Nonequilibrium phase transitions in a simple three-state lattice gas

We investigate the dynamics of a three-state stochastic lattice gas consisting of holes and two oppositely charged species of particles, under the influence of an electric field at zero total charge. Interacting only through an excluded-volume constraint, particles exchange with holes and, on a slower time scale, with each other. Using a combination of Monte Carlo simulations and meanfield equations of motion, we study a set of suitably defined order parameters, their histograms and fluctuations, as well as the current through the system. With increasing particle density and drive, the system first orders into a charge-segregated state, and then disorders again near complete filling. The transition is first order at low densities and turns second order at higher ones. The finite-size and aspect-ratio dependence of characteristic quantities is discussed at the mean-field level.     

Superfluidity and phase transitions in a resonant Bose gas

The atomic Bose gas is studied across a Feshbach resonance, mapping out its phase diagram, and computing its thermodynamics and excitation spectra. It is shown that such a degenerate gas admits two distinct atomic and molecular superfluid phases, with the latter distinguished by the absence of atomic off-diagonal long-range order, gapped atomic excitations, and deconfined atomic π-vortices. The properties of the molecular superfluid are explored, and it is shown that across a Feshbach resonance it undergoes a quantum Ising transition to the atomic superfluid, where both atoms and molecules are condensed. In addition to its distinct thermodynamic signatures and deconfined half-vortices, in a trap a molecular superfluid should be identifiable by the absence of an atomic condensate peak and the presence of a molecular one.     

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.     

2D NMR studies of structural phase transitions in ABF6.6D2O systems

²D NMR studies in single crystals of CoSiF₆.6D₂O, CoTiF₆.6D₂O and ZnTiF₆.6D₂O reveal that all of them transform from rhombohedral to monoclinic symmetry through an intermediate phase as the temperature is lowered from 300 to 100K. The results throw light on the hydrogen bond network in different phases.     

Rotational phase transitions and melting in a 2D system of hard cyclic pentamers

Mechanical simulations reveal four qualitatively different structures in a 2D system of hard cyclic pentamers. Dynamical increase of molecular symmetry from five-fold to six-fold is observed at high densities. At lower densities, rotator and fluid phases occur.     

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.     

On phase transitions of semimetals in a strong magnetic field

A simple model of semimetals in a strong magnetic field is studied in the Hartree-Fock approximation by taking into account both the excitonic and liquid-gas type transitions simultaneously.     

Quantum and thermal transitions out of the supersolid phase of a 2D quantum antiferromagnet

We investigate the thermodynamic properties of a field-induced supersolid phase in a 2D quantum antiferromagnet model. Using quantum Monte Carlo simulations, a very rich phase diagram is mapped out in the temperature-magnetic-field plane, with an extended supersolid region where a diagonal (solid) order coexists with a finite XY spin stiffness (superfluid). The various quantum and thermal transitions out of the supersolid state are characterized. Experimental consequences in the context of field-induced magnetization plateau materials are briefly discussed.     

On phase transitions in cold nuclear matter

Real conditions for the formation of cold subhadronic matter are considered with allowance for nontrivial properties of the QCD vacuum. It is shown that a steady state of this matter is attainable, if at all, only in the case where dynamical (massive) quarks exist as rather stable quasiparticles. This state may consist of both a degenerate nearly perfect gas of these particles and a degenerate gas of current quarks in the interior of some (compact) neutron stars. In the latter case, both phases should coexist, and the first phase should occupy a certain space between the second phase and (normal) hadron matter occurring at the periphery of the star.     

Magnetic field induced transitions between quantized hall and insulator states in a dilute 2D electron gas

We report on the observation of a new phenomenon: a sequence of magnetic field induced transitions between well defined quantum Hall effect states, with a Hall resistance quantized as integer fractions of h/e² and a vanishingly small longitudinal resistance, and insulator states with longitudinal resistance exceeding 2×10⁹ Ω. This phenomenon is observed in extremely high mobility Si MOSFETs, in a range of electron concentrations corresponding to a dilute 2D electron gas in or near an activated electronic transport regime. We attribute this effect to a modulation of the metal-insulator transition by the quantum Hall effect or to the formation of a pinned Wigner solid.     

On the structural phase transitions in the incommensurate Cs2CdBr4

Abstract

In the series of incommensurate A₂BX₄ halides with the β-K₂SO₄ type structure, Cs₂CdBr₄ exhibits an unusual behaviour since the “lock-in” phase transition occurs at the centre of the Brillouin zone. The observed phase sequence is the following: Pnma (Z = 4)?INC(k₀ ≈ 1/6a*)?P2_1/n(Z = 4)?P1 (Z = 4). These phase transitions have been studied by means of Raman scattering and ultrasonic measurements. It is shown that the Pnma?INC?P2_1/n sequence is governed by order-disorder processes due to CdB2-₄ tetrahedra reorientations coupled with translations of the Cs⁺ cations, and that the low-temperature P2_1/n?P1 transition is of a displacive nature, governed by a soft optical mode. The “pseudo-proper” ferroelastic character of these transformations is clearly established. A model potential developed in the framework of Landau theory is proposed; this model is able to reproduce the general trends observed in the temperature dependence of the soft-modes and of the elastic constants in the different phases.     

Laser-induced gratings in the gas phase excited by Raman-active transitions

We report on a new time-resolved coherent Raman technique that is based on the generation of thermal gratings following a population change among molecular levels that is induced by stimulated Raman pumping. This is achieved by use of spatially and temporally overlapping intensity interference patterns generated independently by two lasers. When this technique is used in carbon dioxide, making use of transitions that belong to the Q branches of the v(1)/2v Fermi dyad, it is possible to investigate molecular energy transfer processes. A further unique feature of this method is spectral resolution that is better than that achieved by coherent anti-Stokes Raman scattering.     

On the order of itinerant antiferromagnetic phase transitions and superconducting phase transitions in an exchange field

The coefficients of the second and fourth order terms in a Landau expansion of the free energy are evaluated for an itinerant antiferromagnetic transition. The choice of spatial dependence is found to play an important role. A first order transition is obtained for a range of values of the band structure parameters. However if values appropriate to Cr are used it is found that the contribution from the shift in the Fermi level is sufficiently large to give second order transitions even when magnetostrictive effects are included. In the mathematically similar problem of a superconductor in an exchange field it is found that the transition from normal to superconducting states is first order near the triple point with an upper and possibly a lower critical point where the transition changes to second order.     

Phase transitions in a vortex gas

It has been shown recently that the motion of solitons at couplings around a critical coupling can be reduced to the dynamics of particles (the zeros of the Higgs field) on a curved manifold with potential. The curvature gives a velocity-dependent force, and the magnitude of the potential is proportional to the distance from a critical coupling. In this paper we apply this approximation to determining the equation of state of a gas of vortices in the abelian Higgs model. We derive a virial expansion using certain known integrals of the metric, and the second virial coefficient is calculated, determining the behaviour of the gas at low densities. A formula for determining higher-order coefficients is given. At low densities and temperatures T ⪢ λ the equation of state is of the Van der Waals form (P + bN²/A²) (A - aN) = NT with a = 4π and b = -4.89πλ where λ is a measure of the distance from critical coupling. It is found that there is no phase transition in a low-density type-I1 gas, but there is a transition in the type-I case between a condensed and gaseous state. We conclude with a discussion of the relation of our results to vortex behaviour in superconductors.     

Magnetic phase transitions and phase diagrams of DyMn2Ge2

The experimental studies of magnetic phase transitions in the layered tetragonal intermetallic compound DyMn₂Ge₂ are continued. The existence of spontaneous phase transitions is confirmed by the results of measurements of the temperature dependences of lattice parameters and the initial magnetic susceptibility. The measurements in strong (up to 50 T) and ultrastrong (up to 150 T) fields revealed two new field-induced magnetic transitions. The inclusion of the exchange interaction between next-to-nearest layers of manganese and the crystal field effects for the rare-earth subsystem along with the antiferromagnetic exchange interaction of nearest Mn layers has made it possible to describe the magnetic properties of DyMn₂Ge₂ in a wide range of magnetic fields. The parameters of these interactions are determined from a comparison of the experimental and theoretical magnetization curves and H-T phase diagrams.     

Vortex phase transitions in 21/2 dimensions

A phase diagram is mapped out for a 21/2-dimensional vortex lattice model in which vortex filaments lie in a plane, while both the velocity field and the Green function are three-dimensional. Both positive and negative temperatures are considered. Various qualitative properties of turbulent states and of the super-fluid transition are well verified within the limitations of the model; the percolation properties of vortex transitions are exhibited; the differences between superfluid and classical vortex motion are highlighted, as is the importance of topological constraints in vortex dynamics; an earlier model of intermittency is verified.     

Monte Carlo study of the phase transitions in 2D ferro- and antiferromagnetic Potts models on a triangular lattice

The phase transitions in 2D ferro- and antiferromagnetic Potts models with number of spin states q = 3 on a triangular lattice are investigated by the cluster and classical Monte Carlo methods. Systems with linear sizes L = 20–120 are considered. Fourth-order Binder cumulants and histogram data analysis are used to show that second- and first-order phase transitions are observed in the ferromagnetic and antiferromagnetic Potts models, respectively. The static critical indices are calculated for specific heat α, susceptibility γ, magnetization β, and correlation length ν on the basis of finite-size scaling theory for a ferromagnetic Potts model.