“Supersolid” phase in spin-1 easy-plane antiferromagnetic

The phase states are investigated for the spin-1 two-sublattice antiferromagnetic with strong easy-plane single-ion anisotropy, exceeding the exchange interaction, in an external magnetic field. Two cases of field orientation are considered: perpendicular to the easy-plane, and in-plane orientation. The analysis of spectra of elementary excitations allows determining the lines of the phase transitions and their types. The (H, D) phase diagram is built for both orientations of the magnetic field. It is shown that the magnetization of sublattices reaches saturation only at infinitely large field, if it is oriented in-plane. This is related with the fact that the spin magnetic moment is not integral of motion in such geometry.     

“Statistical” symmetry with applications to phase transitions

Hermann proposed that mesomorphic media should be classified by assigning certain statistical symmetry groups to each possible partially ordered array. Two translational groups introduced were called superordinate and subordinate. We find that the average density in such a partially ordered medium has the superordinate symmetry ₁, while the pair correlation function has the subordinate symmetry ₂. A complete listing is made of all compatible combinations of ₁ and ₂ in two and three dimensions. This leads to more possible symmetries than Hermann obtained, e.g., also to nonstoichiometric crystals. The order parameter space for the systems is found to be the quotient space ₁/₂. In most cases it is identical to the order parameter space of low-dimensionalXY spin systems. The Landau free energy is expanded as functional of the two-particle correlation functionK; the translation group is found to be ₁×₂. A Landau mean-field theory can then be carried out by expanding the system free energy into a series of invariants of the active irreducible representations ofK and mapping the free energy onto that for anXY planar spin system. We predict novel critical behavior for transitions between mesomorphic phases and go nogo selection rules for continuous transitions. We give the structure factors for X-ray scattering so changes in all such phase transitions are observable. The statistical symmetry groups, which describe point and translational symmetries of the mesophases, are classified. Proposals are made to include quasi-long-range or topological order in the classification scheme.This work supported in part by National Science Foundation (Division of International Programs), the PSC-BHE—Faculty Research Award CUNY and Deutsche Forschungsgemeinschaft.     

A review of: “Advances in phase transitions”

From specific heat, thermal expansion, and dielectric constant measurements between 150 and 300 K we have confirmed the low temperature phase transition in barium chloride dihydrate (BaCl₂.2H₂O) at 195 ± 1 K. This transition appears to be associated with the local ordering (or displacements) of the two water molecules in the crystal. A “pseudospin” type model is proposed to explain the transition mechanism.     

Weak “phase transitions” in asymptotic properties of lattice sums

Two classes ofn-dimensional lattice sums are shown to exhibit a weak form of a phase transition in their asymptotic properties. Both classes depend on two parameters such that the leading term in an asymptotic limit of one parameter is independent of the structure of the lattice in one domain of the second parameter and dependent on the structure in an adjacent domain, with a boundary point, or transition temperature, between the two domains.     

The “screening phase transitions” in the two-dimensional Coulomb gas

The Mayer series of a Coulomb gas with fixed ultraviolet cutoff is studied in two dimensions. In particular, we show the existence of infinitely many thresholdsT _n =(e ²/8k)(1-1/2n)^–1 k=Boltzmann's constant,e=electric charge,n=1, 2,..., which are conjectured to reflect a sequence of transitions from pure multipole phase (the Kosterlitz-Thouless region) to a plasma phase (the Debye screening region) via an infinite number of intermediate phases. Mathematically we prove that the Mayer series' coefficients of order up to 2n are finite if the temperatureT is <T _n. ForT<T all the coefficients are finite and the gas can be formally interpreted as a multipole gas with multipoles with finite activity.The first author was supported in part by NSF grant No. MCS-8108814 (A03).     

Generation of phase nuclei by solitons of the new “undulator” type in martensitic phase transitions of crystalline materials

The microdynamics of large-amplitude nonlinear lattice vibrations of plutonium and uranium materials has been investigated at high reactor temperatures in the ranges of martensitic phase transitions. Topologically new large-amplitude solitons of the “undulator” type have been revealed. Transverse and longitudinal “undulator” solitons in crystals with hexagonal and cubic symmetry, depending on the direction of motion, have different kinematic and amplitude characteristics, which differ from the characteristics of the previously known solitons. The transverse “undulator” solitons, like electrons in undulators, are observed with periodic atomic displacements orthogonal to the direction of soliton propagation. The longitudinal “undulator” solitons with displacements of atoms in the direction of soliton propagation are characterized by periodic delays with two-step velocities on the trajectory in a certain analogy with two-period engineering undulator devices. It has been shown that, at high energies, such “undulator” solitons of two types generate nuclei of a new phase in early stages of structural phase transitions.     

Ferroelectricity associated with the metastable phase “III” of AgNO3

The dielectric constant, dc resistance, D-E ferroelectric hysteresis loop and dilatometric analysis of the three phases I, II, and III of AgNO₃ single crystals has been studied over the temperature range 100–200° C. A ferroelectric behaviour of the metastable phase III was detected here for the first time similar to what happened in KNO₃. The ferroelectric is attributed here to Ag⁺-ion vacancy formation in the unit cell of AgNO₃. The energy activating the process of vacancy formation was found to beE _v=2.6 eV. It was found that an ionic shift from one lattice point to another requires an amount of energy to overcome a potential barrierE _m=0.1 eV. A model is suggested to explain such behaviour. Dilatometric analysis indicated that this metastable phase transition III is accompanied by an expansion of the unit cell.     

Distorted axi-planar superfluid phase of 3He in the “nematically ordered” aerogel

Stimulated by the results of NMR experiments with superfluid ³He in “nematically ordered” aerogel [R. Sh. Askhadullin, V. V. Dmitriev, D. A. Krasnikhin, et al., JETP Lett. 95, 326 (2012)] we report on the results of phenomenological analysis of stability of different phases of superfluid ³He subjected to a strong homogeneous uniaxial anisotropy. On a basis of this analysis we suggest a form of the order parameter for the new ESP2 phase observed in the quoted experiments. In the weak coupling limit the suggested order parameter approaches that of the axi-planar phase. We discuss a possible experimental check of the proposed identification of the new phase.     

Effective method of phase conjugation of multifrequency radiation in “pseudo”-four-wave interaction

The possibility is considered of formation of a multifrequency-radiation wave front (with a wide line spectrum in the interval , +), reversed with respect to the wave front of a signal wave of frequency ₀ unequal to the frequencies in the indicated spectrum. The wave front reversal (phase conjugation) is achieved by diffraction of a multifrequency light wave in a direction counter to that of the signal, on a three-dimensional dynamic hologram produced by electromagnetic fields of frequency ₀ in a resonant medium (with thermal nonlinearity) that is fully transparent to the multifrequency radiation. A single direction of the radiation diffraction at all frequencies of the spectrum is ensured by preliminary passage of the initial multifrequency wave through a simple dispersive prism that deflects the direction in each spectral line in proportion to the frequency change. The basic equations describing a similar phase conjugation method are derived, and the efficiency of the conjugation are estimated using as an example multifrequency emission of a chemical cw HF laser.P. N. Lebedev Instiute of Physics. Translated from Preprint No. 205 of the lebedev Institute of Physics of the Academy of Sciences of the USSR, Moscow, 1989.     

Electronic “phase” transition of hydrogen molecules in the electron density functional method

Using the electronic density functional method (EDFM), it is shown that as hydrogen atoms approach to interatomic distance of 2.4a ₀ a bond charge forms in the spin polarized (antiferromagnetic) biradical state, after which there is a jump-like electronic phase transition to the spincompensated (covalent) state typical for a chemical bond. The electronic phase transition is calculated in two approximations for the electronic density: superposition of atomic densities and bond charge. The formation of a bond charge stabilizes the hydrogen molecule due to the decrease in the nonelectrostatic (quantum) energy of electrons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 24–27, October, 1982.     

The problem with “The problem of the Pegg–Barnett phase operator”

We show that the physically absurd state obtained by Vorontsov and Rembovsky [Phys. Lett. A 254 (1999) 7] results from an unphysical choice of approximate phase measurement and not from the phase formalism itself as claimed. Thus their assertion that it is impossible to perform even an approximate measurement of phase is unfounded.     

Wavefront motion in the vicinity of a phase dislocation: “optical vortex”

In the scalar approximation, an analysis is made of the light field structure in the vicinity of a line of the ring phase dislocation corresponding to the zero value of the field formed by the interference of two uniaxial Gaussian beams. The formation of an “optical vortex” or the toroidal motion of a portion of a light flow around a ring phase dislocation is shown.     

Determining theCP-violating phase γ

The weak phase γ is conventionally probed by theB _s→ρ ⁰ mode. The predicted rate is tiny. Even if aB _s→ρ ⁰ K _s rate difference could be established, it would not be clear that sin 2γ had been measured, because amplitudes with other weak phases may contribute significantly. Non-CP eigenstates, such asB _s →D _s ^± K ^?, have a two-fold advantage overB _s→ρ ⁰ K _s. Their rates are orders of magnitude above that forB _s→ρ ⁰ K _s, and they probe theCP-violating phase γ, without any contamination from other weak phases. Detailed time-dependent studies of non-CP eigenstates remove possible final-state phases and extract the weak phase γ.     

Features of sound propagation in the vicinity of “symmetry-dictated” isostructural phase transitions in ferroelastics

Using a proper ferroelastic phase transition of the tension-compression type as an example, it is shown that, if the order parameter characterizing a structural phase transition allows the existence of a third-power invariant in the Landau potential, then there must be “symmetry-dictated” isostructural phase transition lines in the vicinity of the line of that structural phase transition. These isostructural transitions may manifest themselves both directly and as supercritical anomalies in the behavior of elastic moduli and lattice parameters. These effects are discovered and investigated without invoking the perturbation theory in terms of which the second-order phase transitions are commonly described. A hypothesis is made on the basis of the results obtained that the sound velocity anomalies observed in orthoclase and sanidine crystals are due to the super-critical behavior of the lattice parameters in the vicinity of a symmetry-dictated isostructural phase transition in the prototype phase of these crystals.     

Comments on “power-law falloff in the Kosterlitz-Thouless phase of a two-dimensional lattice Coulomb gas”

A simple argument is presented by which one can show that the critical inverse temperature _c of a two-dimensional Coulomb gas (standard or hard-core) with activityz satisfies , where in the low-activity limit. Previous results yield .     

Motion of a 3D exciton in a magnetic field: Exciton-magnetoexciton “phase” transition

A rearrangement of the ground state of a Wannier-Mott exciton upon an increase in its momentum is considered. The phase diagram of the electron and the hole experiencing the Coulomb interaction on the magnetic momentum-external magnetic field plane is investigated. A jumplike exciton-magnetoexciton “phase” transition is observed upon an increase in the momentum in fields B weaker than a certain value B<B _tr1. As momentum P increases above a certain critical value P _tr(B), the ground state of the system changes from the hydrogen-like state polarized by the Lorentz force to the magnetoexciton state in which the average distance 〈 r〉 between the electron and the hole increases jumpwise in the transverse direction relative to the field. As the exciton momentum increases, its wave function is extended along the magnetic field, acquiring the shape of a strongly prolate ellipsoid. It is interesting that the momentum of the transition tends to a finite value P ₀>0 even for B→0. At the point of transition, the exciton energy-momentum relation changes jumpwise from a quadratic law to a relation virtually independent of the momentum. For B<B _tr1, the exciton-magnetoexciton transition becomes blurred.     

Can an electric field induce an antiferroelectric phase out of a ferroelectric phase?

It has been widely accepted that electric fields favor the ferroelectric phase with parallel electric dipoles over the antiferroelectric phase. With detailed measurements in polycrystalline ceramics of Pb(0.99)Nb(0.02[(Zr(0.57)Sn(0.43)(1-y)Ti(y)](0.98)O(3), we demonstrate in this Letter that electric fields can induce an antiferroelectric phase out of a ferroelectric phase, i.e., trigger an apparently unlikely ferroelectric-to-antiferroelectric phase transition. We suggest that it is caused by the volume contraction from the converse piezoelectric effect at the coercive field with a reversed polarity.     

Nonequilibrium crystal-to-amorphcs phase transitions in “high pressure” phases SiOz and Ge(A1)

Abstract

The pressure dependences of the temperatures of the crystal-to-amorphous transformations for SiO₂ and Geo.7Alo.9 are presented. The field of the existence of amorphous phases of both substances vanishes at high pressure.

Les variations avec P de la temperature de transformations cristal-to-amorphous de SiO₂ et Ga. 7 Ala. 9 sont presentes. Le champ d'existence de phase amorphe de ces substances desparait a haute pression.     

“Magnetodipole” self-organization of charge carriers in high-T c superconductors and the kinetics of phase transition

A phenomenological model describing “magnetodipole” self-organization of charge carriers (the formation of so-called stripe-structures and the energy gap in the spectrum of states) was suggested to interpret the data of nonstationary nonlinear spectroscopy of high-T _c superconductors. It was shown that, after rapidly heating a superconducting sample, the kinetics of the succeeding phase transition depended on initial temperature T. At small “overheatings” T*<T<T _m x≈(1.4?1.5)T* (T _c and T*≈T _c are the temperatures of the transition to the superconducting state and the formation of stripe-structures) and the optimal level of doping, the decay of stripe-structures (and of the gap in the spectrum of states) occurred at a low rate (in times above to 10^?9 s) in spite of the virtually instantaneous disappearance of superconductivity.