Restrictions on the phase diagrams for a large class of multisite interaction spin systems

Through the proof of two very general theorems involving Ising spin systems with multisite interactions, specific regions of the complexh plane, whereh is the external magnetic field, are shown to be free of zeros of the partition function. Hence in these regions the partition function is analytic and phase transitions are absent. As an example: for systems with ferromagnetic multisite interactions involving even numbers of sites, no phase transition occurs outside of an interval centered on the origin of the realh axis and of the form (–C(T),C(T)), whereT is the temperature. For T0,C(T)0 and phase transitions can occur only ath=0.     

A quenching scenario for lattice gauge theories

We study lattice gauge theories with complex, random and quenched couplings. Such theories are argued to have the same continuum limits as the annealed case. The first-order phase transitions are shown to be absent and the smoother cross-over behavior of the quenched theory leads to the universal scaling law.     

Boundary induced phase transitions in driven lattice gases with metastable states

We study the effect of metastability onto boundary induced phase transitions in a driven lattice gas. The phase diagram for open systems, parameterized by the input and output rates, consists of two regions corresponding to the free flow and jammed phase. Both have been entirely characterized. The microscopic states in the high density phase are shown to have an interesting striped structure, which undergoes a coarsening process and survives in the thermodynamic limit.     

Relaxation in establishing a steady-state stochastic process with 1/<Emphasis Type="Italic">f</Emphasis> spectrum and low-frequency spike statistics

According to theory, fluctuations with a power spectrum inversely proportional to frequency (1/f processes) may arise when dissimilar phase transitions simultaneously take place in physical systems with intense white noise. In this work, relaxation effects in establishing a steady-state stochastic process with non-equilibrium phase transitions are described in terms of two nonlinear stochastic differential equations. The results thus obtained carry information on the statistics of large-scale low-frequency spikes. Step “forgetting” of initial conditions is noted. It is numerically shown that the distributions of the durations and maximal values of extreme low-frequency spikes have a power-type form.     

Landau theory of shape phase transitions in the cranked interacting boson model

Landau theory of phase transitions is applied to quadrupole shapes of rotating atomic nuclei within the interacting boson model (IBM) with cranking. It is shown that the coherent-state method must be generalized to allow for non-Hermitian quadrupole tensors of the coherent-state coefficients, which results in important modifications of the cranking shape-phase diagram compared to previous non-IBM studies of rotating nuclei. The parameter space has two surfaces of the first-order phase transitions and a curve of the second-order phase transition at their intersection. The phase structure of the cranked IBM closely resembles systems with competing superconducting and normal phases.     

Phase Transitions and Topology Changes in Configuration Space

The relation between thermodynamic phase transitions in classical systems and topological changes in their configuration space is discussed for two physical models and contains the first exact analytic computation of a topologic invariant (the Euler characteristic) of certain submanifolds in the configuration space of two physical models. The models are the mean-field XY model and the one-dimensional XY model with nearest-neighbor interactions. The former model undergoes a second-order phase transition at a finite critical temperature while the latter has no phase transitions. The computation of this topologic invariant is performed within the framework of Morse theory. In both models topology changes in configuration space are present as the potential energy is varied; however, in the mean-field model there is a particularly strong topology change, corresponding to a big jump in the Euler characteristic, connected with the phase transition, which is absent in the one-dimensional model with no phase transition. The comparison between the two models has two major consequences: (i) it lends new and strong support to a recently proposed topological approach to the study of phase transitions; (ii) it allows us to conjecture which particular topology changes could entail a phase transition in general. We also discuss a simplified illustrative model of the topology changes connected to phase transitions using of two-dimensional surfaces, and a possible direct connection between topological invariants and thermodynamic quantities.     

Stick-slip regime of melting of boundary lubrication taking into account spatial inhomogeneity

The processes of boundary friction between two atomically smooth solid surfaces with an ultrathin layer of lubricant between them are studied in the context of the model of the first-order phase transitions, taking into account the spatial inhomogeneity. The stick-slip regime of motion, which is often observed experimentally for such systems, is considered. Such a regime is represented as the periodic first-order phase transitions between the structural states of the lubricant. It is shown that during motion, the lubricant tends to assume a homogeneous structure over the sliding plane, which results in the periodicity of time dependences of the basic parameters in the stick-slip regime. The dependence of the order parameter on the shear rate is analyzed and it is shown that this dependence has the same shape for all the regions on the contact plane.     

广义等温等压系综-分子动力学模拟全原子水的气液共存形貌

探测相变过程中瞬时共存相的形貌等特征对理解其微观机制十分重要.本文应用广义等温等压系综-分子动力学模拟方法,研究全原子水模型的气液两相平衡及相变的中间过程.研究发现,此广义系综方法能够通过持续降温,连续地历经从气态、气液共存到液态的整个相变过程,通过持续升温历经其相反过程,而不会发生标准正则系综中的过饱和热滞现象.该方法不需要使用副本交换等增强抽样方法,因此可以用于较大体系的研究,多个独立的模拟即能获得整个气态液态区的平衡性质及共存相特征.本文还提出了计算气液共存界面面积的新方法,给出了水的气液共存界面形状随温度、压强的变化规律.结果表明,低压时水的气液共存界面因其较大的表面张力而接近球面,符合经典成核理论的描述,但随着压强升高接近其临界压强时,气态和液态的差别减小,界面的表面张力变小,界面形状变为无规则的枝杈结构,表现为二阶相变特征.     

Anomalies of dielectric properties of cs2hgci4 crystals in the vicinity of phase transitions

A short review of the published experimental data on various physical properties and some new results of the measurement of dielectric and electromechanical properties of Cs₂HgCI₄ crystals in a wide temperature range are presented. The crystal is shown to reveal anomalies at seven temperatures associated with structural phase transitions and hence to have eight phases. In two temperature regions there exists a small spontaneous polarization and the anomalous dielectric permittivity is a nonlinear function of the electric biasing field. Within the phase followed by the first polar phase on cooling the crystal, the effects of thermal, electrical and mechanical memory, specific for incommensurate phases, are observable.     

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     

Quantum phase transitions in mesoscopic systems

Quantum phase transitions in mesoscopic systems are studied. It is shown that the main features of phase transitions, defined for infinite number of particles, N--> infinity, persist even for moderate N approximately 10. A Landau analysis of first order transitions is done and a "critical" exponent at the spinodal point is defined. Two order parameters are introduced to distinguish first from second order transitions. Applications to atomic nuclei, molecules, atomic clusters, and finite polymers are mentioned. Experimental evidence in atomic nuclei is presented.     

Entropy-driven phase transitions with influence of the field-dependent diffusion coefficient

We present a comprehensive study of phase transitions in a single-field reaction-diffusion stochastic systems with a field-dependent mobility of a power-law form and internal fluctuations. Using variational principles and mean-field theory we have shown that the noise can sustain spatial patterns and leads to phase transitions type of “order-disorder”. These phase transitions can be critical and non-critical in character. Our theoretical results are verified by computer simulations.     

Dynamics of order-disorder-type ferroelectrics and anti-ferroelectrics

The dynamics of order-disorder-type ferroelectrics and anti-ferroelectrics is discussed with reference to recent theoretical and experimental investigations. Following a short review of the lattice dynamical theory of structural phase transitions the pseudo-spin formalism appropriate for order-disorder-type systems is introduced. It is shown that most dynamical properties of such systems can be described by a pseudo-Ising model in a transverse tunnelling field. Finally, the predictions of the pseudo-spin wave theory are compared with some recent experiments.     

光学多稳系统的相变及临界点分类

参照朗道相变理的基本精神,确定光学多稳性系统的相变与临床界现象,揭示了相变的多样性,以及多稳系统通向完全单稳状态态的不同路径。多稳性的级次愈高,相变的式样也就愈多。     

The sequence of structural phase transitions in systems with a four-well potential

The theory of order-disorder phase transitions is discussed for systems with a four-well potential. It is shown that the model can be generalized to the case of the well-known Ising model in a transverse field. It is also shown that two or three crystalline phases can emerge as the temperature is reduced. Fiz. Tverd. Tela (St. Petersburg) 39, 2217–2219 (December 1997)     

Quantum–classical crossover of the escape rate in the two-parameter doubly periodic potential

The transition from the quantum tunneling to classical hopping for a two-parameter doubly periodic potential is investigated. According to the Chudnovsky's criterion for the first-order transition, it is shown that there is the first- or second-order transition depending on different parameters regions. The phase boundary lines between first- and second-order transitions are calculated, and a complete phase diagram is presented.     

Yang-Lee theory for a nonequilibrium phase transition

To analyze phase transitions in a nonequilibrium system, we study its grand canonical partition function as a function of complex fugacity. Real and positive roots of the partition function mark phase transitions. This behavior, first found by Yang and Lee under general conditions for equilibrium systems, can also be applied to nonequilibrium phase transitions. We consider a one-dimensional diffusion model with periodic boundary conditions. Depending on the diffusion rates, we find real and positive roots and can distinguish two regions of analyticity, which can be identified with two different phases. In a region of the parameter space, both of these phases coexist. The condensation point can be computed with high accuracy.