New Type Of Phase Transition Research Articles

Phase diagram and spectrum of gauge-fixed Abelian lattice gauge theory

We consider a lattice discretization of a covariantly gauge-fixed abelian gauge theory. The gauge fixing is part of the action defining the theory, and we study the phase diagram in detail. As there is no BRST symmetry on the lattice, counterterms are needed, and we construct those explicitly. We show that the proper adjustment of these counterterms drives the theory to a new type of phase transition, at which we recover a continuum theory of (free) photons. We present both numerical and (one-loop) perturbative results, and show that they are in good agreement near this phase transition. Since perturbation theory plays an important role, it is important to choose a discretization of the gauge-fixing action such that lattice perturbation theory is valid. Indeed, we find numerical evidence that lattice actions not satisfying this requirement do not lead to the desired continuum limit. While we do not consider fermions here, we argue that our results, in combination with previous work, provide very strong evidence that this new phase transition can be used to define abelian lattice chiral gauge theories.

The μSR-technique as a new tool to study magnetic properties of nanocrystalline ferromagnetic metals

Possible applications of slow and low-energy muons to study important questions in ferromagnetism are considered, namely, appearance of a magnetic order in nanostructures. It is shown, that the μSR-technique gives possibilities to determine both magnetic texture and the influence of domain walls on some magnetic characteristics in nanostructures produced by different techniques. The μSR-examination of nanostructures enables the solution of important questions like vanishing ferromagnetic order when the sizes of grains are too small and, maybe, a study of a new type of phase transition.

Polaron and charge transfer vibronic exciton phenomena in ferroelectrics

New phenomena induced by Jahn-Teller polarons (JTP) and charge transfer vibronic excitons (CTVE) in ferroelectric oxides are considered. The important role of CTVE was shown in the analysis of the drastic temperature shift of the fundamental absorption edge in ferroelectric oxides. This effect was explained on the basis of “band-intermediate radius CTVE” transitions. The critical increasing of the absolute diffraction efficiency of transient gratings near the ferroelectric phase transition (PT) was predicted and detected in KTN. It was shown that the mechanism of this phenomenon is connected with the excitation of CTVE. A new type of PT in solid solution with double substitution, like (SrTiO3)x(KTaO3)i-x is proposed.

Model of isostructural dynamical phase transition in anharmonic crystal with possible relevance to SrTiO3

A new type of phase transition is discussed which corresponds to a pairing of phonons of different lattice modes due to their anharmonic attraction in a crystal. It is shown that the main features of the isostructural phase transition observed in SrTiO3 at T≃37 K can be explained qualitatively by the phonon pairing phenomenon.

Thermodynamics of adatoms diffusing on a surface with two different sites: a new type of phase transition

The chemical surface diffusion coefficient D has been determined for a lattice-gas model where diffusion proceeds through two non-equivalent lattice sites with different activation energies, E1 and E2. By calculating the thermodynamic factor as well as jump diffusion rate we obtain the coverage dependence of D as a function of the energy difference E1 − E2. No lateral interactions are considered. Depending on the sign of E1 − E2, the diffusion coefficient shows either a maximum or a step-like increase at a particular coverage, both of which develop into discontinuities in the limits of either E1 − E2 → ± ∞. In the case of the step-like behaviour of D, the variation of D can be classified as being due to a kind of diffuse phase transition between a low and a high diffusivity state, which in the limits is shown to be of first order.

Appropriate and inappropriate statistical mechanical treatments for phase transitions at electrified interfaces

The prerequisites for a statistical mechanical treatment of phase transitions of adsorbed layers on electrode surfaces to give results compatible with both classical thermodynamics and experiment are studied. It is shown that the use of the charge density as the independent electrical variable, either in the canonical ensemble or the grand ensemble partition function of simple models, leads to artifacts which are at variance with thermodynamics. In contrast, the generalised ensemble Δ, presented in Nikitas et al. (J. Electroanal. Chem., 317 (1991) 43), which uses the potential drop across the layer which undergoes the transition as independent variable, gives results compatible with both experiment and thermodynamics under all circumstances. The results obtained are compared with the predictions of Guidelli and Aloisi's three-dimensional lattice model of TIP4P water molecules. It is shown that in this case the choice of the charge density as independent variable leads to an incomplete description of the two-dimensional phase transition properties, since the vertical steps obtained in the adsorption isotherms at constant charge density may either indicate a new type of phase transition which has not been observed experimentally yet or they are artifacts which do not represent the properties of the phase transition.

Destruction of superconductivity by magnetic ordering in Ho 1.2Mo 6S 8

An entirely new type of phase transition has been found in the superconducting compound Ho 1.2Mo 6S 8. The compound becomes superconducting at 1.2K and returns to the normal state through the newly found, very sharp phase transition at 0.64K. The latter transition is attributed to the magnetic ordering of the Ho 3+ ions in the compound.

Monte Carlo simulation of a coupled XY model.

Employing the Monte Carlo simulation technique, we have investigated several phase transitions exhibited by a coupled XY model in two dimensions. The model is based on the Hamiltonian proposed by Bruinsma and Aeppli [Phys. Rev. Lett. 48, 1625 (1982)]. The simulation results demonstrate the existence of a new type of phase transition in which two different orderings are simultaneously established through a continuous transition. The unique nature of this phase transition provides a plausible explanation for our recent high-resolution heat-capacity data near the smectic- A-hexatic-B transition of two-layer free-standing films

Gravitational phase transition: An origin of the large-scale structure in the Universe?

It is shown that a new type of phase transition, namely, a pure gravitational one, could exist for a certain range of parameters of the tree-level gravitational Lagrangian. The symmetry-breaking mechanism responsible for the transition is due to vacuum-polarization effects of matter fields. It is argued that if the Universe had undergone this transition at late times this could be the source of structures in the Universe.

New types of phase transitions in physisorbed films

Abstract We present an overview of the most outstanding results during the past few years concerning phase transition studies in thin physisorbed films on homogenous surfaces. Reawakening of interest in this field is mainly a consequence of the availability of a new homogeneous substrate type, namely magnesium oxide (100). The symmetry difference between the square lattice of MgO (100) surface potential wells and the hexagonal close packed structure which is the natural crystalline arrangement of atoms (or spherical molecules) in two dimensions (2d) induces for the first time observation of “frustrated” (2d) solids. New structures and phase transitions have been observed, as for instance the methane square (2d) solid (Coulomb, Madih, Croset and Lauter, 1985) and the “one dimensional” melting (Degenhardt, Madih, Lauter and Coulomb, to be published). This unusual (2d) melting gives rise to an original “crystal liquid” like (2d) phase which is characterized by two kinds of order, a short-range one ((2d) liquid-like phase) in a particular direction and a long-range one ((2d) crystal-like phase) in the perpendicular direction. Another interesting property of the MgO (100) surface is its strong periodic electrical field E. As a consequence, the interactive potential MgO-gas and its variation along the surface (energetic corrugation) depend largely on the electrical characteristics of the adsorbate. MgO, for instance, is a weaker substrate than the so-called graphite for the rare gases but it is a much more attractive one for CO2 (a molecule with a strong quadrupole moment). Another topic concerns the surface-induced phase transitions which have been recently observed in bulk material in the vicinity of their melting point. Thin physisorbed films which are nice examples of “growth at thermodynamic equilibrium” are particularly well suited for the analysis of such surface phase transitions, like premelting (Krim, Coulomb and Bouzidi, 1987; Bienfait, 1987) and roughening (Larher and Angerand, 1988). Finally coadsorption of two gases constitutes the third type of new phase transition phenomena in physisorption studies (Bouchdoug, Menaucourt and Thorny, 1986). The recent better understanding of (2d) mixed film properties (Hommeril and Mutaftschiev, 1989) will certainly motivate new experimental investigations.

VORTICES IN THE LATTICE MODEL OF PLANAR NEMATIC

The vortices in the planar nematic are investigated using a field-theoretical description in terms of RP2 σ-model. In the limit of strong-coupling expansion the vortices interaction considered in the framework of mean-field theory describes a new type of phase transition.

Influence of internal degrees of freedom on the structure of one-dimensional systems

The structure of one-dimensional systems with a half-filled band is discussed. In addition to lattice coordinates several intramonomer coordinates are considered. It is found that the system distorts and dimerizes (Peierls distortion). The distortions can be caused by lattice coordinates, by intramonomer coordinates or by both types of coordinates simultaneously. Thereby the various intramonomer degrees of freedom act cooperatively. The interplay between the intramonomer and lattice coordinates may induce new types of phase transitions with critical temperatures below the usual Peierls transition. Depending on the values of the effective matrix elements up to two of these new transitions may occur for a given system. The impact of these additional phase transitions on the specific heat, energy gap and lattice structure is discussed.

Phase transitions associated with dynamical properties of chaotic systems.

A new type of phase transition associated with a singularity in the spectrum of generalized entropies, or in a corresponding free energy, is shown to exist in dynamical systems. Candidates for exhibiting such transitions are intermittent chaotic systems where certain generalized entropies vanish. This new transition may coexist with one associated with the static properties of the system.

Equilibrium nucleation - a new type of phase transition

In the framework of an equilibrium theory the transition between a low-temperature pure phase and a mixture of that pure phase and a macroscopic fraction of a high-temperature phase is investigated. Peculiarities of the thermodynamic characteristics near the nucleation point are discussed.

On the classification of the homogeneous 16-vertex models on a square lattice: II. Connection with a new type of phase transition

In the previous paper we have shown that the classification problem of the homogeneous real 16-vertex models on a square lattice as proposed by Gaaff and Hijmans is closely connected with the similar problem corresponding to the boson Bogoliubov transformation in quantum mechanics. This insight leads us in the present paper to use arguments and examples from papers dealing with the latter problem in a further study of the classification problem of the real 16-vertex models. We also use a somewhat modified concept of “standard (or normal) 16-vertex model” which was introduced by Hijmans and Schram in a similar study concerning the complex 16-vertex models. We show that, rather than the single family found by Hijmans and Schram for the complex variant of the problem, the set of equivalence classes of real 16-vertex models can be divided up into seven families. This strongly suggests the existence of a new type of phase transitions which manifest themselves when the physical system in the 16-parameter space “moves” from one family to another.

Symmetry breaking and phase transitions in general statistics

A Higgs-type symmetry-breaking mechanism with general statistics is described in scalar phi/sup 4/ theories. It is shown that the minimum of the potential depends upon statistics and the parameter space is subdivided into separate regions, each with a particular statistics as a phase variable. In the model we describe, a phase diagram is constructed in parameter space, suggesting a new type of phase transition between domains of different statistics.

Phase transition and chaos in electrical avalenche breakdown caused by weak photoexcitation at 4.2 K in n-GaAs

Periodic oscillations and turbulence of the electrical avalanche breakdown have been observed in high-purity n-GaAs at 4.2 K, under the pulse voltage ( ∼ 4 V/cm) and with the weak photoexcitation around band-gap energy. The oscillatory behaviors follow the chaotic regime in the fluid flow system, and give a first example of the onset of chaos which originates in a new type of phase transition in semiconductors.

Chaotic Motions in the Electrical Avalanche Breakdown Caused by Weak Photoexcitation in n-GaAs

Periodic oscillations and turbulence in the electrical avalanche breakdown caused by the weak photoexcitation at 4.2 K in n-GaAs have been presented as a first example of the onset of chaos which originates in a new type of phase transition in semiconductors.

Periodic Spinodal Decomposition in Solid Binary Mixtures

Phase separation is studied when the temperature is made to oscillate around a critical value near a second-order phase transition. In such a case the structure factor is calculated for a one-component system with conserved order parameter using a computational method of Langer, Bar-on and Miller. The structure factor tends to a periodic function when the average of the oscillating temperature is higher than a new critical value which is lower than the equilibrium critical temperature. However, its maximum grows without limit when the average temperature is lowered below the critical value. This is a new type of phase transition where the fluctuations are much enhanced above the thermal level. Besides solid binary mixtures the results of this paper are applicable to uniaxial ferromagnets under strong ultra-sounds in which the spin component along the easy axis is conserved.

Observation and study of new types of phase transitions in Co(ClO4)2.6H2O and Ni(ClO4)2.6H2O by EPR

A detailed EPR study of Mn2+ is carried out from melting temperature to 77K. In Co(ClO4)2.6H2O, two new structural phase transitions are detected at T1 approximately 354K and T2 approximately 243K in addition to the previously known transition at T3 approximately 155K. In Ni(ClO4)2.6H2O the recently reported new phase transitions at T1 approximately 361K and T2 approximately 311K are confirmed and a new phase transition at T3 approximately 245K is observed. The phase transition occurring at T3 in both the crystals seems to be purely a structural one, while all the other phase transitions in both crystals involve changes in both structure and configurational disorder of the perchlorate ions. The space group symmetry C2v7 (Pmn21) of the crystal remains unchanged from melting temperature to T3 in Co(ClO4)2.6H2O and from melting temperature to 100K in Ni(ClO4)2.6H2O. The transitions at T1 and T2 in both crystals involve changes in the water-perchlorate sublattice symmetry. The two crystals are isostructural in the phases between the melting temperature and T1, and between T1 and T2.