Equilibrium configurations and phase transitions in dipole lattices

Two-dimensional square and hexagonal lattices of magnetic dipoles with the number of rows 1–4 have been studied. Based on the numerical analysis, equilibrium stable domain configurations, including the minimum number of lattice dipoles, have been revealed; the conditions for the creation and destruction of domains have been determined; and their associated changes in the magnetic moment of the lattice and in the energy of the dipole interaction have been found. The conditions for the occurrence of phase transitions that change the configuration of the lattices have been investigated and the conditions for unidirectional propagation of the front of the phase transition have been established. A comparative analysis of different square and hexagonal lattices has been performed in terms of the specific features of the formed domains and the observed orientation phase transitions.     

Trapped states and transitions between them in double-well pseudopotentials

We analyze a model of a double-well pseudopotential (DWPP), based in the 1D Gross-Pitaevskii equation with a spatially modulated self-attractive nonlinearity. In the limit case when the DWPP structure reduces to the local nonlinearity coefficient represented by a set of two delta-functions, analytical solutions are obtained for symmetric, antisymmetric and asymmetric states. In this case, the transition from symmetric to asymmetric states, i.e., a spontaneous-symmetry-breaking (SSB) bifurcation, is subcritical. Numerical analysis demonstrates that the symmetric states are stable up to the SSB point, while emerging asymmetric states (together with all antisymmetric solutions) are unstable in the delta-function model. In a general model, which features a finite width of the nonlinear-potential wells, the asymmetric states quickly become stable, simultaneously with the switch of the bifurcation into the supercritical type. Antisymmetric solutions may also enjoy stabilization in the finite-width DWPP structure, demonstrating a bistability involving the asymmetric states. The symmetric states require a finite norm for their existence. A full diagram for the existence and stability of the trapped states is produced for the general model.     

Disordered magnetic phases and magnetic transitions in non anisotropic frustrated systems

Numerous studies have been devoted to disordered magnetic phases which show the existence of several types of disorder in non-anisotropic systems. Semi-disordered systems which retain at least partially long-range order (reentrant properties and randomly canted structures) and fully disordered systems (spin cluster and soft transition systems, true spin glass state) are shortly reviewed. Several characteristic experiments and results are presented and commented on, such as alternative, nonlinear and static susceptibilities, thermoremanence, ageing effects, neutron diffraction and Mössbauer spectroscopy. The possible types of disordered magnetic phases are discussed as a function of a sharp (or soft) transition and of a more or less fast dynamics. In true spin glasses, some problems are still open while in the other disordered phases the unresolved questions are numerous.     

Transient dynamical behavior and phase transitions in magnetic systems

It was recently suggested that transient dynamical properties were of some use to predict equilibrium critical properties of 2D and 3D models of statistical mechanics on the lattice. We investigate such dynamical properties for three related models with competitive interactions, namely the ANNNI model, the brickwork model, and the BNNNI model. In spite of known differences in their equilibrium phase diagrams, our simulations display similar transient dynamical behaviors for all three models. The reliability of this method for probing equilibrium properties seems therefore questionable even for rather simple magnetic models without any structural disorder.On leave from Centre de Physique Théorique, Ecole Polytechnique, 91128, Palaiseau Cedex, France.     

On the “phase transitions” between quantum and classical behavior in magnetic mesoscopic systems

The results of theoretical and numerical investigation of thermally stimulated tunneling depinning in mesoscopic systems are presented using Mn₁₂Ac and CrNi₆ as examples. It is shown that the boundary between the regions of tunneling and thermally activated depinning interpreted by some authors as a phase transition of the first order is not sharply defined. The effect of absolute spin delocalization in transverse fields weaker than the anisotropy field is studied for the first time.     

Equilibrium configurations of systems of neutrons with allowance for nuclear and gravitational interactions

A study is made of equilibrium neutron configurations with allowance for nuclear interactions and two theories of gravitation—Einstein's and the scalar—tensor. The equation of state of neutron matter is obtained on the basis of the model of single-boson exchange in the Hartree-Fock approximation. Various parameters of neutron stars are obtained and the influence of the choice of the coupling constant G _NN ² and the masses of scalar mesons on these parameters investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 22–26, November, 1976.     

Pressure-induced magnetic phase transitions in Yb Kondo lattice systems

In this paper we give a brief overview of the effect of pressure on the magnetic and electronic properties of Yb Kondo lattices using the ¹⁷⁰Yb Mössbauer technique, electrical resistance and X-ray diffraction. The selected materials were either nonmagnetic (YbCu₂Si₂ and Yb₂Ni₂Al) or ferromagnetic (YbNiSn). We show that pressure induces a first order transition to a magnetic ground state in both YbCu₂Si₂ and Yb₂Ni₂Al. In the former compound, the transition is accompanied by a valence change towards Yb³⁺ state. The behavior of both YbCu₂Si₂ and Yb₂Ni₂Al can be understood as resulting from a pressure enhancement of the RKKY interaction which finally dominates the Kondo effect. We demonstrate that the ground state properties of YbNiSn are governed by a volume dependent competition between anisotropic exchange interactions and crystal field anisotropy rather than by a direct competition between Kondo and RKKY interactions.     

Multiphonon optical transitions in size-limited systems in a magnetic field

A study has been performed of optical multiphonon transitions in undoped size-limited systems in a magnetic field aligned with the spatial quantization axis. A theory is proposed which allows one to describe the half-width of the luminescence curve for isolated quantum wells and investigate the frequency and temperature dependence of the light absorption coefficient in the long-wavelength region. Zh. éksp. Teor. Fiz. 116, 2069–2078 (December 1999)     

Dynamics and phase transitions in biased ladder systems with magnetic flux

The ground states and the dynamics of a biased two-leg flux ladder in the presence of a gravitational field are discussed. In the absence of the gravitational field, the ground states and the critical condition of phase transition are obtained analytically. We identify the Meissner phase, Vortex phase, and interestingly, two new Plane Wave phases, that break both $Z_2$ and time-reversal symmetry, characterized by the imbalance particle density distribution, asymmetry double well energy band structure in Plane Wave I (PWI) phase and asymmetry single well energy band structure in Plane Wave II (PWII) phase, respectively. In the presence of a longitudinal dc gravitational field, rich chiral Bloch oscillation and Landau-Zener tunneling are predicted theoretically and confirmed numerically. The characteristics of the chiral Bloch oscillation can distinguish the novel phases intuitively. Our work gives an interesting way to discuss the quantum phase transitions in a dynamical way.     

Intraexcitonic transitions in two-dimensional systems in a high magnetic field

The internal transitions of two-dimensional (2D) excitons in a high magnetic field B exhibit features due to the coupling of the internal and center-of-mass motions. A study is made of these features, and it is shown that for magnetoexcitons with a center-of-mass momentum K ≠0 the energies of the strong transitions decrease with increasing K, and the absorption spectra show weakly resolved transitions, whose total intensity depends strongly on the exciton statistics (distribution function). Pis’ma Zh. éksp. Teor. Fiz. 66, No. 9, 588–593 (10 November 1997)     

Thermodynamics around magnetic phase transitions in alternating double-chain spin systems

The thermodynamics and quantum phase transitions of two typically alternating double-chain systems are investigated by Green＇s function theory.（i） For the completely antiferromagnetic（AFM） alternating double-chain, the low-temperature antiferromagnetism with gapped behavior is observed, which is in accordance with the experimental result. In a magnetic field, we unveil the ground state phase diagram with zero plateau, 1/2 plateau, and polarized ferromagnetic（FM） phases,as a result of the intra-cluster spin-singlet competition. Furthermore, the Gr ¨uneisen ratio is an excellent tool to identify the quantum criticality and testify various quantum phases.（ii） For the antiferromagnetically coupled FM alternating chains,the 1/2 magnetization plateau and double-peak structure of specific heat appear, which are also observed experimentally.Nevertheless, the M–h curve shows an anomalous behavior in an ultra-low field, which is ascribed to the effectively weak Haldane-like state, demonstrated by the two-site entanglement entropy explicitly.     

Thermal expansion study of magnetic phase transitions in Kondo-lattice systems

High precision measurements of the thermal expansion coefficient, (T), of the Kondo lattice systems CeAl₂ and CeB₆ reveal the coexistence of two second-order phase transitions at the Néel temperaturesT _N =3.9 K and 2.35 K, respectively. These results are discussed with regard to the complex antiferromagnetic order known from neutron diffraction experiments on both compounds. For CeIn₃, a Kondo lattice with simpler magnetic structure, only one discontinuity in (T) occurs atT _N 10 K.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Equilibrium configurations of a jet of an ideally conducting liquid in an external nonuniform magnetic field

Possible equilibrium configurations of the free surface of a jet of an ideally conducting liquid placed in a nonuniform magnetic field are considered. The magnetic field is generated by two thin wires that are parallel to the jet and bear oppositely directed currents. Equilibrium is due to a balance between capillary and magnetic forces. For the plane symmetric case, when the jet deforms only in the plane of its cross section, two one-parameter families of exact solutions to the problem are derived using the method of conformal mapping. According to these solutions, a jet with an initially circular cross section deforms up to splitting into two separate jets. A criterion for jet splitting is derived by analyzing approximate two-parameter solutions.     

Families of solutions to the generalized Ginzburg-Landau equation and structural transitions between them

Solutions to the generalized Ginzburg-Landau equations for superconductors are obtained for a Ginzburg-Landau parameter κ close to unity. The families of solutions with arbitrary number n of flux quanta in a unit cell are analyzed. It is shown that under certain conditions, a cascade of phase transitions between different structures in a magnetic field appears near T _c . Algebraic equations are derived for determining the boundaries of coexistence of different phases on the {T, H ₀} plane.     

The effect of spin fluctuations on phase transitions in magnetic systems

In this paper we study the problem of admitting an exact solution of the effect of spin fluctuations on phase transition when modeling a multiferroic superconducting system in a strong magnetic field. New results are obtained for phase portraits of the system of equations for amplitudes of spin density and temperature waves. The possibility is justified for the transition of the system to a phase in which superconductivity and antiferromagnetic ordering coexist, particularly via slowly fluctuating spin-density waves.     

Valence fluctuations between two magnetic configurations: Bethe Ansatz solution

It is shown that a model hamiltonian that describes valence fluctuations between two magnetic configurations is completely integrable.     

Experiments and low-order modelling of intermittent transitions between clockwise and anticlockwise spinning thermoacoustic modes in annular combustors

Annular combustion chambers of gas turbines and aircraft engines are subject to unstable azimuthal thermoacoustic modes leading to high amplitude acoustic waves propagating in the azimuthal direction. For certain operating conditions, the propagating direction of the wave switches randomly. The strong turbulent noise prevailing in gas turbine combustors is a source of random excitation for the thermoacoustic modes and can be the cause of these switching events. A low-order model is proposed to describe qualitatively this property of the dynamics of thermoacoustic azimuthal modes. This model is based on the acoustic wave equation with a destabilizing thermoacoustic source term to account for the flame’s response and a stochastic term to account for the turbulent combustion noise. Slow-flow averaging is applied to describe the modal dynamics on times scales that are slower than the acoustic pulsation. Under certain conditions, the model reduces formally to a Fokker-Planck equation describing a stochastic diffusion process in a potential landscape with two symmetric wells: One well corresponds to a mode propagating in the clockwise direction, the other well corresponds to a mode propagating in the anticlockwise direction. When the level of turbulent noise is sufficient, the stochastic force makes the mode jump from one well to the other at random times, reproducing the phenomenon of direction switching. Experiments were conducted on a laboratory scale annular combustor featuring 12 hydrogen-methan flames. System identification techniques were used to fit the model on the experimental data, allowing to extract the potential shape and the intensity of the stochastic excitation. The statistical predictions obtained from the Fokker–Planck equation on the mode’s behaviour and the direction switching time are in good agreement with the experiments.