On the chiral phase transition in hadronic matter

A qualitative analysis of the chiral phase transition in QCD with two massless quarks and nonzero baryon density is performed. It is assumed that, at zero baryonic density, ρ=0, the temperature phase transition is of the second order. Due to a specific power dependence of baryon masses on the chiral condensate, the phase transition becomes of the first order at the temperature T=T_ph(ρ) for ρ>0. At temperatures T_cont(ρ)> _T>_Tph(ρ), there is a mixed phase consisting of the quark phase (stable) and the hadron phase (unstable). At the temperature T=T_cont(ρ), the system experiences a continuous transition to the pure chirally symmetric phase.     

Critical point coupling and proximity effects in ^{4}he at the superfluid transition

We report measurements of the superfluid fraction ρ_{s}/ρ and specific heat c_{p} near the superfluid transition of ^{4}He when confined in an array of (2 μm)^{3} boxes at a separation of S=2 μm and coupled through a 32.5?nm film. We find that c_{p} is strongly enhanced when compared with data where coupling is not present. An analysis of this excess signal shows that it is proportional to the finite-size correlation length in the boxes ξ(t,L), and it is measurable as far as S/ξ～30-50. We obtain ξ(0,L) and the scaling function (within a constant) for ξ(t,L) in an L^{3} box geometry. Furthermore, we find that ρ_{s}/ρ of the film persists a full decade closer to the bulk transition temperature T_{λ} than a film uninfluenced by proximity effects. This excess in ρ_{s}/ρ is measurable even when S/ξ>100, which cannot be understood on the basis of mean field theory.     

低密度微孔聚合物泡沫的制备

 介绍了一种利用聚合物和溶剂的相分离，经过冷冻干燥来制备聚合物泡沫（特别是聚苯乙烯类泡沫）的通用技术。采用该技术制备的具有开放状孔洞结构的聚苯乙烯泡沫的密度为0.02~0.1g/cm3，平均孔径为1～20μm，而且泡沫具有各向同性，孔径非常均匀。通过对聚合物泡沫的表征，对影响泡沫形貌和密度的因素，如聚合物/溶剂体系，冷却速率等进行了研究，结果表明：所用溶剂对于该聚合物的Θ温度（溶剂与聚合物作用力为零的温度）必须高于该溶剂的熔点；采用单轴冷却方式可以减小由于溶液中心处和外围处的温差而引起的热应力对泡沫形貌的影响，冷却速率为100℃/min。     

Direct measurements of the dynamical correlation length indicate its divergence at an athermal glass transition

The supercooled N3 model exhibits an increasingly slow dynamics as density approaches the random closest packing density. Here, we present a direct measurement of the dynamical correlation function G4(r,t), showing the emergence of a growing length scale ξ(4) across which the dynamics is correlated. The correlation length measured, up to 120 lattice sites, power law diverges as the density approaches ρ(t), the density at which the fluid phase of the model is predicted to terminate. The four-point susceptibility, often used as an agent to estimate ξ(4), does not depend simply on the latter. Rather, it depends strongly on the short-range behavior of G4(r,t). Consequently, χ(4) peaks before ξ(4) reaches its maximal value. The two quantities should therefore be studied independently.     

Reaction‐Induced Phase‐Separation in Polyoxyethylene/Polystyrene Blends. II. Structure Development

Abstract

The morphology development in model polymer blends was investigated in relation to the processing pathway. Reaction‐induced phase separation was used to make polyoxyethylene (POE) and polystyrene (PS) blends from a solution of POE/styrene. As the styrene underwent polymerization by photo‐initiation with ultraviolet light, phase separation, and phase inversion were induced, whereby the POE became the matrix phase. Optical microscopy showed that liquid–liquid (L–L) phase separation occurred soon after the styrene polymerization was initiated. Nucleation and growth was identified as the mechanism of L–L phase separation. Polystyrene/styrene‐rich domains formed in a POE/styrene‐rich matrix. The domain size developed until arrested by the POE liquid–solid phase separating and crystallizing, since the experiments were conducted below the melt temperature of POE. The POE crystal growth process also followed a nucleation and growth mechanism. The time to the onset of crystallization was observed to decrease as the POE content increased, until the POE formed a saturated solution in styrene. As the crystallization onset time decreased, the PS‐rich domain size also decreased. The phase diagram previously established can now be used to describe (and predict) the number density and size of the PS‐rich domains in the POE matrix of the blends.     

Unexpected scenario of glass transition in polymer globules: an exactly enumerable model

We introduce a lattice model of glass transition in polymer globules. This model exhibits ergodicity breaking in which the disjoint regions of phase space do not arise uniformly, but as small chambers whose number increases exponentially with polymer density. Chamber sizes obey power law distribution, making phase space similar to a fractal foam. This clearly demonstrates the importance of the phase space geometry and topology in describing any glass-forming system, such as semicompact polymers during protein folding.     

Cosmic curvature from de Sitter equilibrium cosmology

I show that the de Sitter equilibrium cosmology generically predicts observable levels of curvature in the Universe today. The predicted value of the curvature, Ω(k), depends only on the ratio of the density of nonrelativistic matter to cosmological constant density ρ(m)(0)/ρ(Λ) and the value of the curvature from the initial bubble that starts the inflation, Ω(k)(B). The result is independent of the scale of inflation, the shape of the potential during inflation, and many other details of the cosmology. Future cosmological measurements of ρ(m)(0)/ρ(Λ) and Ω(k) will open up a window on the very beginning of our Universe and offer an opportunity to support or falsify the de Sitter equilibrium cosmology.     

Reaction‐Induced Phase Separation in Polyoxyethylene/Polystyrene Blends. I. Ternary Phase Diagram

Abstract

Reaction‐induced, phase separation has been studied in polymer blends. A model crystalline‐amorphous system consisted of semicrystalline polyoxyethylene (POE) dissolved in the monomer styrene, which was used as a reactive solvent to ease processing. When the styrene was polymerized to polystyrene (PS) in the mold, phase separation and phase inversion are induced, and a polymer blend was formed. Polyoxyethylene was selected with a molar mass, M _n  = 8578 g mol^?1 and a polydispersity of 1.19, as determined by using gel permeation chromatography. The polymerization of styrene was initiated by using 1 wt% benzoin methyl ether and 0.2 wt% 2,2′‐azobisisobutyronitrile under ultraviolet light. The polymerization kinetics were determined by monitoring the reduction in the intensity of the C?C stretching vibration band at 1631 cm^?1 in the Raman spectrum of styrene. The onset times for the liquid–solid (L–S) phase separation and crystallization of POE from styrene/PS were observed by using simultaneous small‐angle x‐ray scattering (SAXS) and wide‐angle x‐ray scattering. Onset times for L–S phase separation determined from the SAXS data were combined with the styrene polymerization kinetics to plot the L–S phase separation data onto a ternary phase diagram for the reactive system POE/styrene/PS at 45°C and 50°C.     

Quantitative evaluation of evaporation rate during spin-coating of polymer blend films: Control of film structure through defined-atmosphere solvent-casting

Thin films of polymer mixtures made by spin-coating can phase separate in two ways: by forming lateral domains, or by separating into distinct layers. The latter situation (self-stratification or vertical phase separation) could be advantageous in a number of practical applications, such as polymer optoelectronics. We demonstrate that, by controlling the evaporation rate during the spin-coating process, we can obtain either self-stratification or lateral phase separation in the same system, and we relate this to a previously hypothesised mechanism for phase separation during spin-coating in thin films, according to which a transient wetting layer breaks up due to a Marangoni-type instability driven by a concentration gradient of solvent within the drying film. Our results show that rapid evaporation leads to a laterally phase-separated structure, while reducing the evaporation rate suppresses the interfacial instability and leads to a self-stratified final film.     

受限一维无自旋费米子系统的性质研究

本文借助于一维自旋1/2-XXZ模型的Bethe-ansatz精确解, 利用局域密度近似(LDA), 讨论了谐振势中一维无自旋费米子的密度分布, 得出了ρ-u相图(这里的ρ为无量纲的粒子数密度 变量u为相互作用强度)对相图的分析表明, 随着原子密度和近邻相互作用的变化, 系统出现五个不同的混合量子相通过对热力学硬度S_ρ的计算, 发现其可作为体系的序参量, 其奇异点可用以度量受限体系中量子相变的发生     

On the Distribution of the Wave Function for Systems in Thermal Equilibrium

For a quantum system, a density matrix ρ that is not pure can arise, via averaging, from a distribution μ of its wave function, a normalized vector belonging to its Hilbert space ?. While ρ itself does not determine a unique μ, additional facts, such as that the system has come to thermal equilibrium, might. It is thus not unreasonable to ask, which μ, if any, corresponds to a given thermodynamic ensemble? To answer this question we construct, for any given density matrix ρ, a natural measure on the unit sphere in ?, denoted GAP(ρ). We do this using a suitable projection of the Gaussian measure on ? with covariance ρ. We establish some nice properties of GAP(ρ) and show that this measure arises naturally when considering macroscopic systems. In particular, we argue that it is the most appropriate choice for systems in thermal equilibrium, described by the canonical ensemble density matrix ρ_β = (1/Z) exp (?β H). GAP(ρ) may also be relevant to quantum chaos and to the stochastic evolution of open quantum systems, where distributions on ? are often used.     

低密度聚环己基乙烯泡沫的制备

低密度CH聚合物多孔材料是惯性约束聚变(ICF)的重要靶材料,利用热致相分离原理对低密度聚环己基乙烯泡沫的制备进行了研究。首先通过聚苯乙烯(PS)氢化反应制备了聚环己基乙烯(PVCH),经过溶剂选择,确定以环己烷/1,4-二氧六环为溶剂体系,经热致相分离和冷冻干燥技术制备出低密度PVCH泡沫。通过分析溶液浓度对泡沫密度的影响,确定了泡沫密度与聚合物溶液质量浓度之间的关系,在0.04~0.15 g/cm3范围之内可实现对泡沫密度的有效控制。泡沫孔结构测试结果表明随着密度的增加,平均孔径有升高的趋势,孔径分布趋于单一化,孔径范围为23.63~0.83μm。     

Quantum critical transport near the Mott transition

We perform a systematic study of incoherent transport in the high temperature crossover region of the half filled one-band Hubbard model. We demonstrate that the family of resistivity curves displays characteristic quantum critical scaling of the form ρ(T, δU) = ρ(c)(T)f(T/T?(δU)), with T?(δU) ~ |δU|(zν), and ρ(c)(T) ~ T. The corresponding β function displays a "strong coupling" form β ~ ln(ρ(c)/ρ), reflecting the peculiar mirror symmetry of the scaling curves. This behavior, which is surprisingly similar to some experimental findings, indicates that Mott quantum criticality may be acting as the fundamental mechanism behind the unusual transport phenomena in many systems near the metal-insulator transition.     

Understanding liquid-liquid immiscibility and LCST behaviour in polymer solutions with a Wertheim TPT1 description

The aim of the work presented in this paper is to help in the understanding of the lower critical solution temperature (LCST) fluid phase behaviour exhibited by polymer solutions. It is well recognized that the LCST in polymer solutions is a consequence of density (compressibility) effects; the solvent is much more compressible than the polymer and the increasing difference in compressibility when the temperature is increased leads to a negative volume of mixing. The separate roles that the repulsive and attractive intermolecular interactions play in this regard are less well understood. In this study we use the Wertheim first-order thermodynamic perturbation theory (TPT1) [Wertheim, M. S., 1987, J. chem. Phys., 87, 7323; Chapman, W. G., Jackson, G., and Gubbins, K. E., 1988, Molec. Phys., 65, 1057] to describe the phase equilibria of model polymer solutions of hard spheres and hard-sphere chains where the diameter of the solvent and the polymeric segments are the same (symmetrical system). The thermodynamic functions (volume, enthalpy, entropy and Gibbs function) of mixing are determined to assess the possibility of a demixing instability in such a system. No fluid-fluid phase separation is found for the purely repulsive (athermal) system, regardless of the chain length of the polymer. The role of the attractive interactions is then investigated by incorporating attractive interactions at the mean-field level; the simplest system with equivalent (symmetric) solvent-solvent, solvent-polymer segment, and polymer segment-polymer segment interaction energies is examined. The attractive interactions are found to be essential in describing the liquid-liquid phase separation; LCST behaviour is found for mixtures with ‘polymer’ chains of seven segments or more. In this case we show that the phase behaviour is driven by an unfavourable (negative) entropy of mixing due to an increase in the density of the solvent on addition of small amounts of polymer. We also determine the thermodynamic properties of mixing for a system of spherical molecules of the same size with directional interactions that give rise to LCST and closed-loop behaviour. As expected the mechanism for phase separation in such systems is very different to that in polymer solutions.     

Simulation studies of gas-liquid transitions in two dimensions via a subsystem-block-density distribution analysis

The finite-size scaling analysis of the density distribution function of subsystems of a system studied at constant total density is studied by a comparative investigation of two models: (i) the nearest-neighbor lattice gas model on the square lattice, choosing a total lattice size of 64×64 sites. (ii) The two-dimensional off-lattice Lennard-Jones system (truncated at a distance of 2.5 σ, σ being the range parameter of the interaction) withN=4096 particles, applying the NVT ensemble. In both models, the density distribution functionP _L (ρ) is obtained forL×L subsystems for a wide range of temperaturesT, subblock linear dimensionsL and average densities <ρ>. Particular attention is paid to the question whether accurate estimates of critical temperatureT _c and critical density ρ _c can be obtained. In the lattice gas model these critical parameters are known exactly and the limitations of the approach can thus be definitively asserted. The final estimates for the Lennard Jones problem areT _c =0.47±0.01 (in units of the Lennard Jones energy ε) and ρ _c (in units of σ²), a comparison with previous estimates is made.     

Charged ρ photoproduction in the energy range 2.8–4.8 GeV

Measurements of the photoproduction processes γρ→ρ⁺ n and γρ→ρ^-Δ⁺⁺ (1236) are reported in the energy range 2.8 to 4.8 GeV. The data show shrinkage of the differential cross section in this energy region for the process γρ→ρ^-Δ⁺⁺ (1236); no shrinkage is observed for the ρ⁺ n process. The energy dependences of the ρ⁺ n and ρ^-Δ⁺⁺ (1236) total cross sections are much steeper than current model prediction. The ρ spin density matrices for each process are also presented.     

New Approach for Solving Master Equations in Quantum Optics and Quantum Statistics by Virtue of Thermo-Entangled State Representation

By introducing a fictitious mode to be a counterpart mode of the system mode under review we introduce the entangled state representation <η |, which can arrange master equations of density operators ρ(t) in quantum statisticsas state-vector evolution equations due to the elegant properties of <η |. In this way many master equations (respectively describing damping oscillator, laser, phase sensitive, and phase diffusion processes with different initial density operators) can be concisely solved. Specially, for a damping process characteristic of thedecay constant κ we find that the matrix element of ρ (t) at time t in <η| representation is proportional to that of the initial ρ₀ in the decayed entangled state <ηe^-κt| representation, accompanying with a Gaussian damping factor. Thus we have a new insight about the nature of the dissipative process. We also set up the so-called thermo-entangled state representation of density operators, ρ=∫(d²η /π)< η | ρ> D(η), which is different from all the previous known epresentations.     

Do extreme conditions of nuclear matter influence particle properties? summary of the HADES project

Heavy ion collisions at beam energies of 1–2 GeV/u (SIS at GSI, Darmstadt) offer a unique possibility to investigate the properties of hot, compressed nuclear matter. Baryon densities of 2–3ρ ₀ and temperature up to 100 MeV can be reached during a collision phase. Due to a high density of the phase new particles are produced in multiple baryon-baryon collisions. Objects decaying into electron-positron pairs play an important role in this context. In contrast to hadrons, dileptons carry away undistorted by final-state interactions information about the hot and dense phase. The two-body decays of the vector mesons ρ, ω and ? are of special interest since width and masses of the resonances are accessible in the dielectron mass spectra. A High Acceptance DiElectron Spectrometer (HADES) has been proposed for the SIS accelerator at GSI in Darmstadt and currently is constructed by the HADES Collaboration in many places of Europe. A Ring Imaging Cerenkov Counter (RICH), blind for hadrons, together with Multiplicity Electron Trigger Array (META) containing Shower detector and Time-of-Flight wall will serve for the electron identification. The momentum of dileptons will be determined in 2 sets of Mini-Drift Chambers (MDC) placed in front of and behind a six-fold super-conducting coil. An expected invariant mass resolution in the ρ/ω region less than 1% (σ) assures a clear separation of the ω and ? from ρ. For the difficult case of the 1 GeV/u Au+Au collision we expect a 10:1 signal-to-background ratio at the ρ/? region. The performance of the spectrometer was studied by detailed simulations using GEANT. Results of simulations and tests of prototypes assure that with its high counting rate capability and large geometrical acceptance HADES will be able to measuree ⁺ e ^? pairs for the heaviest systems at the highest SIS energies. First results are expected in 1999.     

Stars of strange matter?

We investigate suggestions that quark matter with strangeness per baryon of order unity may be stable. We model this matter at nuclear matter densities as a gas of close packed Λ-particles. From the known mass of the Λ-particle we obtain an estimate of the energy and chemical potential of strange matter at nuclear densities. These are sufficiently high to preclude any phase transition from neutron matter to strange matter in the region near nucleon matter density. Including effects from gluon exchange phenomenologically, we investigate higher densities, consistently making approximations which underestimate the density of transition. In this way we find a transition density ρ_tr≳7ρ₀, where ρ₀ is nuclear matter density is not far from the maximum density in the center of the most massive neutron stars that can be constructed. Since we have underestimated ρ_tr and still find it to be ∼7ρ₀, we do not believe that the transition from neutron to quark matter is likely in neutron stars. Moreover, measured masses of observed neutron stars are ≅1.4 M_⊙, where M_⊙ is the solar mass. For such masses, the central (maximum) density is ρ_c<5ρ₀. Transition to quark matter is certainly excluded for these densities.     

Breakdown of dynamic scaling in thin film binary liquids undergoing phase separation

The kinetics of phase separation in thin polymer blend films displaying discrete and bicontinuous domain morphologies are examined. For discrete domains, the correlation length xi grows as t(1/3), in agreement with a coalescence model. By plotting xi/d vs t/t(i) (initiation time), universal growth behavior is obtained for thickness values (d) from 1000 to 190 nm. In contrast, bicontinuous domains grow with a decreasing exponent, 0.62 to 0.28, as d decreases from 900 to 90 nm (i.e., no universal growth). This slowing down with reduced dimensionality suggests suppression of lateral hydrodynamic pumping.