'vibrational' phase, characterized by enhanced orientational order, might arise as a consequence of the mixed two- and three-dimensional nature of the flat plasma crystals. Alternatively, it may indicate the existence of a new intermediate state in melting transitions more generally.

Models of traps and glass phenomenology

Abstract: We study various models of independent particles hopping between energy `traps' with a density of energy barriers , on a d-dimensional lattice or on a fully connected lattice. If decays exponentially, a true dynamical phase transition between a high-temperature `liquid' phase and a low-temperature `aging' phase occurs. More generally, however, one expects that for a large class of , `interrupted' aging effects appear at low enough temperatures, with an ergodic time growing faster than exponentially. The relaxation functions exhibit a characteristic shoulder, which can be fitted as stretched exponentials. A simple way of introducing interactions between the particles leads to a modified model with an effective diffusion constant in energy space, which we discuss in detail.

Vortex formation in neutron-irradiated superfluid 3He as an analogue of cosmological defect formation

Abstract: TOPOLOGICAL defects formed during a rapid symmetry-breaking phase transition in the early Universe1,2 could be responsible for seeding large-scale structure, for the anisotropy of the microwave background radiation, and for the predominance of matter over antimatter3,4. The theory describing this cosmological phase transition is formally analogous to that describing the transition to the superfluid state in liquid 3He, so that in principle the process of cosmological defect formation can be modelled in the laboratory. Here we report the results of an experiment in which the 'primordial fireball' is mimicked using a neutron-induced nuclear reaction (n + 3He → p + 3He + 0.76 MeV) to heat small regions of superfluid 3He above the superfluid transition temperature. These bubbles of normal liquid cool extremely rapidly, and we find that their transition back to the superfluid state is accompanied by the formation of a random network of vortices (the superfluid analogue of cosmic strings). We monitor the evolution of this defect state by rotating the superfluid sample, allowing vortices to escape from the network and thus be probed individually. Our results provide clear confirmation of the idea that topological defects form at a rapid second-order phase transition, and give quantitative support to the Kibble–Zurek mechanism5,6 of cosmological defect formation.

Laboratory simulation of cosmic string formation in the early Universe using superfluid 3He

Abstract: TOPOLOGICAL defects in the geometry of space–time (such as cosmic strings) may have played an important role in the evolution of the early Universe, by supplying initial density fluctuations which seeded the clusters of galaxies that we see today1. The formation of cosmic strings during a symmetry-breaking phase transition shortly after the Big Bang is analogous to vortex creation in liquid helium following a rapid transition into the superfluid state; the underlying physics of this cosmological defect-forming process (known as the Kibble mechanism1) should therefore be accessible to experimental study. Superfluid vortices have been observed in 4He following rapid quenching to the superfluid state2, lending qualitative support to Kibble's contention that topological defects are generated by such phase transitions. Here we quantify this process by using an exothermic neutron-induced nuclear reaction to heat small volumes of super-fluid 3He above the superfluid transition temperature, and then measuring the deficit in energy released as these regions of normal liquid pass back into the superfluid state. By ascribing this deficit to the formation of a tangle of vortices, we are able to infer the resulting vortex density; we find that this agrees very well with the predictions of Zurek's modification3 of the original Kibble mechanism1.

Experimental investigation of the melting transition of the plasma crystal.

Abstract: Measurements of the melting transition of a Coulomb crystal consisting of dust particles immersed in an rf parallel plate discharge in helium were performed. The dust crystal is shown to be solid at higher gas pressure (120 Pa) and low discharge power (10--20 W). Reducing the gas pressure or increasing the discharge power leads to fluid states of the dust ensemble. Even gaslike states are observed at low pressures of about 40 Pa. The transition is attributed to an increasing effective particle temperature. The phase transition is compared with one-component-plasma and Yukawa models, and with basic predictions of theories for two-dimensional melting. \textcopyright{} 1996 The American Physical Society.

A re-examination of the phase diagram of hard spherocylinders

Abstract: The phase transitions exhibited by systems of hard spherocylinders, with a diameter D and cylinder length L, are re‐examined with the isothermal–isobaric Monte Carlo (MC‐NPT) simulation technique. For sufficiently large aspect ratios (L/D) the system is known to form liquid crystalline phases: isotropic (I), nematic (N), smectic‐A (Sm A), and solid (K) phases are observed with increasing density. There has been some debate about the first stable liquid crystalline phase to appear as the aspect ratio is increased from the hard‐sphere limit. We show that the smectic‐A phase becomes stable before the nematic phase as the anisotropy is increased. There is a transition directly from the isotropic to the smectic‐A phase for the system with L/D=3.2. For larger aspect ratios, e.g., L/D=4, the smectic‐A phase is preceded by a nematic phase. This means that the hard spherocylinder system exhibits I–Sm A–K and I–N–Sm A triple points, the latter occurring at a larger aspect ratio. We also confirm the simulation results of Frenkel [J. Phys. Chem. 92, 3280 (1988)] for the system with L/D=5, which exhibits isotropic, nematic, smectic‐A, and solid phases. All of the phase transitions are accompanied by a discontinuous jump in the density, and are, therefore, first order. In the light of these new simulation results, we re‐examine the adequacy of the Parsons [Phys. Rev. A 19, 1225 (1979)] scaling approach to the theory of Onsager for the I–N phase transition. It is gratifying to note that this simple approach gives an excellent representation of both the isotropic and nematic branches, and gives accurate densities and pressures for the I–N phase transition. As expected for such a theory, the corresponding orientational distribution function is not accurately reproduced at the phase transition. The results of the recent Onsager/DFT theory of Esposito and Evans [Mol. Phys. 83, 835 (1994)] for the N–Sm A bifurcation point are also in agreement with the simulation data. It is hoped that our simulation results will be used for comparisons with systems with more complex interactions, e.g., dipolar hard spherocylinders and hard spherocylinders with attractive sites.

In situ X-ray observations of the coesite-stishovite transition: reversed phase boundary and kinetics

Abstract: Using a DIA-type, cubic-anvil, high-pressure apparatus (SAM-85) in conjunction with in situ X-ray diffraction, we have investigated phase relations between coesite and stishovite up to 12 GPa and 1530 °C using synthetic powders of the two phases as the starting materials. The phase transition between coesite and stishovite was identified by observing the first appearance of a phase that did not already exist or by a change in the relative intensity of the two patterns. In most experiments, the diffraction patterns on samples were collected within 10 minutes after reaching a pressure and temperature condition. On this time scale, two phase boundaries associated with the coesite-stishovite transition have been determined: (1) for the stishovite-to-coesite transition, observations were made in the temperature range of 950–1530 °C, and (2) for the coesite-to-stishovite transition from 500 to 1300 °C. These observations reveal that there exists a critical temperature of about 1000 °C to constrain the coesite-stishovite equilibrium phase boundary. Above this temperature, both boundaries are linear, have positive dP/dT slopes, and lie within a pressure interval of 0.4 GPa. Below this temperature, the dP/dT slope for the stishovite-to-coesite phase boundary becomes significantly larger and that for the coesite-tostishovite phase boundary changes from positive to negative. As a result, an equilibrium phase boundary can only be determined from the results above 1000 °C and is described by a linear equation P (GPa)=6.1 (4)+ 0.0026 (2) T (°C). This dP/dT slope is in good agreement with that of Zhang et al. (1993) but more than twice that of Yagi and Akimoto (1976). For the kinetics of the phase transition, preliminary rate data were obtained for the stishovite-to-coesite transition at 1160 and 1430 °C and are in agreement with the simple geometric transformation model of Avrami and Cahn.

Vortex-Lattice Phase Transitions in Bi 2 Sr 2 CaCu 2 O 8 Crystals with Different Oxygen Stoichiometry

Abstract: The vortex-lattice phase transitions in Bi 2Sr2CaCu2O8 crystals with various oxygen stoichiometry are studied using local magnetization measurements. Three new findings are reported: The first-order phase transition line at elevated temperatures shifts upward for more isotropic overdoped samples. At lower temperatures another sharp transition is observed that results in enhanced bulk pinning in the second magnetization peak region. The two lines merge at a multicritical point at intermediate temperatures forming apparently a continuous phase transition line that is anisotropy dependent. PACS numbers: 74.60.Ec, 74.60.Ge, 74.60.Jg, 74.72.Hs The mixed state of high-temperature superconductors (HTSC) has a very complicated phase diagram. The nature of the different vortex phases and the thermodynamic transitions between them are of fundamental interest, and are the subject of substantial recent theoretical and experimental efforts [1 ‐ 10]. It is generally accepted that the high anisotropy plays a crucial role in the richness of the phase diagram of HTSC. Nevertheless, a systematic study of the anisotropy effects is quite complicated and was limited so far by the lack of a well defined phase boundary that could be monitored as a function of the anisotropy. A recent advance in local measurements has revealed a sharp step in magnetization due to a first-order vortex-lattice phase transition in Bi2Sr2CaCu2O8 (BSCCO) crystals [10]. Such a clearly defined fundamental transition is thus a natural candidate for an investigation of the anisotropy effects in HTSC, and this paper presents a first study in this direction. Another intriguing feature of the phase diagram of many HTSC crystals, and BSCCO in particular, is the anomalous second magnetization peak at lower temperatures. The associated increase of magnetization with magnetic field has been attributed to surface barrier effects [11], crossover from surface barrier to bulk pinning [12], sample inhomogeneities [13], dynamic effects [14], and 3D to 2D transitions [15‐ 17]. Our local measurements indicate that a very abrupt upturn in the bulk critical current occurs at the onset of the second peak in BSCCO. We postulate that this behavior is triggered by an underlying thermodynamic phase transition of the flux-line lattice. Furthermore, for the different anisotropy crystals the two phase transition lines are found to form, apparently, one continuous transition line that changes from first to possibly second order at

Generalized gradient theory for silica phase transitions.

Abstract: Density functional theory based on the generalized gradient approximation to the exchange and correlation energy is shown to correct a qualitative error of the local density approximation in describing a high-pressure phase transition of ${\mathrm{SiO}}_{2}$. Advantages of an adaptive curvilinear coordinate method for such generalized gradient calculations are discussed.

Rigid-unit phonon modes and structural phase transitions in framework silicates

Abstract: The rigid-unit mode model provides many new insights into the stability and physical properties of framework silicates. In this model the SiOo and AlO4 tetrahedra are treated as very stifl to a first approximation as completely rigid, in comparison with intertetrahedral forces. In this paper we apply the model to several important examples. The model is reviewed by a detailed study of quarlz, and it is shown that the a-B phase transition involves a rigid-unit mode that preserves the Si-O-Si bond angle. The model is used to explain the phase transitions in cristobalite and the diferent feldspar, sodalite, and leucite structures. We also use the model to explain the nature of the high-temperature disordered phases of cristobalite and tridymite, to interpret the observations of streaks of diffuse scattering in electron diffraction patterns, to interpret the structures in the kalsilite-nepheline solid solution, to explain volume anomalies in the cubic leucite structures, and to explain qualitatively the negative linear thermal expansion in cordierite. The results for the highest symmetry sodalite structure show that there is a rigid-unit mode at every wave vector, a finding with significant implications for the understanding of the sorption and catalytic behavior of zeolites.

A general macroscopic description of the thermomechanical behavior of shape memory alloys

Abstract: The paper presents a macroscopic description that allows the simulation of the global thermomechanical behavior of shape memory alloys (SMA). Use is made of the thermodynamics of irreversible processes framework. Two internal variables are taken into account: the volume fraction of self-accommodating (pure thermal effect) and oriented (stress-induced) product phase. A specific free energy, valid in the total range of phase transition, is defined with particular attention paid to the interaction term. A study of the thermodynamic absolute equilibrium during phase transition proves its instability, and hence explains the hysteretic behavior of SMA. The kinetic equations for the internal variables are written in such a general way that the model could comply with the second law of thermodynamics. The postulate of five yield functions (each of them being related to one process) permits the phase transition criteria to be defined and the kinetic equations related to each process through consistency equations to be derived. The parameters of the model have been identified for three particular SMA, and the simulated results show good agreement with experiments.

First-principles theory of iron up to earth-core pressures: Structural, vibrational, and elastic properties.

Abstract: {ital Ab} {ital initio} electronic-structure calculations, based on density-functional theory and a full-potential linear-muffin-tin-orbital method, have been used to predict crystal-structure phase stabilities, elastic constants, and Brillouin-zone-boundary phonons for iron under compression. Total energies for five crystal structures, bcc, fcc, bct, hcp, and dhcp, have been calculated over a wide volume range. In agreement with experiment and previous theoretical calculations, a magnetic bcc ground state is obtained at ambient pressure and a nonmagnetic hcp ground state is found at high pressure, with a predicted bcc {r_arrow} hcp phase transition at about 10 GPa. Also in agreement with very recent diamond-anvil-cell experiments, a metastable dhcp phase is found at high pressure, which remains magnetic and consequently accessible at high temperature up to about 50 GPa. In addition, the bcc structure becomes mechanically unstable at pressures above 2 Mbar (200 GPa) and a metastable, but still magnetic, bct phase ({ital c}/{ital a} {approx_equal} 0.875) develops. For high-pressure nonmagnetic iron, fcc and hcp elastic constants and fcc phonon frequencies have been calculated to above 4 Mbar. These quantities rise smoothly with pressure, but an increasing tendency towards elastic anisotropy as a function of compression is observed, and this has important implications for themore » solid inner-core of the earth. The fcc elastic-constant and phonon data have also been used in combination with generalized pseudopotential theory to develop many-body interatomic potentials, from which high-temperature thermodynamic properties and melting can be obtained. In this paper, these potentials have been used to calculate full fcc and hcp phonon spectra and corresponding Debye temperatures as a function of compression. {copyright} {ital 1996 The American Physical Society.}« less

Structural and dielectric studies of Pb(Mg1/3Nb2/3)O3–PbTiO3 ferroelectric solid solutions around the morphotropic boundary

Abstract: Solid solutions (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 with 0.20≤x≤0.40 were studied using impedance measurements and high resolution x‐ray powder diffraction. The dependence of the real and imaginary parts of the dielectric constant with temperature was investigated on unpoled and poled ceramics, showing several transition points. The temperature dependence of the unit‐cell parameters showed the onset of two successive spontaneous phase transitions: cubic→tetragonal →rhombohedral. A coexistence of both ferroelectric phases was evidenced in a large temperature and composition range. The dielectric and structural properties were interpreted in terms of relaxor→ferroelectric spontaneous phase transition. An intermediate behavior between a relaxor and a real ferroelectric was evidenced. A partial phase diagram was proposed including both structural and dielectric informations.

Bose-Einstein condensation in disordered exclusion models and relation to traffic flow

Abstract: A disordered version of the one-dimensional asymmetric exclusion model where the particle hopping rates are quenched random variables is studied. The steady state is solved exactly by use of a matrix product. It is shown how the phenomenon of Bose condensation whereby a finite fraction of the empty sites are condensed in front of the slowest particle may occur. Above a critical density of particles a phase transition occurs out of the low-density phase (Bose condensate) to a high-density phase. An exponent describing the decrease of the steady-state velocity as the density of particles goes above the critical value is calculated analytically and shown to depend on the distribution of hopping rates. The relation to traffic flow models is discussed.

Magnetocaloric properties of doped lanthanum manganite films

Abstract: LaMnO3 films doped with Ca, Ba, or Sr have been fabricated using the metalorganic decomposition technique. These films exhibit paramagnetic‐to‐ferromagnetic phase transitions at 250, 300, and 350 K, respectively. By measuring the film magnetization as a function of field and temperature we have determined the entropy change associated with these transitions. The large magnetization of these materials results in a total entropy change a factor of five less than that of gadolinium, the prototypical high‐temperature magnetocaloric material. Improvements in film morphology and composition may provide a further increase in the magnetization and total entropy change in these materials.

Origins of colossal magnetoresistance in perovskite-type manganese oxides (invited)

Abstract: Phenomena of colossal magnetoresistance (MR) or magnetic field induced insulator–metal (I–M) transitions have been investigated for single crystals of perovskite‐type manganese oxides with controlled carrier density and one‐electron bandwidth. In addition to the canonical MR behavior near the Curie temperature, the first‐order phase transition accompanying several orders of magnitude change in resistivity has been observed under an external magnetic field for many of the composition‐controlled crystals as an intrinsic bulk phenomenon. It was proved by the systematic experimental investigations that the field‐induced destruction of the charge‐ordered state accompanying the lattice structural as well as metamagnetic transition is a major origin of such a colossal MR. Versatile MR phenomena and I–M phase diagrams in the T–H plane are presented with their interpretation.

Pressure-induced structural transformations in Si nanocrystals: Surface and shape effects.

Abstract: The kinetics of solid-solid phase transitions are explored using pressure-induced structural transformations in Si nanocrystals. In agreement with the predictions of homogeneous deformation theories, large elevations in phase transition pressure are observed in nanocrystals as compared to bulk Si, and high pressure x-ray diffraction peak widths indicate an overall change in nanocrystal shape upon transformation. In addition, unlike the BC8 phase recovered in bulk Si, amorphous Si nanoclusters are obtained upon release of pressure, providing an example of kinetic size control over solid phases. {copyright} {ital 1996 The American Physical Society.}

Theory of Branching and Annihilating Random Walks

Abstract: Nonequilibrium models with an extensive number of degrees of freedom whose dynamics violates detailed balance occur in studies of many biological, chemical and physical systems. Like equilibrium systems, their stationary states may exhibit phase transitions which in many cases appear to fall into distinct classes characterized by universal quantities such as critical exponents. One of the most common such classes is that exemplified by directed percolation (DP) [1]. This represents a transition from a nontrivial ‘active’ steady state to an absorbing ‘inactive’ state with no fluctuations. Many nonequilibrium phase transitions appear to belong to this universality class, e.g., the contact process [2], the dimer poisoning problem in the ZGB model [3], and auto–catalytic reaction models [4]. The universal properties of the DP transition are theoretically well understood in the context of a renormalization group (RG) analysis based on an expansion around mean field theory below the upper critical dimension dc = 4 [5]. More recently a class of models has been studied which, in certain cases, appear as exceptions to the general rule that such transitions should fall into the DP universality class. These include a probabilistic cellular automaton model [6], certain kinetic Ising models [7,8], and an interacting monomer–dimer model [9]. In one dimension the dynamics of these is equivalent to a class of models called branching and annihilating random walks (BARWs) [10–12], which also have a natural generalization to higher dimensions. In the language of reaction– diffusion systems, BARWs describe the stochastic dynamics of a single species of particles A undergoing three basic processes: diffusion, often modeled by a random walk on a lattice and characterized by a diffusion coefficient D; an annihilation reaction A + A → ⊘ when particles are close (or on the same site), at rate λ; and a branching process A → (m + 1)A (where m is a positive integer), at rate σm. The above–mentioned one– dimensional models all correspond to the case m = 2. For the kinetic Ising model, the particles A are to be identified with the domain walls, and the transition to the inactive state corresponds to the ordering of the Ising spins [7,8]. In general, this new universality class has been observed in d = 1 for even values of m, when the number of particles is locally conserved modulo 2. When m is odd, the DP values of the exponents appear to be realized. (It should be remarked that several of the models which have been studied do not contain three independent parameters corresponding to D, λ, and σm so that it may occur that the actual transition is inaccessible. This appears to be so for the simplest lattice BARW model with m = 2, which is always in the inactive phase [10].) Besides the appearance of a new universality class, another issue which clearly requires theoretical explanation is the occurrence of a transition at a finite value of σm. For the mean field rate equation for the average density u n(t) = −2λ n(t) 2 + mσm n(t) (1)

Phase transitions in driven diffusive systems with random rates

Abstract: We study a one-dimensional driven lattice gas model in which quenched random jump rates are associated with the particles. Under suitable conditions on the distribution of jump rates the model displays a phase transition from a high-density `laminar' phase with product measure to a low-density `jammed' phase in which the interparticle spacings have no stationary distribution. Using a waiting time representation the phase transition is shown to be equivalent to a pinning transition of directed polymers with columnar defects. The phenomenon is argued to have a natural realization in traffic flow.

Charge‐Density‐Wave Conductors

Abstract: When metals are cooled, they often undergo a phase transition to a state exhibiting a new type of order. Metals such as iron and nickel become ferromagnetic below temperatures of several hundred degrees Celsius; electron spins order to produce a net magnetization in zero field. Other metals, such as lead and aluminum, become superconductors at cryogenic temperatures; electrons form Cooper pairs of opposite spin and momentum, leading to electrical conduction with zero resistance and to expulsion of magnetic fields.

Thermal analysis using a micromechanical calorimeter

Abstract: A fast and extremely sensitive method for reversible thermal analysis of picoliter volumes of solid samples is presented. Using a micromechanical calorimeter based on the deflection of a bimetallic cantilever, enthalpy changes at phase transitions in n‐alkanes (paraffins) are determined. The key role of volume changes associated with phase transitions is discussed together with a method to separate thermal and volume‐induced stresses associated with these phase transitions.

Low-Temperature Phase Transition of Water Confined in Mesopores Probed by NMR. Influence on Pore Size Distribution

Abstract: The NMR signal intensity vs temperature (IT curve) of water confined in mesoporous materials (pore radius lager than 10 A) reveals one or more “high-temperature” transitions above 222 K, which are dependent on pore size, and an additional transition temperature below 209 K, which is independent of pore dimension. This latter transition shows no hysteresis effect or discontinuity and contributes to more than 65% of the total water content of the porous materials investigated and is explained as interfacial water in contact with the surface of the matrix and the solid ice phase. The thickness of this interface water is estimated to be 5.4 ± 1.0 A (cylindrical pores). It is shown that the observed NMR intensity of water associated with the “high-temperature” transition phases has to be corrected in order to present the actual amount of water within these phases. It is further demonstrated that these intensity corrections must be implemented in the pore size distribution functions to give quantitative results...

High-temperature structural phase transitions in perovskite

Abstract: High-temperature powder x-ray diffraction data are presented for perovskite between 293 and 1523 K. The temperature-dependence of superlattice intensities and cell parameters suggests a sequence of phase transitions from the room temperature orthorhombic (Pbnm) structure to a tetragonal (I4/mcm) polymorph at temperatures in the range 1373 - 1423 K, followed by transformation to the cubic aristotype at . The intensity of the diffuse background increases on transformation to the cubic structure and is associated with disorder (and anionic mobility) of the oxygen sub-lattice. The I4/mcm - Pbnm transition induces a large spontaneous strain, but the tetragonal spontaneous strain in the I4/mcm phase due to the - I4/mcm transition is small, below the resolution of this experiment. These results add weight to suggestions from recent computer simulations that orthorhombic may transform to a tetragonal (rather than a cubic) polymorph under the conditions of the Earth's mantle, in which case the effects on electrical conductivity would not be expected to be as great as for a transition to a cubic polymorph, although the consequences for elastic properties may be more significant.