Surface phase transitions of multiple-site associating fluids

Surface phase transitions of Lennard–Jones (LJ) based two- and four-site associating fluids have been studied for various associating strengths using grand-canonical transition matrix Monte Carlo simulations. Our results suggest that, in the case of a smooth surface, represented by a LJ 9-3-type potential, multiple-site associating fluids display a prewetting transition within a certain temperature range. However, the range of the prewetting transition decreases with increasing associating strength and increasing number of sites on the fluid molecules. With the addition of associating sites on the surface, a quasi-2D vapor–liquid transition may appear, which is observed at a higher surface site density for weaker associating fluids. The prewetting transition at lower associating strength is found to shift towards the quasi-2D vapor–liquid transition with increasing surface site density. However, for highly associating fluids, the prewetting transition is still intact, but shifts slightly towards the lower temperature range. Adsorption isotherms, chemical potentials and density profiles are used to characterize surface phase transitions.     

量子相变

量子相变是一种发生在绝对零度,由量子涨落而非热涨落导致的相变现象,满足著名的海森堡不确定关系。通过零温量子临界点的研究,可获知物质系统更广泛范围的行为,包括稀土磁性绝缘体,高温超导体和二维电子气体等。     

Cosmic phase transitions

The sequence of phase transitions during the hot history of the universe is followed within a phenomenological framework. Particular emphasis is put on the QCD confinement transition, which is at reach under earth laboratory conditions. A tepid inflationary scenario on the GUT scale with bubble growth at moderate supercooling is discussed.     

Structural phase transitions in ionic molecular solids

An overview is presented of our studies on the nature of structural instabilities in relatively complex ionic solids. These are based on parameter-free interionic potentials based on the Gordon-Kim modified electron gas formalism extended to molecular ions.

We describe the manner in which there emerge from these studies quite general concepts of “size” and “shape” as structural determinants. In particular, we discuss how these, and the approximate symmetries that they can produce, can provide a relatively simple structure-based explanation of the origins of incommensurate phases in these systems. However, we also emphasize that the existence of such symmetries does not guarantee an incommensurate phase. This can only be realized if long-range correlations are sufficiently strong to overcome random local disordering. Thus, either the molecular units are partially linked and/or there exist long-range Coulomb interactions between individual units.     

Nonequilibriurn discontinuous phase transitions in a fast ionic conductor model: Coexistence and spinodal lines

Two-dimensional lattice-gas models with attractive interactions and particle-conserving hopping dynamics under the influence of a very large external electric field along a principal axis are studied in the case of off-critical densities. We describe the corresponding nonequilibrium first-order phase transitions, evaluate coexistence and spinodal lines, and make some comparisons with experimental observations on fast ionic conductors.See Ref. 1 (henceforth referred to as II) for references.     

保真率与量子相变

量子相变是量子多体理论中的一个重要概念,保真度则是量子信息学的重要概念.文章简单介绍了一个量子系统的基态对系统参量的响应,即保真率,在量子相变中的行为.作为理解量子相变的一个新的视角,保真度方法的优势在于它是一个纯粹的几何学量,所以在研究相变过程中不需要考虑任何预设的序参量.文章用通俗的语言介绍了基态保真度、保真率、量子绝热维度以及它们的物理意义.为便于理解,文章以Lipkin-Meshkov-Glick模型与Kitaev蜂巢模型为例,对保真率在这两个模型中的性质做了简单介绍.     

Thermodynamics and phase transitions in the Overhauser model

We analyze the thermodynamics of the Overhauser model and demonstrate rigorously the existence of a phase transition. This is achieved by extending techniques previously developed to treat the BCS model in the quasi-spin formulation. Additionally, we compare the thermodynamics of the quasi-spin and full-trace BCS models. The results are identical up to a temperature rescaling.     

Kitaev模型与拓扑量子相变

文章通过在一种准一维路径上引入自旋算符的约当-维格纳（Jordan—Wigner）变换，证明了Kitaev自旋模型完全等价于一个不含任何非物理自由度的自由Majorana费米子模型。通过对偶变换，进一步证明了这个系统中存在的量子相变可用非定域的拓扑序参量来描述；并且，这些非定域的拓扑序参量在对偶空间变成为定域的朗道类型的序参量。文章作者的工作揭示了传统的量子相变和拓扑量子相变的内在关系，扩展了朗道二级相变理论的适用范围。     

Reentrant phase transitions in liquid crystals

A comprehensive review of the recent developments regarding the phenomenon of reentrant phase transitions (RPT) in liquid crystals is presented. In addition to liquid crystals this phenomenon has been observed in amazingly diverse systems. A critical assessment of the experimental investigations concerning single and multiple reentrances is given. A brief account of the theoretical efforts is also given. The article ends with the identification of the factors which impede the proper understanding of the phenomenon.     

Adsorption isotherms for fluids with strong interparticle repulsions

We derive adsorption isotherms for an adsorbate of hard-sphere particles with sticky interactions at any fluid density being adsorbed onto a plane, sticky surface. The theory is based on the Percus-Yevick theory for bulk fluids and explicitly includes the equilibrium between the adsorbed fluid and the bulk adsorbate. The theory predicts a surface condensation at low temperatures and low bulk densities in good agreement with surface condensations found in experimental studies of adsorption of gases onto graphite. An approximate law of corresponding states for these transitions is developed. At higher bulk densities and room temperatures, the adsorption isotherms can show a maximum, in accord with recent experimental work.Supported by the Australian Research Grants Commission.     

Structural phase transitions in elpasolite- and cryolite-type fluorides

The occurrence of phase transitions in elpasolite- and cryolite-type fluorides depends mainly on two factors: the electronic structure of the involved transition element and/or the value of the tolerance factor (t) close to the lower limit of the stability range. The phase transitions have been followed using low- and hightemperature X-ray diffraction. EPR signals of Ni¹¹¹ and Pd¹¹¹ compounds are characteristic of a doublet ground state. The trivalent species are located in axially elongated octahedra and a static ← dynamic Jahn-Teller transition can be observed in some of these phases.     

Characteristics of phase transitions via intervention in random networks

We present a percolation process in which the classical Erdo¨s–Re′nyi(ER) random evolutionary network is intervened by the product rule(PR) from some moment t0. The parameter t0is continuously tunable over the real interval [0, 1].This model becomes the random network under the Achlioptas process at t0= 0 and the ER network at t0= 1. For the percolation process at t0≤ 1, we introduce a relatively slow-growing point, after which the largest cluster begins growing faster than that in the ER model. A weakly discontinuous transition is generated in the percolation process at t0≤ 0.5.We take the relatively slow-growing point as the lower pseudotransition point and the maximum gap point of the order parameter as the upper pseudotransition point. The critical point can be approximately predicted by each fitting function of the two points about t0. This contributes to understanding the rapid mergence of the large clusters at the critical point.The numerical simulations indicate that the lower pseudotransition point and the upper pseudotransition point are equal in the thermodynamic limit. When t0> 0.5, the percolation processes generate a continuous transition. The scaling analyses of several quantities are presented, including the relatively slow-growing point, the duration of the relatively slow-growing process, as well as the relatively maximum strength between the percolation percolation at t0< 1 and the ER network about different t0. The presented mechanism can be viewed as a two-stage percolation process that has many potential applications in the growth processes of real networks.     

Frustration and quantum phase transitions in an anisotropic antiferromagnet on a square lattice

We study the effect of frustration between nearest and next-nearest neighbors of the quantum S=1 anisotropic Heisenberg model on a square lattice using the bond operator technique. A single-site anisotropy term induces a quantum phase transition in the system. We calculate the effect of zero-temperature quantum fluctuations on the magnetization for the Néel and collinear antiferromagnetic phases.     

Quantum phase transitions in antiferromagnets and superfluids

We present a general introduction to the non-zero temperature dynamic and transport properties of low-dimensional systems near a quantum phase transition. Basic results are reviewed in the context of experiments on the spin-ladder compounds, insulating two-dimensional antiferromagnets, and double-layer quantum Hall systems. Recent large N computations on an extended t–J model (Phys. Rev. Lett. 83 (1999) 3916) motivate a global scenario of the quantum phases and transitions in the high-temperature superconductors, and connections are made to numerous experiments.     

Diffuse ferroelectric phase transitions in cubically sabilized perovskites

Typical examples of ferroelectrics with diffuse phase transitions (relaxor ferroelectrics), like Pb(Mg_1/3Nb_2/3)O₃ are actually non transforming. The paraelectric phase is fully stablized against a ferroelectric phase transition in this case. A phase transition can be induced, however, by an electric field with appropriate orientation below the temperature of the dielectric constant maximum. The analogy with stress-induced martensitic phase transitions in metallic alloys is pointed out. Pecularities of the properties and of the polarization reversal of such systems are demonstrated. Actual diffuse ferroelectric phase transitions in disordered solid solutions and mixed compounds with a partially stabilized parent phase are compared with athermal martensitic transformations. With particular regard to technical ceramics based on PZT, the influence of interfaces between transformed regions and remnants of the parent phase which have to distinguished from domain walls, and of the reduced stability of the ferroelectric phase on the properties of these systems is discussed. Some effects usually explained solely by domain processes may be understood also from this point of view.

Stabilization of a parent phase against an order-disorder-type phase transition is supposed to be caused by glass-like freezing caused by inelastic cooperative interactions between disordered molecular groups.     

Analytic continuation at first-order phase transitions

We study the analytic structure of thermodynamic functions at first-order phase transitions in systems with short-range interactions and in particular in the two-dimensional Ising model. We analyze the nature of the approximation of the d=2 system by anN × strip. Investigation of the structure of the eigenvalues of the transfer matrix in the vicinity of H=0 in the complexH plane allows us to define a new function which provides rapidly convergent approximations to the stable free energyf and its derivatives for allH 0. This new function is used for numerical calculation of the coefficients C_n in the power series expansions of the magnetizationm in the form m(H)=1 + C_n(H-H ₀ )ⁿ for various H₀ 0. The resulting series are studied by conventional methods. We confirm recent series analysis results on the existence of the droplet model type essential singularity at H=0. Evidence is found for a spinodal at H=H_sp(Ti < 0.     

Coarse-graining approach to first-order phase transitions

On an example of a simple spin system with two ground states and no symmetry, we show how to control low-temperature systems near first-order phase transitions by a straightforward renormalization group argument. The method, as opposed to the Pirogov-Sinai approach, also works for complex Hamiltonians.     

Evidence for vacancy-driven phase transitions in BaMnF 4 at elevated temperatures

Anomalies near 370 K and/or 400 K in dielectric constant, diffuse x-ray scattering, C₁₁ elastic coefficient, electrical conductivity, and piezoelectric resonance have been reported for BaMnF ₄. Particularly curious is the dielectric constant along the ac-diagonal, which rises from 15 at 300 K to a sharp peak of 73 at 400 K. These anomalies are discussed in terms of 2D ionic conduction mechanisms (fluorine vacancy hopping), and the disappearance of short-range antiferroelectric order at temperatures above 400 K. At 650 K the crystals lose all piezoelectricity, which is interpreted as a 3D vacancy-driven C_2v – D_2h, transition.     

Structural phase transitions in KMnF3

The structural transitions of the perovskite KMnF₃ are studied with an energy-dispersive X-ray diffractometer. It is found that the 83 K transition is of first order, though the transition related to the condensation of a M₃-soft phonon mode is considerably affected by crystallographic domain-walls occurring below the 186 K transition. The latter transition is observed at 88.0 K (first-order), and 92.0 K (second-order) in different single crystals, respectively. The difference of the transition temperature and the transition order is interpreted in terms of inner strains appeared in the domain walls.