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.     

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。     

磁性量子相变

量子相变广泛存在于关联电子材料体系中,与非常规超导和奇异金属行为有着紧密的联系。近年来,人们对量子相变的认识正在不断深入,不同类型的量子相变相继被发现。揭示量子相变的普适分类,发展和完善量子相变理论,探索量子临界点附近的呈展量子态及其产生机理是当前量子相变研究的热点。文章简要介绍磁性量子相变的一些最新研究进展以及面临的挑战。     

Commensurate incommensurate phase transitions

there are two types of Commensurate-Incommensurate Phase Transitions : t_I between the basic structure and the incommensurate phasis and t_L between the inc. phasis and the lock in phasis. They both have connected structures. So that, they must verify the relation between crystallographic groups in accordance with Landau's Theory. But we have to consider the three phases in the superspace - as we do at an ordered-disordered magnetic transition-because the inc. phasis ins't a crystal in the physical space; e.g., in order to cancel the “middle range order” of the inc. phasis at t_I, we are led to assume the grey (point) group to the basic structure in the superspace. Now both inc. phase and basic structure verify the connection between point-groups in the superspace in the same way as both para and ferro phases do at a ferroic transition in the physical space. We also show that the same type of relation is possible at t_L and we give the order parameter at both t_I and t_L.     

Picturing quantum phase transitions

We discuss the quantum phase transitions (QPT) in N-spin chains from the point of view of collective observables. We show that the measurement space representation is a convenient tool for the analysis of phase transitions, allowing the determination of an appropriate set of macroscopic order parameters (for a given Hamiltonian). Quantum correlations in the vicinity of the critical points are analyzed both in the ground states and low temperature thermal states.     

On tensor phase transitions

Within the Landau theory, phase diagrams are obtained for second-order phase transitions with order parameter in the form of a symmetric traceless tensor of rank at most six that is transformed in accordance with a one-dimensional representation of a crystal group. The case of two-dimensional representation is analyzed for rank equal to three.     

Confinement and phase transitions

Using a density dependent quark mass to express non-perturbative interaction effects, we calculate the critical density for a phase transition between nuclear and quark matter atT=0.     

Line of second-order phase transitions in the four-dimensional Z2 gauge theory with matter fields

Strong evidence is presented that the phase transition between the free-charge and the screening region of the four-dimensional Z₂ lattice gauge theory with Z₂ matter fields is second order with mean field exponents. The quantity best suited for the analysis is an order parameter that tests the existence of charged states. Both its scaling and finite-size scaling properties are determined by performing a Monte Carlo simulation.     

Structural phase transitions in ZnS

X-ray diffraction techniques have been used to investigate structural phase transitions in ZnS between 20 and 1200°C. These measurements imply that the transition from the cubic 3C structure to the hexagonal 2H structure is a first-order phase transition while transitions between the 2H, 4H, and the 6H(33) hexagonal structures were found to obey the symmetry rules of second-order phase transitions. Direct transitions from the cubic 3C structure to the 4 or 6H hexagonal structures are not observed.     

Electron microscopy of phase transitions

The possibilities of electron microscopy for the study of phase transitions are briefly outlined and illustrated by some particular aspects of phase transitions in Pb₃V₂O₈, Ba₂NaNb₅O₁₅ and Au3+xZn. Lead orthovanadate is used as an example for the application of a procedure to distinguish between the static or dynamic nature of diffuse intensity in diffraction patterns occurring near the transition temperature. For Ba₂NaNb₅O₁₅ the behaviour of the discommensurations in the vicinity of the ferroelastic transition has been studied and the low-temperature transition around 105 K is proven to be due to a doubling of the baxis and not to a restoration of the fourfold symmetry. The decomposition mechanism of the metastable <2³3> type long period super-lattices in the binary Au_3+xZn alloys is described in terms of the withdrawal of nodes of four <3> domains.     

Pressure-induced phase transitions in gypsum

Abstract

We report high-pressure Raman scattering spectroscopy and energy dispersive X-ray diffraction investigations on gypsum, CaSO₄ · 2H₂O, at room temperature in a diamond cell. With increasing pressure, measurements indicate that CaSO₄ · 2H₂O undergoes two stages of crystalline-state phase transitions at 5 and 9 GPa, and then converts to a disordered phase above 11 GPa. The structures of the three high-pressure phases of gypsum have not been determined yet. These phases are tentatively named as “post-gypsum-I” (PG-I), “post-gypsum-II” (PG-II) and “disordered” according to the sequence of their appearance with pressure.

Gypsum shows anisotropic compressibility along three crystallographic axes with b > c > a below 5 GPa. The difference in the behavior of the two OH stretching modes in gypsum is attributed to the different reduction rate in the hydrogen bonding distances by the anisotropic axial compressibility.     

Intrinsic magnetostriction and phase transitions

A new possibility of first-order magnetic phase transitions in simple systems, based on the concept of intrinsic volume-magnetostriction Ω_s ≡ ? V_n/V_n, is suggested. The net volume V_n is defined as the difference between the total lattice volume and the sum of the individual atomic volumes. A magneto-volume equation of state for the net volume: Ω_s = Q₀η^s with s < 2, as well as the dependence of the effective exchange integral on the long-range order parameter η, are postulated. The value s ? 1.5 accounts well for the experimental values of the critical exponents β and δ, as well as for the deviation from linearity of the Arrott—Belov plots obtained for the ferromagnetic transition metals. The model leads to an order—order transition just below the order—disorder transition, thus providing an explanation for the two experimentally found temperatures, the forromagnetic Curie temperature and the paramagnetic Curie—Weiss temperature.     

On structural phase transitions from a hypothetical phase

Examples known so far of structural phase transitions which can be treated as an instability of a hypothetical parent phase are enumerated. Physical requirements for being reasonable to consider such hypothetical phase are pointed out. The possibility of using this concept for explanation of successive structural phase transitions in Rochelle salt and langbeinites is discussed.The author thanks Dr. V.Janovec for many valuable remarks to this paper.     

Elastic phase transitions in plutonium

The rich phase diagram of plutonium with a large number of different transitions in a narrow temperature interval has been puzzling scientists for decades. We offer a theoretical proof that most of the structural transformations in plutonium at temperatures exceeding the Debye temperature are the elastic phase transitions. The proof is given in the framework of the Landau theory of phase transitions and space group theory taking into account the anomalously small value of the elastic shear constants related to tetragonal and orthorhombic lattice deformations.     

Fracture and second-order phase transitions

Using the global fiber bundle model as a tractable scheme of progressive fracture in heterogeneous materials, we define the branching ratio in avalanches as a suitable order parameter to clarify the order of the phase transition occurring at the collapse of the system. The model is analyzed using a probabilistic approach suited to smooth fluctuations. The branching ratio shows a behavior analogous to the magnetization in known magnetic systems with second-order phase transitions. We obtain a universal critical exponent beta approximately = 0.5 independent of the probability distribution used to assign the strengths of individual fibers.     

Isostructural phase transitions in solids

A model of electrons interacting with lattice vibrations is shown to exhibit an isostructural phase transition as a function of applied force by relating the Hamiltonian to that of an Ising model in magnetic field.