Ground-state properties of the Falicov-Kimball model in one and two dimensions

A new numerical method is used to study the ground-state properties of the spinless Falicov-Kimball model in one and two dimensions. The resultant solutions are used to examine the phase diagram of the model as well as possibilities for valence and metal-insulator transitions. In one dimension a comprehensive phase diagram of the model is presented. On the base of this phase diagram, the complete picture of valence and metal-insulator transitions is discussed. In two dimensions the structure of ground-state configurations is described for intermediate interactions between f and d electrons. In this region the phase separation and metal-insulator transitions are found at low f-electron concentrations. It is shown that valence transitions exhibit a staircase structure. Received 20 October 2000     

Uniform uniaxial anisotropic systems with coupled vector order parameters

Phase transitions in magnets, described by two coupled, m-component vector order parameters, having uniform uniaxial anisotropies are studied. Using a phenomenological model, it is shown that when both order parameters are anisotropic, phase transitions are always second order, in either the uniaxial or the (m???1)-isotropic phase. This is contrary to the isotropic case of two coupled order parameters, for which phase transitions are fluctuation-induced first order. The transitions are still continuous into the m-isotropic phase even when the only anisotropic order parameter is the one with the lowest mean-field critical temperature. New discontinuous transitions still occur in either the uniaxial or the (m???1)-isotropic phase, when the only anisotropic order parameter has the highest mean-field critical temperature.     

Ising model with a new class of four-spin interactions

The Ising model on a Union-Jack lattice, described by a Hamiltonian with second-neighbor pair-pair, four-spin, infinite-range interactions is considered. The model is solved exactly and the results are compared with MFA predictions. Within the exact treatment two new classes of phase transitions are obtained. The first one includes transitions from a disordered to a metastable, ordered and then to a stable and ordered phase with decreasing temperature. The metastable phase does not appear if the temperature is increased. The second one contains transitions between ordered and partialy ordered, partialy frustrated phases.     

Simulation of α-Zr structural stability under pressure using the molecular dynamics method

The structural stability of α-Zr was studied using the molecular dynamics method in wide temperature and pressure ranges. The interatomic interaction was described by a pair potential calculated within the Animalu pseudopotential model. The potential parameters were selected using α-Zr phonon spectra. The features in the dynamics of the α-β and α-ω phase transitions at various temperatures and pressures were considered. The calculated hysteresis of forward and backward phase transitions and its pressure and temperature dependences are discussed. The data obtained were used to plot phase equilibrium lines in the P-T phase diagram.     

Kitaev模型与拓扑量子相变

文章通过在一种准一维路径上引入自旋算符的约当-维格纳（Jordan—Wigner）变换，证明了Kitaev自旋模型完全等价于一个不含任何非物理自由度的自由Majorana费米子模型。通过对偶变换，进一步证明了这个系统中存在的量子相变可用非定域的拓扑序参量来描述；并且，这些非定域的拓扑序参量在对偶空间变成为定域的朗道类型的序参量。文章作者的工作揭示了传统的量子相变和拓扑量子相变的内在关系，扩展了朗道二级相变理论的适用范围。     

3-d coulomb coupling and charge ordering in non-conducting chain systems

The charge ordering phenomenon due to 3-d Coulomb forces in linear chains, built up of boxes having two sites but one electron, described by the Zawadowski-Cohen model, is considered. The case is discussed, in which 3-d forces prefer ferroelectric ground state. It is found that interchain coupling leads to a variety of phase transitions, and tricritical behaviour. For a range of parameters two subsequent phase transitions exist. The reason for this behaviour is discussed.     

Field-induced phase transitions of repulsive spin-1 bosons in optical lattices

We study the phase diagram of repulsively interacting spin-1 bosons in optical lattices at unit filling, showing that an externally induced quadratic Zeeman effect may lead to a rich physics characterized by various phases and phase transitions. We find that the main properties of the system may be described by an effective field model, which provides the precise location of the phase boundaries for any dimension, in excellent agreement with our numerical calculations for one-dimensional (1D) systems. Thus, our work provides a quantitative guide for the experimental analysis of various types of field-induced quantum phase transitions in spin-1 lattice bosons. These transitions, which are precluded in spin-1/2 systems, may be realized by using an externally modified quadratic Zeeman coupling, similar to recent experiments with spinor condensates in the continuum.     

Virtual levels,mixed valence phase and electronic phase transitions in rare earth compounds

We discuss the influence of the Coulomb interaction between localized and conduction electrons on the properties of magnetic impurities in metals and on electronic phase transitions such as the γ—α—α' transitions in Ce and the insulator—metal transition in SmS. Due to excitonic pairing between ?-holes and s-electrons, similar to that in excitonic insulators, the virtual ?-levels in metals may acquire an extra width, which, in contrast to the width in the Anderson model, depends upon the position of the ?-level, the width being the largest when the ?-level crosses the Fermi-level. This effect stabilizes the intermediate valence phase. As a result, in the Falicov model we get either a gradual phase transition (like that found in SmTe), or a first order one, followed by the intermediate valence phase (SmS), or, which is most interesting, two successive jump-like transitions with a mixed valence in between, similar to the γ—α—α' transitions in Ce. The mixed valence phase is described here as a kind of an “excitonic insulator”. The theory also predicts the correct slopes of the phase equilibrium lines for both Ce and SmS.     

光学耗散系统的准热力学模型及其在光学双稳性相变问题上的应用

本文以光学耗散系统为范例,论证在满足细致平衡原理的条件下,可以与传统热力学相平行地发展一种“准热力学”模型。然后运用此模型系统而普遍地讨论了光学双稳系统的临界现象与相变。又借助Ginzburg-Landau模型处理了光学双稳性(第二类)临界点附近的涨落与关联。将平衡热力学的Landau相变理论推广到具有双稳性并远离热平衡的耗散系统。     

Surface reconstruction: A physical realization of two-dimensional Ising and XY-models

The reconstruction transitions on certain clean metal surfaces are shown to be isomorphous with two-dimensional models of phase transitions. Specifically, the Pt, Ir, and Au(110) transitions correspond to the 2d-Ising model, the W(100) transition to the 2d?XY model with cubic anisotropy. The Mo (100) reconstruction is described in terms of the Kosterlitz-Thouless picture of the 2d-XY model. Experiments (LEED or He diffraction) are suggested to check theories of 2d systems.     

Data-base for structual phase transitions at high pressures

Abstract

The data on 700 structual phase transitions at high pressure are collected. The 420 fully described phase transitions are reviewed in the form of statistical table showing the number of given change of phase symmetry at the transition. Some other statistical data are presented.     

Phase transitions between orthogonal and tilted hexatic phases

Tilt-driven phase transitions between hexatic smectic phases: SmF-HexB and reversed HexB-SmF have been studied in compounds belonging to two enaminoketone derivative homologue series. The tilt angle order parameter has been measured and its temperature dependence near the phase transition point has been described by applying mean-field model. For both phase sequences the tricritical points have been observed on phase transition lines in binary mixtures of respective materials having first and second order phase transitions between hexatic phases. Received 21 June 1999     

Potential-induced strain relaxation in au mono- and bilayer films on Pt(111) electrode surfaces

In situ scanning tunneling microscopy observations of 1-2 monolayer thick Au films on Pt(111) electrodes are presented, which show a complex, potential-dependent structural phase behavior of the Au film. Starting from a pseudomorphic structure at <0.35 V(Ag/AgCl), a sequence of transitions into dislocation network structures occurs with increasing potential: (1x1)--> "striped phase" --> "hex phase" in the Au bilayer, (1x1)--> striped phase in the Au monolayer. This is explained by a reduction of the in-plane stress in the Au surface layer(s) due to anion adsorption and a strain energy minimization as described by the Frenkel-Kontorova model.     

Domino mechanisms in photoinduced phase transitions

Photoinduced structural phase transitions via excited electronic states are discussed theoretically using a one-dimensional model composed of localized electrons and lattices under the adiabatic or diabatic approximation. We show that the global structural change by photoexcitation only at a site is possible, and we clarify conditions for the occurrence of such phenomena. Spatiotemporal dynamics of nonequilibrium first-order phase transitions is also investigated in detail in terms of photoinduced nucleations and domino processes of the domain boundaries (domain walls), which are in striking contrast to the mean-field dynamics. In the adiabatic regime, after the spontaneous emission of a photon, an initial local structural change (i) remains locally, (ii) induces cooperatively a global structural change, or (iii) disappears and returns to the initial phase. Dynamical features of the case (ii) are characterized by the deterministic (semichaotic) domino process; domain walls between the two phases move determinis-tically at a constant velocity (with changing speed) without further spontaneous emissions in the case of strong (weak) dissipation. In the diabatic regime, similar three types of structural change exist. The domain-wall dynamics is described as the stochastic domino process, which is accompanied by the successive radiative transitions. A new theoretical treatment is also proposed to study crossover between the adiabatic and diabatic regimes.     

182Os 核yrast带相继SU(3)–U(5)–SU(3)结构相变势能曲面的一种可能理解

基于联合实施微观相互作用玻色子模型的最大F旋方案 (sdIBM-F_max)与γ射线能量-自旋曲线 (γ-ray energy over spin curves, E-GOS)方案, 成功描述了¹⁸²Os核yrast带相继的SU(3)–U(5)–SU(3)结构相变, 由于缺少直观解释而显得抽象. 本文借助微观sdIBM-F_max的微观参数与Bohr哈密顿量的势能曲面方程之间存在的泛函关系, 几何地给出了对这种相继相变途径的另外一种可能理解; 并阐述了在完全变形核的高角动量态中, 由于量子效应在高激发态与低激发态之间生成高简并的临界区, 提供了γ振动能量会变得低于转动能量的一个可能途径, 从而实现了SU(3)–U(5)的相变. 关键词： yrast带结构演化 势能曲面 相变临界区 182Os核')" href="#">¹⁸²Os核     

Effect of an electric field on ferroelastic phase transitions

The effect of an external electric field on the proper ferroelastic phase transitions described in a two-dimensional representation of the D _4h class is analyzed theoretically. The electric field induces a Lifshitz invariant. It is shown that this invariant does not lead to the formation of an incommensurate phase. The phase diagram for commensurate transitions occurring under the action of an electric field changes significantly. The second-order phase transition from the initial phase is split by an electric field of specific orientation into a sequence of two second-order phase transitions with close temperatures.     

Study of the 1D anisotropic Kondo necklace model at criticality via an entanglement entropy estimator

We use an estimator of quantum criticality based on the entanglement entropy to discuss the ground state properties of the 1D anisotropic Kondo necklace model. We found that the T=0$T=0$ phase diagram of the model is described by a critical line separating an antiferromagnetic phase from a Kondo singlet state. Moreover we calculate the conformal anomaly on the critical line and obtain that c tends to 0.5 as the thermodynamic limit is reached. Hence we conclude that these transitions belong to Ising universality class being, therefore, second order transitions instead of infinite order as claimed before.     

Critical Behavior of Spatial Evolutionary Game with Altruistic to Spiteful Preferences on Two-Dimensional Lattices

Self-questioning mechanism which is similar to single spin-flip of Ising model in statistical physics is introduced into spatial evolutionary game model. We propose a game model with altruistic to spiteful preferences via weighted sums of own and opponent's payoffs. This game model can be transformed into Ising model with an external field. Both interaction between spins and the external field are determined by the elements of payoff matrix and the preference parameter. In the case of perfect rationality at zero social temperature, this game model has three different phases which are entirely cooperative phase, entirely non-cooperative phase and mixed phase. In the investigations of the game model with Monte Carlo simulation, two paths of payoff and preference parameters are taken. In one path, the system undergoes a discontinuous transition from cooperative phase to non-cooperative phase with the change of preference parameter. In another path, two continuous transitions appear one after another when system changes from cooperative phase to non-cooperative phase with the prefenrence parameter. The critical exponents ν, β, and γ of two continuous phase transitions are estimated by the finite-size scaling analysis. Both continuous phase transitions have the same critical exponents and they belong to the same universality class as the two-dimensional Ising model.