Features Of Phase Diagram Research Articles

Dynamical phase separation on rhythmogenic neuronal networks.

We explore the firing-rate model of excitatory neurons with dendritic adaptation (the Feldman-Del Negro model [J. L. Feldman and C. A. Del Negro, Nat. Rev. Neurosci. 7, 232 (2006)10.1038/nrn1871; D. J. Schwab et al., Phys. Rev. E 82, 051911 (2010)10.1103/PhysRevE.82.051911] interacting on a fixed, directed Erdős-Rényi network. This model is applied to the dynamics of the pre-Bötzinger complex, the mammalian central pattern generator with N∼10^{3} neurons, which produces a collective metronomic signal that times inspiration. In the all-to-all coupled variant of the model, there is spontaneous symmetry breaking in which some fraction of the neurons becomes stuck in a high-firing-rate state, while others become quiescent. This separation into firing and nonfiring clusters persists into more sparsely connected networks. In these sparser networks, the clustering is influenced by k cores of the underlying network. The model has a number of features of the dynamical phase diagram that violate the predictions of mean-field analysis. In particular, we observe in the simulated networks that stable oscillations do not persist in the high-sensitivity limit, in contradiction to the predictions of mean-field theory. Moreover, we observe that the oscillations in these sparse networks are remarkably robust in response to killing neurons, surviving until only approximately 20% of the network remains. This robustness is consistent with experiment.

The Mechanism of Doping and the Features of Phase Diagrams of HTSC Cuprates and Ferropnictides

We propose a generalized model of electronic structure modification in HTSC cuprates and ferropnictides under doping. In this model the role of doping consists in only a local change in the electronic structures of the parent phases of cuprates and ferropnictides due to the formation of trion complexes comprising a doped carrier localized in unit cell and charge transfer (CT) excitons around it. These CT excitons emerge in CuO4 or AsFe4 plaquettes in the CuO2 or FeAs basal planes (CT plaquettes) under the influence of doped carrier, restricting its itinerancy. As the dopant concentration is increased, CT plaquettes combine into clusters of the so called CT phase. It is this CT phase that is related in the model to the HTSC phase. In support of this assumption, we determined the ranges of dopant concentrations conforming to the existence of percolation clusters of the CT phase; these ranges were shown to coincide with the positions of the superconducting domes on the phase diagrams of these compounds. The model also perfectly describes subtle features of the phase diagrams of various cuprates and ferropnictides including the 1/8 anomaly, narrow peaks in the dependences of the London penetration depth on the concentration of the dopant, and other specific features. The mechanism of the generation of free carriers in the CT phase, provided by intrinsic self-doping, was considered. The mechanism is not directly related to external doping, but is due to the interaction of band electrons with so called Heitler-London (HL) centres inherently existing in the percolation cluster of CT phase and representing pairs of adjacent CuO4 or AsFe4 CT plaquettes in the CuO2 or FeAs basal planes. Material in CT phase was shown to represent a medium, in which the mechanism of excitonic superconductivity, specified by the interaction of band electrons with HL centres, can be realized.

Phase diagram and quantum criticality of Heisenberg spin chains with Ising anisotropic interchain couplings

Motivated by recent progress on field-induced phase transitions in quasi-one-dimensional quantum antiferromagnets, we study the phase diagram of $S=1/2$ antiferromagnetic Heisenberg chains with Ising anisotropic interchain couplings under a longitudinal magnetic field via large-scale quantum Monte Carlo simulations. The interchain interactions is shown to enhance longitudinal spin correlations to stabilize an incommensurate longitudinal spin density wave order at low temperatures. With increasing field the ground state changes to a canted antiferromagnetic order until the magnetization fully saturates above a quantum critical point controlled by the $(3+2)$D XY universality. Increasing temperature in the quantum critical regime the system experiences a fascinating dimension crossover to a universal Tomonaga-Luttinger liquid. The calculated NMR relaxation rate $1/T_1$ indicates this Luttinger liquid behavior survives a broad field and temperature regime. Our results determine the global phase diagram and quantitative features of quantum criticality of a general model for quasi-one-dimensional spin chain compounds, and thus lay down a concrete ground to the study on these materials.

Thermodynamic analysis of the classical lattice-hole model of liquids

A complete thermodynamic analysis of the classical 2-parameter lattice-hole model of liquids is presented. To our knowledge, no such analysis was available before. It is shown that the model depicts a van der Waals like behavior. The calculated phase diagram features a coexistence line between a condensed and a gas phases ending at a critical point. The model is not able to describe a transition between two condensed phases such as melting. Model parametrization to simulate an archetypal fragile glass forming liquid, the ortho-terphenyl, reveals an only qualitative/semi-quantitative agreement with available experimental information. The extreme simplicity of this 2-parameter model restricts its ability to describe a real liquid. Taking into account the existence of a volume difference between a hole and a molecule, inserting a temperature and pressure dependencies of the physical parameters and allowing the energy parameter to depend on the hole concentration are the most promising modeling options.

Two-colour QCD phases and the topology at low temperature and high density

We delineate equilibrium phase structure and topological charge distribution of dense two-colour QCD at low temperature by using a lattice simulation with two-flavour Wilson fermions that has a chemical potential μ and a diquark source j incorporated. We systematically measure the diquark condensate, the Polyakov loop, the quark number density and the chiral condensate with improved accuracy and j → 0 extrapolation over earlier publications; the known qualitative features of the low temperature phase diagram, which is composed of the hadronic, Bose-Einstein condensed (BEC) and BCS phases, are reproduced. In addition, we newly find that around the boundary between the hadronic and BEC phases, nonzero quark number density occurs even in the hadronic phase in contrast to the prediction of the chiral perturbation theory (ChPT), while the diquark condensate approaches zero in a manner that is consistent with the ChPT prediction. At the highest μ, which is of order the inverse of the lattice spacing, all the above observables change drastically, which implies a lattice artifact. Finally, at temperature of order 0.45Tc, where Tc is the chiral transition temperature at zero chemical potential, the topological susceptibility is calculated from a gradient-flow method and found to be almost constant for all the values of μ ranging from the hadronic to BCS phase. This is a contrast to the case of 0.89Tc in which the topological susceptibility becomes small as the hadronic phase changes into the quark-gluon plasma phase.

Природа псевдощелевой фазы ВТСП купратов

The pseudogap phase of HTSC cuprates is associated with the formation of a system of quantum electron-hole (EH) dimers similar to the Anderson RVB-phase. We considered the specific role of electron-lattice relaxation in the formation of metastable EH dimers in cuprates with T- and T′-structures. In the model of charge triplets and S = 1 pseudospin formalism, the effective spin-pseudospin Hamiltonian of the cuprate CuO2 plane is introduced. In the framework of the molecular field approximation (MFA) for the coordinate representation, the main MFA phases were found: an antiferromagnetic insulator, a charge density wave, a bosonic superconductor with d-symmetry of the order parameter, and two metal Fermi-phases forming the phase of the "strange" metal. We argue that the MFA can correctly reproduce all the features of the typical cuprate phase diagrams. As for typical s = 1/2 quantum antiferromagnet the actually observed cuprate phases such as charge order and superconductivity reflect "physical" ground state, which is close to MFA-phases but with strongly reduced magnitudes of the local order parameters.

Estimation of microscopic variables for materials system optimization using Cluster Variation Method

Stability of various phases present in the alloy system under thermodynamic conditions of temperature and composition estimated through experiments requires enormous number of tests and huge manpower. The study of these phases required equilibrium diagrams developed from experimental data. These diagrams are road maps for understanding structural stability of materials system under prevailed service conditions. Development of sound thermodynamic model is essential requirement to reduce the experimental cost, effort and time. Therefore theoretical computation of phase stability in materials system reduces the experimental burden. Although various thermodynamic models such as CALPHAD etc. are developed and using for estimating the materials systems, a reliable and versatile physical based thermodynamic model is the requirement in current day materials system development. Cluster Variation Method (CVM) is one such familiar model but algebraically complex to implement. Therefore the immediate task required to attempt is to reduce the algebraic complexity of the model by devising some simplified procedures for estimating materials systems using CVM. In the proposed work, procedures have been devised in order to reduce the algebraic complexity of Cluster Variation Method and make it viable for computation of materials systems. In the process of developing these procedures one of the important task is to estimate initial guess of microscopic variables for model materials system which is viable to carry out optimization of the materials system. As a part of these procedures this work estimates initial values of microscopic variables using which simultaneous optimization process can be carried successfully by developing procedures and approximating the proper initial guess values of these variables. A typical materials data has been selected to validate these procedures based on special features of phase diagrams like miscibility gap and congruent extreme etc. in this work.

An Analysis of Thermodynamic Characteristics of Metal-Hydride Systems for Hydrogen Storage, Using a Modified Scheme of the Perturbation Theory

The use of hydrides of intermetallic compounds (IMC) to implement the working processes of thermo sorption compressors, heat pumps, storage systems, cleaning, and programmable supply of hydrogen is due to a number of unique properties of these hydrogen sorbents. These include, first of all, the fact that the saturation of IMCs with a large sorption capacity occurs under relatively mild thermodynamic conditions, as well as the selectivity of sorption processes and the presence of the effect of thermodesorptional activation of atoms and molecules of hydrogen isotopes. The work is devoted to the description of phase equilibria in IMC hydrides. The proposed approach to the problem of calculating phase equilibria in metal hydrides consists in determining the properties of the lattice gas of H atoms and the H2 molecular phase (which is in equilibrium with it) within the framework of a single method, the modified perturbation theory. The thermodynamic description of the hydrogen subsystem in the region of disordered α-, β-phases is carried out on the basis of the model of the non-ideal (interacting) lattice gas of hydrogen atoms. In this case, both the direct interaction between hydrogen atoms and the indirect deformation contributions to the potential energy due to the expansion of the lattice during hydrogen dissolution are taken into account. Simulation of phase transitions in IMC-hydrogen systems based on the modified perturbation theory scheme gives a correct description of the main features of phase diagrams in a wide range of hydrogen pressures. From the condition of the equality of the chemical potentials of the H-subsystem of the hydride and H2-phase (calculated on the H atom), equations are obtained, relating the pressure of the gaseous H2 phase to the hydride parameters c and T (phase diagrams). The proposed computational procedure does not use adjustable parameters or empirical correlations. It relies on the atomic characteristics of the hydrogen subsystem and metal matrix, which have an unambiguous physical meaning. The object of this research is the LaNi5 intermetallic hydride. Of particular interest is the position of the critical point of the β→α-transition in the LaNi5-hydrogen system, for which there are no experimentally obtained parameter values. The paper presents the calculated values of the β→α-transition critical parameters Tc=445 K, pc=87 atm. The calculated data on the thermodynamic parameters of the α→β-transition (enthalpy, entropy, and pressure on the isotherm plateau) make it possible to describe the solubility of hydrogen in LaNi5 at pressures up to 500 atm and are in good agreement with the available experimental data in the literature.

Quantum Criticality and Formation of a Singular Fermi Liquid in the Attractive SU(N>2) Anderson Model.

While much is known about repulsive quantum impurity models, significantly less attention has been devoted to their attractive counterparts. This motivated us to study the attractive SU(N) Anderson impurity model. While for the repulsive case the phase diagram features mild N dependence and the ground state is always a Fermi liquid, in the attractive case a Kosterlitz-Thouless charge localization phase transition is revealed for N>2. Beyond a critical value of attractive interaction, an abrupt jump appears in the number of particles at the impurity site, and a singular Fermi liquid state emerges, where the scattering of quasiparticles is found to exhibit power law behavior with fractional power. The capacity diverges exponentially at the quantum critical point, signaling the Kosterlitz-Thouless transition.

Impact of reduced dimensionality on the correlation length and magnetization dynamics of the spin chain cobaltite Ca3Co2O6

We have recently applied the magnetic field and temperature dependence of magnetic entropy change as a probe of the magnetic states of a single crystal of Ca3Co2O6 to establish a more comprehensive magnetic phase diagram for this exotic system. The results are consistent with the spin-density wave description of the ground state and indicate the suppression of the modulated state in favor of a ferrimagnetic up-up-down arrangement of the spin chains with the application of a moderate field. We demonstrate, in this study, that the features of the static magnetic phase diagram are preserved when the grain size of Ca3Co2O6 is reduced to 440 nm (below the maximum c-axis correlation length of ~550 nm in Ca3Co2O6), although the polycrystalline materials show a non-saturating behavior, rounded steps, and reduced magnetization when compared to the single crystal. Our new finding of the multiple low-temperature magnetization steps in the 440 nm sample, together with conflicting reports concerning such steps in nanostructured samples, suggests that correlations in the ab plane are more important than correlations along the c-axis in the appearance of the plateaus. The relaxation processes in the system are studied through the decay of remanent magnetization, ac susceptibility, and Argand diagrams. The introduction of grain boundaries in Ca3Co2O6 weakens the slow-dynamic state, widens the low-temperature distribution of relaxation times, and lowers activation energies within the Arrhenius-like relaxation regime.

Research on geometric features of phase diagram and crack identification of cantilever beam with breathing crack

Geometric features of the phase diagram of a cantilever beam with a breathing crack are investigated. Influences of crack properties on the phase diagram geometry are studied and a method for crack identification based on phase diagram features is proposed. The pixel method is used to estimate different phase diagram areas. Multiple crack identification indexes are defined under the conditions of primary and super-harmonic resonance. The effects of crack location and depth on each index are analyzed. The contour lines for multiple identification indexes are used to estimate crack parameters. Numerical calculations indicate that the phase diagrams for super-harmonic resonance are more sensitive to crack parameters and that the proposed method is both feasible and effective.

Order-disorder transition in a two-dimensional associating lattice gas.

We study an associating lattice gas (ALG) using Monte Carlo simulation on the triangular lattice and semianalytical solutions on Husimi lattices. In this model, the molecules have an orientational degree of freedom and the interactions depend on the relative orientations of nearest-neighbor molecules, mimicking the formation of hydrogen bonds. We focus on the transition between the high-density liquid (HDL) phase and the isotropic phase in the limit of full occupancy, corresponding to chemical potential μ→∞, which has not yet been studied systematically. Simulations yield a continuous phase transition at τ_{c}=k_{B}T_{c}/γ=0.4763(1) (where -γ is the bond energy) between the low-temperature HDL phase, with a nonvanishing mean orientation of the molecules, and the high-temperature isotropic phase. Results for critical exponents and the Binder cumulant indicate that the transition belongs to the three-state Potts model universality class, even though the ALG Hamiltonian does not have the full permutation symmetry of the Potts model. In contrast with simulation, the Husimi lattice analyses furnish a discontinuous phase transition, characterized by a discontinuity of the nematic order parameter. The transition temperatures (τ_{c}=0.51403 and 0.51207 for trees built with triangles and hexagons, respectively) are slightly higher than that found via simulation. Since the Husimi lattice studies show that the ALG phase diagram features a discontinuous isotropic-HDL line for finite μ, three possible scenarios arise for the triangular lattice. The first is that in the limit μ→∞ the first-order line ends in a critical point; the second is a change in the nature of the transition at some finite chemical potential; the third is that the entire line is one of continuous phase transitions. Results from other ALG models and the fact that mean-field approximations show a discontinuous phase transition for the three-state Potts model (known to possess a continuous transition) lends some weight to the third alternative.

A potential justifying superconductivity and pseudogap formation in high-Tc superconductors

Copper and Iron based high temperature superconductors exhibit d-wave type superconducting gap and order parameter and posses a universal phase diagram. Here a potential is introduced that accounts for high temperature superconductivity, justifies d-wave symmetric behavior, and successfully explains phase diagram’s salient features. This potential is stipulated by principles of special relativity and arises from the difference between the electric potential of moving electrons and the potential of stationary nuclei. In quasi-two-dimensional materials this difference results in an uncompensated angular dependent attraction force in preferred directions of motion and a repulsion force in the perpendicular directions. The attraction force causes d-wave angular dependent superconducting gap and order parameter at high temperatures for d or p orbitals, which are the orbitals involved in Copper and Iron based superconductors. The repulsion force justifies the existence of angular dependent pseudogap and since the attraction and repulsions forces confine electrons to two directions of motion the number of allowed momentum states are reduced resulting in anti-ferromagnetic Mott-insulator behavior. The combination of the attraction and repulsion forces is shown to create charge density waves in these quasi-two-dimensional materials. This potential is able to justify the main features of the universal phase diagram self-consistently.

Molecular partitioning in ternary solutions of cellulose

Neutron scattering measurements on the structure and dynamics of ternary solutions of microcrystalline cellulose (MC) in mixtures of an ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate and a polar organic solvent dimethylformamide (DMF) have shown that MC can be fully dissolved in solvent mixtures. Data also show the molecular partitioning of IL into coexisting states. The structure partitioning is manifested as IL adsorbed to cellulose molecules with additional IL self-assembled to form clusters in solution, while the dynamics partitioning shows dynamical heterogeneities of the IL with slow dynamics resembling neat IL and fast dynamics being coupled with the solvent. The composition dependence of the molecular partitioning results in a solubility gap in dilute cellulose solutions and a phase boundary criterion of the molar ratio of IL / MC ∼ 3 in more concentrated regimes. The two characteristics together define the main features of the dissolution phase diagram of ternary cellulose mixtures of MC / IL / DMF at the room temperature.

Classical and Quantum Liquids Induced by Quantum Fluctuations.

Geometrically frustrated interactions may render classical ground states macroscopically degenerate. The connection between classical and quantum liquids and how the degeneracy is affected by quantum fluctuations is, however, not completely understood. We study a simple model of coupled quantum and classical degrees of freedom, the so-called Falicov-Kimball model, on a triangular lattice and away from half-filling. For weak interactions the phase diagram features a charge disordered state down to zero temperature. We provide compelling evidence that this phase is a liquid and show that it is divided by a crossover line that terminates in a quantum critical point. Our results offer a new vantage point to address how quantum liquids can emerge from their classical counterparts.

Nanostructure stability and swelling of ternary block copolymer/homopolymer blends: A direct comparison between dissipative particle dynamics and experiment

ABSTRACTTernary block copolymer (BCP)‐homopolymer (HP) blends offer a simple method for tuning nanostructure sizes to meet application‐specific demands. Comprehensive dissipative particle dynamic (DPD) simulations were performed to study the impact of polymer interactions, molecular weight, and HP volume fraction (φHP) on symmetric ternary blend morphological stability and domain spacing. DPD reproduces key features of the experimental phase diagram, including lamellar domain swelling with increasing φHP, the formation of an asymmetric bicontinuous microemulsion at a critical HP concentration , and macrophase separation with further HP addition. Simulation results matched experimental values for and lamellar swelling as a function of HP to BCP chain length ratio, α = NHP/NBCP. Structural analysis of blends with fixed φHP but varying α confirmed that ternary blends follow the wet/dry brush model of domain swelling with the miscibility of HPs and BCPs depending on α. Longer HPs concentrate in the center of domains, boosting their swelling efficiencies compared to shorter chains. These results advance our understanding of BCP‐HP blend phase behavior and demonstrate the value of DPD for studying polymeric blends. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 794–803

Fluctuations in cool quark matter and the phase diagram of quantum chromodynamics

We consider the phase diagram of hadronic matter as a function of temperature, T , and baryon chemical potential, mu. Currently the dominant paradigm is a line of first order transitions which ends at a critical endpoint. In this work we suggest that spatially inhomogenous phases are a generic feature of the hadronic phase diagram at nonzero mu and low T . Familiar examples are pion and kaon condensates. At higher densities, we argue that these condensates connect onto chiral spirals in a quarkyonic regime. Both of these phases exhibit the spontaneous breaking of a global U(1) symmetry and quasi-long range order, analogous to smectic liquid crystals. We argue that there is a continuous line of first order transitions which separate spatially inhomogenous from homogenous phases, where the latter can be either a hadronic phase or a quark-gluon plasma. While mean field theory predicts that there is a Lifshitz point along this line of first order transitions, in three spatial dimensions strong infrared fluctuations wash out any Lifshitz point. Using known results from inhomogenous polymers, we suggest that instead there is a Lifshitz regime. Non-perturbative effects are large in this regime, where the momentum dependent terms for the propagators of pions and associated modes are dominated not by terms quadratic in momenta, but quartic. Fluctuations in a Lifshitz regime may be directly relevant to the collisions of heavy ions at (relatively) low energies, sqrt(s)/A : 1 to 20 GeV.

Uncertainty quantification and propagation in CALPHAD modeling

Design is about making decisions bounded by a quantifiable degree of certainty. In the context of alloy design, Integrated Computational Materials Engineering (ICME) provides the framework whereby performance requirements are ultimately transformed into alloy/processing specifications through the combination of (complex) computational models connecting process-structure-property-performance relationships and experiments. Most ICME approaches consider the models used as deterministic and thus do not provide the means to make alloy design decisions with proper confidence measures. At the root of ICME lie CALPHAD models that describe the thermodynamics and phase stability of phases under specific thermodynamic boundary conditions. To date, the vast majority of efforts within the CALPHAD community have been deterministic in that thermodynamic models and the resulting thermodynamic properties and phase diagram features do not explicitly account for the uncertainties inherent in the model formulation or in the experimental/computational data used. In this contribution, we provide an overview of the state of the field. We review major efforts thus far and we then provide a (brief) tutorial on basic concepts of uncertainty quantification and propagation (UQ/UP) in CALPHAD. We discuss the major features of frequentist and Bayesian interpretations of uncertainty and proceed with a discussion of recent case studies in which UQ has been used to parameterize models for the thermodynamic properties of phases. We follow our discussion by presenting frameworks and demonstrating the propagation of uncertainty in thermodynamic properties and phase diagram predictions and briefly discuss how we can use Bayesian frameworks for rigorous model selection as well as for model fusion. We close our contribution by providing context for what has been done and what remains to be accomplished in order to fully embrace the management of uncertainty in CALPHAD modeling, a foundational element of ICME.

Chiral Y junction of quantum spin chains

We study a Y junction of spin-1/2 Heisenberg chains with an interaction that breaks both time-reversal and chain exchange symmetries, but not their product nor SU(2) symmetry. The boundary phase diagram features a stable disconnected fixed point at weak coupling and a stable three-channel Kondo fixed point at strong coupling, separated by an unstable chiral fixed point at intermediate coupling. Using non-abelian bosonization and boundary conformal field theory, together with density matrix renormalization group and quantum Monte Carlo simulations, we characterize the signatures of these low-energy fixed points. In particular, we address the boundary entropy, the spin conductance, and the temperature dependence of the scalar spin chirality and the magnetic susceptibility at the boundary.

Sensing and Quantifying a New Mechanism for Vehicle Brake Creep Groan

This paper investigates the creep groan of a vehicle’s brake experimentally, analytically, and numerically. Experimentally, the effects of acceleration on caliper and strut, noise, brake pressure, and tension are measured. The results show that the measured signals and their relevant spectra broadly capture the complex vibrations of creep groan. This includes the simple stick‐slip, severe stick‐slip vibrations/resonances, multiple harmonics, half‐order harmonics; stick‐slip‐induced impulsive vibrations, steady/unstable vibrations, and their transitions. Analytically, a new mathematical model is presented to capture the unique features of half‐order harmonics and the connections to fundamental stick‐slip/resonant frequency and multiple harmonics. The analytical solution and the experimental results show that the vibro‐impact of the brake pad‐disc system can be triggered by severe stick‐slip vibrations and is associated with instable, impulsive stick‐slip vibration with wideband. The induced stick‐slip vibro‐impact can evolve into a steady and strong state with half‐order, stick‐slip fundamental, and multiple‐order components. This new mechanism is different from all previously proposed mechanisms of creep groan in that we also view some type of creep groan as a stick‐slip vibration‐induced vibro‐impact phenomenon in addition to conventional stick‐slip phenomena. The new mechanism comprehensively explains the complex experimental phenomena reported in the literature. Numerically, the salient features of phase diagrams of instable stick‐slip and vibro‐impact are examined by using a seven‐degree‐of‐freedom brake system model, which shows that the phase diagrams of the dynamics of creep groan with and without vibro‐impact are substantially different. The phase diagram of the dynamics with vibro‐impact is closer to the experimental results. In contrast to existing mechanisms, the proposed new mechanism encompasses the instable stick‐slip nature of creep groan and elaborates the inherent connections and transition of the spectrogram. The new knowledge can be used to attain critical improvements to brake noise and vibration analysis and design. By applying the proposed new model in addition to existing models, all experimental phenomena in creep groan are elaborated and quantified.