Asymmetrically shaped hysteresis loop in exchange-biased FeNi/FeMn film

The magnetization reversal of the bilayer polycrystalline FeNi(50 Å)/FeMn(50 Å) film sputtered in a magnetic field has been studied by magnetic and magneto-optical techniques. The external magnetic fields were applied along the easy or hard magnetization axis of the ferromagnetic permalloy layer. The asymmetry of hysteresis loop has been found. Appreciable asymmetry and the exchange bias were observed only in the field applied along the easy axis. The specific features of magnetization reversal were explained within the phenomenological model that involves high-order exchange anisotropy and misalignment of the easy axes of the antiferromagnetic and ferromagnetic layers. It has been shown that the film can exist in one of three equilibrium magnetic states in the field applied along the easy axis. The transitions between these states occur as first-order phase transitions. The observed hysteresis loop asymmetry is related to the existence of the metastable state.     

The mixed-spins 1/2 and 3/2 Blume—Capel model with a random crystal field

The random crystal field(RCF) effects are investigated on the phase diagrams of the mixed-spins 1/2 and 3/2 Blume-Capel(BC) model on the Bethe lattice.The bimodal random crystal field is assumed and the recursion relations are employed for the solution of the model.The system gives only the second-order phase transitions for all values of the crystal fields in the non-random bimodal distribution for given probability.The randomness does not change the order of the phase transitions for higher crystal field values,i.e.,it is always second-order,but it may introduce first-order phase transitions at lower negative crystal field values for the probability in the range about 0.20 and 0.45,which is only the second-order for the non-random case in this range.Thus our work claims that randomness may be used to induce first-order phase transitions at lower negative crystal field values at lower probabilities.     

Optically driven Mott‐Hubbard systems out of thermodynamic equilibrium

We consider the Hubbard model at half filling, driven by an external, stationary laser field. This stationary, but periodic in time, electromagnetic field couples to the charge current, i.e. it induces an extra contribution to the hopping amplitude in the Hubbard Hamiltonian (photo‐induced hopping). We generalize the dynamical mean‐field theory (DMFT) for nonequilibrium with periodic‐in‐time external fields, using a Floquet mode representation and the Keldysh formalism. We calculate the non‐equilibrium electron distribution function, the density of states and the optical DC conductivity in the presence of the external laser field for laser frequencies above and below the Mott‐Hubbard gap. The results demonstrate that the system exhibits an insulator‐metal transition as the frequency of the external field is increased and exceeds the Mott‐Hubbard gap. This corresponds to photo‐induced excitations into the upper Hubbard band.     

Ionic transitions and oscillator strengths in Debye plasma - a case study

Stability and energy of the excited states of the ion in plasmas are investigated theoretically using the Debye model. The transition energies of and transitions are seen to follow completely opposite trends of variation with the plasma screening strength. The dependence of absorption oscillator strength values on the screening strength is also discussed. Received: 22 October 1997 / Accepted: 9 January 1998     

Charge correlations in the weakly doped t-J model calculated by projection technique

We study frequency- and wave-vector dependent charge correlations in weakly doped antiferromagnets using Mori-Zwanzig projection technique. The system is described by the two-dimensional t-J model. The ground state is expressed within a cumulant formalism which has been successfully applied to study magnetic properties of the weakly doped system. Within this approach the ground state contains independent spin-bag quasiparticles (magnetic polarons). We present results for the charge-density response function and for the optical conductivity at zero temperature for different values of t / J. They agree well with numerical results calculated by exact diagonalization techniques. The density response function for intermediate and large momenta shows a broad continuum on energy scales of order of several t whereas the optical conductivity for is dominated by low energy excitations (at 1.5-2J). We show that these weak-doping properties can be well understood by transitions between excited states of spin-bag quasiparticles. Received: 10 July 1997 / Revised: 19 March 1998 / Accepted: 3 April 1998     

Chiral phase transition at finite temperature in the linear sigma model

We study the chiral phase transition at finite temperature in the linear sigma model by employing a self-consistent Hartree approximation. This approximation is introduced by imposing self-consistency conditions on the effective meson mass equations which are derived from the finite temperature one-loop effective potential. It is shown that in the limit of vanishing pion mass, namely when the chiral symmetry is exact, the phase transition becomes a weak first order accompanying a gap in the order parameter as a function of temperature. This is caused by the long range fluctuations of meson fields whose effective masses become small in the transition region. It is shown, however, that with an explicit chiral symmetry breaking term in the Lagrangian which generates the realistic finite pion mass the transition is smoothed out irrespective of the choice of coupling strength. Recieved: 19 September 1997 / Revised version: 30 October 1997     

Magnetic transitions in layered triangular antiferromagnets

Magnetic states and phase transitions of the layered triangular antiferromagnets in an applied field are studied. It is shown that in compounds like VBr₂ and VCl₂ quantum effects change the ground-state structure and cause successive phase transitions as the magnetic field increases. Coplanar structures of different spin configuration are realized far from the saturation field and a noncoplanar structure of umbrella-type configuration is realized near this field. The ground-state phase diagram is constructed, and a finite region of fields where the collinear phase is also possible is indicated.     

Edison revisited: Electro mechanical effects in wet porous materials

Edison discovered that the coefficient of friction between a metallic plate and a porous material moistened with a dilute electrolyte could be modulated by an electric field. In experiments on the same kind of contacts (clays or chalks on carbon or metals) but without continuous tangential relative motion we measure two electro-mechanical effects at frequencies of the order of 10 kHz. An alternating field induces an alternating normal force between the porous material and the conducting base. The force is lagging by versus the field. A forced normal relative motion induces through the contact a current nearly in phase with the motion. For an explanation we start from Helmholtz theory of stationary electrophoretic phenomena. We present a model in which liquid motions obey the Helmholtz laws. It explains decently the phase relations between causes and effects, and approximately the values of the effects. In optical experiments on contacts between a wet clay and the transparent conducting coating of a glass plate we measure in the frequency range 1-100 kHz a modulation of reflecting power induced by an alternating potential. The decrease of reflecting power is lagging by an angle close to behind the field. We think the modulation observed is induced by a modulation of the amount of liquid in the film present between glass and clay. In friction experiments this alternating liquid lubrication acting exclusively at the very places where friction occurs may have significant effect. Received: 22 July 1997 / Revised: 21 October 1997 / Accepted: 21 November 1997     

The role of power law nonlinearity in the discrete nonlinear Schrödinger equation on the formation of stationary localized states in the Cayley tree

We study the formation of stationary localized states using the discrete nonlinear Schr?dinger equation in a Cayley tree with connectivity K. Two cases, namely, a dimeric power law nonlinear impurity and a fully nonlinear system are considered. We introduce a transformation which reduces the Cayley tree into an one dimensional chain with a bond defect. The hopping matrix element between the impurity sites is reduced by . The transformed system is also shown to yield tight binding Green's function of the Cayley tree. The dimeric ansatz is used to find the reduced Hamiltonian of the system. Stationary localized states are found from the fixed point equations of the Hamiltonian of the reduced dynamical system. We discuss the existence of different kinds of localized states. We have also analyzed the formation of localized states in one dimensional system with a bond defect and nonlinearity which does not correspond to a Cayley tree. Stability of the states is discussed and stability diagram is presented for few cases. In all cases the total phase diagram for localized states have been presented. Received: 18 September 1997 / Revised: 31 October and 17 november 1997 / Accepted: 19 November 1997     

Bias field effects on the dielectric properties of 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 single crystals with different orientations

The domain states and phase transitions in 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ single crystals were investigated by studying their relative permittivity under various dc bias at constant heating and cooling rates. The orientation dependence of the bias field effect was revealed by examining the temperature dependence of relative permittivity as a function of crystal orientation (the 〈111〉, 〈011〉 and 〈001〉 directions) and dc bias field. The crystals basically have a macrodomain rhombohedral ferroelectric state in the ferroelectric phase under zero dc bias. External bias field could modulate the domain state and induce a stable macrodomain state in the crystals. Also, it is proposed that the dc bias applied along the 〈001〉 or 〈011〉 direction could induce a tetragonal ferroelectric phase or an orthorhombic ferroelectric phase, respectively, in an intermediate temperature range.     

Nonequilibrium states of the soft mode anharmonic phonons

A theory of the nonequilibrium state of the soft mode optical phonons in ferroelectrics exposed to the pumping electromagnetic field has been developed. The self-consistent field equation, which determines the phase transition temperature shift and the dielectric permeability change due to a deviation of the system from the equilibrium state is derived. A two-temperature model describing the energy transport in the presence of pumping is suggested. Analytic solutions of this model both for evolving in time and for stationary states are obtained. Spatial oscillations of the soft mode temperature are predicted for the observation time approximately equal to the energy relaxation time. It is shown that nonlinear effects can be a reason of forming of the bottleneck in the energy relaxation process. An exactly solvable model of a breather: the breather with the one-site non-linearity has been constructed and investigated.     

The mesoscopic chiral metal-insulator transition

Sharp localization transitions of chiral edge states in disordered quantum wires subject to a strong magnetic field are shown to be driven by crossovers from two-to one-dimensional localization of bulk states. As a result, the two-terminal conductance is found to exhibit discontinuous transitions at zero temperature between exactly integer plateau values and zero, reminiscent of first-order phase transitions. We discuss the corresponding phase diagram. The spin of the electrons is shown to result in a multitude of phases when the spin degeneracy is raised by the Zeeman energy. The width of conductance plateaus is found to depend sensitively on the spin flip rate 1/τ_s.     

Limiting Gibbs States and Phase Transitions of a Bipartite Mean-Field Hubbard Model

In the frame of operator-algebraic quantum statistical mechanics we calculate the grand canonical equilibrium states of a bipartite, microscopic mean-field model for bipolaronic superconductors (or anisotropic antiferromagnetic materials in the quasispin formulation). Depending on temperature and chemical potential, the sets of statistical equilibrium states exhibit four qualitatively different regions, describing the normal, superconducting (spin-flopped), charge ordered (antiferromagnetic), and coexistence phases. Besides phase transitions of the second kind, the model also shows phase transitions of the first kind between the superconducting and the charge ordered phases. A unique limiting Gibbs state is found in its central decomposition for all temperatures, even in the coexistence region, if the thermodynamic limit is performed at fixed particle density (magnetization).     

Structure Phase Transitions of the (D + 1)-Dimensional Double sine-Gordon Field Theory

Using the thermal coherent state approach, we investigate the finite temperature quantum double sine-Gordon (DsG) field theory. From the stability conditions of the vacuum states of effective potential, the exact soliton-like solution of the field equation and the resemblance between the DsG model andλφ⁴ model, we show that there exist two types of structure phase transitions among four structures of the (D+1)-dimensional DsG model similar to the symmetry restored phase transition of λφ⁴ model. For the first type of phase transition, the small kink-like solutions will disappear at the critical temperature while the large kinklike solutions will be changed continuously at the critical temperature. For the second type of the phase transitions, the metastable localized soliton-like solution will disappear, while the stable kink-like solution exists for all the temperatures and their exact form is also independent of the temperature being under or over the critical temperature.     

Non-equilibrium phase transition of the fully frustrated square lattice Coulomb gas model

The non-equilibrium phase transitions of the fullyfrustrated (f = 1/2) square lattice Coulomb gas (CG) modeldriven by external electrical fields are studied in the frameworkof the short-time dynamic scaling approach. The criticaltemperature T_c, the static and dynamic critical exponents2β/ν, ν, and z are obtained for several smalldriving fields. The results show that T_c decreases with theincrease of electric field, and 2β/ν and z arestrongly dependent on the external electric field. Interestingly,contrary to the equilibrium case, in the presence of smallelectric field, the calculated exponent ν is close to that inpure 2D Ising model, which provides numerical evidence thatexternal electric field may change the universality class of thef = 1/2 CG system.     

单模光腔中N个二能级原子系统的有限温度特性和相变

本文研究单模光场中N个二能级原子Dicke模型的有限温度特性和相变. 把原子赝自旋转换为双模费米算符, 用虚时路径积分方法推导出系统的配分函数, 对作用量变分求极值得到系统的热力学平衡方程, 及原子布居数期待值和平均光子数随原子-光场耦合强度变化的解析表达式. 重点研究了在量子涨落起主导作用的低温区, 由耦合强度变化产生的从正常相到超辐射相的相变, 指出该相变遵从Landau连续相变理论, 平均光子数可作为序参数, 零值表示正常相, 大于零则为超辐射相. 在零温极限下本文的结果和量子相变理论完全符合. 另外, 本文也讨论了系统的热力学性质, 比较有限温度相变和量子相变的异同. 发现, 在强耦合区低温稳定态的光子数和平均能量都和绝对零度的值趋于一致, 而超辐射相的熵则随耦合强度的增强迅速衰减为零.     

A Note on Fick’s Law with Phase Transitions

Journal of Statistical Physics - We characterize the non equilibrium stationary states in two classes of systems where phase transitions are present. We prove that the interface in the limit is a...     

Editorial

Many excellent articles and textbooks on superconductivity have been published. However, concise and easily-handled data books written for scientists and engineers who work in the field of superconducting materials are very few. This article aims to collect and outline basic properties of superconducting materials and to provide such people with a database. The properties listed include crystal and electronic structures, equilibrium phase diagram, superconducting critical temperature, critical magnetic fields, critical current density, pinning force, specific heat coefficient, Debye temperature, penetration depth, coherence length, GL parameter, energy gap, electronic density of states at the Fermi level, electron-phonon coupling constant, Coulomb pseudopotential, and others. Emphasis is on the materials used in superconductive applications. Structural phase transitions such as martensitic transformations and transitions to the charge-density-wave state in superconducting materials are also reviewed. The properties of superconducting oxides will be summarized in the next article of this series.     

Anomalies of attenuation and phase velocity of surface acoustic waves in YBa2Cu3O7‒δ thin films in the vicinity of Tc

We present measurements of the attenuation and phase velocity of surface acoustic waves in thin YBa₂Cu₃O films as a function of temperature, in magnetic fields up to 3.6 T applied parallel to the c-axis of the films. We have observed anomalies in both, the attenuation and the phase velocity in the vicinity of the superconducting critical temperature which do not depend on the magnetic field. Possible origins of these anomalies, observed, to our knowledge, for the first time in YBa₂Cu₃O thin films, are discussed and compared to bulk acoustic wave experiments. We present a kind of feedback technique for surface acoustic waves which improves the sensitivity of this type of measurement. The actual sensitivity limits are mentioned. Received: 7 August 1997 / Revised: 7 November 1997 / Accepted: 17 November 1997     

Multicritical dynamical phase diagrams of the kinetic Blume–Emery–Griffiths model with repulsive biquadratic coupling in an oscillating field

We study, within a mean-field approach, the stationary states of the kinetic Blume–Emery–Griffiths model with repulsive biquadratic coupling under the presence of a time-varying (sinusoidal) magnetic field. We employ the Glauber-type stochastic dynamics to construct set of dynamic equations of motion. The behavior of the time dependence of the order parameters and the behavior of the average order parameters in a period, which is also called the dynamic order parameters, as functions of the reduced temperature are investigated. The dynamic phase transition points are calculated and phase diagrams are presented in the reduced magnetic field amplitude and reduced temperature plane. The dynamical transition from one regime to the other can be of first- or second order depending on the region in the phase diagram. According to the values of the crystal field interaction or single-ion anisotropy constant and biquadratic exchange constant, we find 20 fundamental types of phase diagrams which exhibit many dynamic critical points, such as tricritical points, zero-temperature critical points, double critical end points, critical end point, triple point and multicritical point. Moreover, besides a disordered and ordered phases, seven coexistence phase regions exist in the system.