Noise-induced dynamical transition in systems with symmetric absorbing states

We investigate the effect of noise strength on the macroscopic ordering dynamics of systems with symmetric absorbing states. Using an explicit stochastic microscopic model, we present evidence for a phase transition in the coarsening dynamics, from an Ising-like to a voter-like behavior, as the noise strength is increased past a nontrivial critical value. By mapping to a thermal diffusion process, we argue that the transition arises due to locally-absorbing states being entered more readily in the high-noise regime, which in turn prevents surface tension from driving the ordering process.     

Debye-like diffusive central mode near the phase transition in ferroelectric lithium tantalate

We report a clear cut evidence of a temperature dependent diffusive central mode associated with the ferroelectric phase transition of LiTaO₃; a detailed linshape analysis shows that this central mode is a Debye-like relaxation. We also show its influence in the dielectric constant through polariton measurements for several temperatures.     

Superfluid phase transition in two-dimensional excitonic systems

We study the superfluid phase transition in the two-dimensional (2D) excitonic system. Employing the extended Falicov–Kimball model (EFKM) and considering the local quantum correlations in the system composed of conduction band electrons and valence band holes we demonstrate the existence of the excitonic insulator (EI) state in the system. We show that at very low temperatures, the particle phase stiffness in the pure-2D excitonic system, governed by the non-local cross correlations, is responsible for the vortex–antivortex binding phase-field state, known as the Berezinskii–Kosterlitz–Thouless (BKT) superfluid state. We demonstrate that the existence of excitonic insulator phase is a necessary prerequisite, leading to quasi-long-range order in the 2D excitonic system.     

Surface effects and diffusive phase transition in ferroelectric thin films: A self-consistent approach

A self-consistent Landau phenomenological approach has been used to study the ferroelectric transition in films in the presence of various surface effects such as depolarization and strain. The polarization distribution of the film is computed and its variation with respect to temperature, thickness and strain is determined. The gradual decrease in polarization across the transition shows the diffusive behavior which is confirmed from the soft mode and the dielectric susceptibility analysis. The critical thickness below which ferroelectricity disappears is also computed. The degree of diffuseness in the transition is obtained from the susceptibility exponent which shows more and more diffusive behavior for smaller and smaller film thickness.     

Berezinskii-Kosterlitz-Thouless phase transition in systems with exotic symmetries

A Berezinksii-Kosterlitz-Thouless phase transition in systems with the exceptional symmetry groups G=E _6,7,8, G ₂, and F ₂ is studied. The critical exponents and the exponents of the logarithmic corrections to the correlation functions at the transition point are found by the renormalization-group method. It is shown that for G=A, D, and E the critical exponents can be expressed in terms of the Coxeter numbers h _G (or the values of the Casimir operator in the adjoint representation K ₂ ^G ). Pis’ma Zh. éksp. Teor. Fiz. 63, No. 9, 743–747 (10 May 1996)     

Concentration phase transition in systems with weak anisotropy

Systems with weak easy-axis anisotropy, which contain im purities of the type of a “random local field,” have been considered. The anisotropy-constant dependence of the critical concentration of impurities at which the long-range order is destroyed has been obtained for the space-dimension range 2 ≤ d ≤ 4.     

Nonequilibrium quantum phase transition in itinerant electron systems

We study the effect of the voltage bias on the ferromagnetic phase transition in a one-dimensional itinerant electron system. The applied voltage drives the system into a nonequilibrium steady state with a nonzero electric current. The bias changes the universality class of the second order ferromagnetic transition. While the equilibrium transition belongs to the universality class of the uniaxial ferroelectric, we find the mean-field behavior near the nonequilibrium critical point.     

Absence of modulated phase transition and spin density wave phase transition in one- and two-dimensional systems

We prove that the modulated phase transition with any wave vector and the spin density wave (SDW) phase transition cannot exist in one- and two-dimensional systems at finite temperatures.     

Phase segregation in driven diffusive systems

Driven diffusive models describe an array of atoms in an external periodic potential, when the motion is damped due to energy exchange with the substrate. The systems of this class have wide application in modeling of charge and mass transport in solids. Recently, the driven diffusive models have been used in tribology, where the driving force emerges due to motion of one of two substrates, which are separated by a thin atomic layer. When a dc force is applied to the atoms, the system exhibits the locked-to-sliding transition. During the transition the system may split in domains of two kinds, the running domains where the atoms move with almost maximum velocity, and the immobile domains (“traffic jams”). We discuss a new model for a 1D chain, where the particles have a complex structure treated in a mean-field fashion: particle collisions are inelastic and also each particle is considered as having its own thermostat. This model exhibits a hysteresis and the “traffic jams” state even at high temperatures due to the clustering of atoms with the same velocity.     

Thermal response in driven diffusive systems

Evaluating the linear response of a driven system to a change in environment temperature(s) is essential for understanding thermal properties of nonequilibrium systems. The system is kept in weak contact with possibly different fast relaxing mechanical, chemical or thermal equilibrium reservoirs. Modifying one of the temperatures creates both entropy fluxes and changes in dynamical activity. That is not unlike mechanical response of nonequilibrium systems but the extra difficulty for perturbation theory via path-integration is that for a Langevin dynamics temperature also affects the noise amplitude and not only the drift part. Using a discrete-time mesh adapted to the numerical integration one avoids that ultraviolet problem and we arrive at a fluctuation expression for its thermal susceptibility. The algorithm appears stable under taking even finer resolution.     

Surface tension and phase transition for lattice systems

We introduce the surface tension for arbitrary spin systems and study its general properties. In particular we show that for a large class of systems, the surface tension is zero at high temperature. We also derive a geometrical condition for the surface tension to be zero at all temperature. For discrete spin systems this condition becomes a criterion to establish the existence of a phase transition associated with surface tension. This criterion is illustrated on several examples.     

Pseudogap formation and quantum phase transition in strongly-correlated electron systems

Pseudogap formation is a ubiquitous phenomenon in strongly-correlated superconductors, for example cuprates, heavy-fermion superconductors, and iron pnictides. As the system is cooled, an energy gap opens in the excitation spectrum before entering the superconducting phase. The origin of formation and the relevancy to the superconductivity remain unclear, which is the most challenging problem in condensed matter physics. Here, using the cuprate as a model, we demonstrate that the formation of pseudogap is due to a massive gauge interaction between electrons, where the mass of the gauge boson, determining the interaction length scale, is the consequence of the remnant antiferromagnetic fluctuation inherited from the parent compounds. Extracting from experimental data, we predict that there is a quantum phase transition belonging to the 2D XY universality class at the critical doping where pseudogap transition vanishes.     

Elastic interaction and the phase transition in coherent metal-hydrogen systems

We study the statistical mechanics of hydrogen dissolved in metals. The underlying model is based on the assumption that the dominant attractive interaction between the protons in the metal is of an elastic nature.

In the first part of the paper we review some general properties of the elastic interaction. We then discuss the importance of boundary conditions for the form of the elastic interaction, which turns out to be of the Curie-Weiss type with macroscopic range.

In the second part we investigate the a-a' (‘gas-liquid’) phase transition in the hydrogen lattice fluid. The long-range part of the elastic interaction is treated in mean field approximation. In the canonical ensemble as opposed to the grand canonical ensemble one finds no co-existing phases near the critical point. Instead there is a continuous transition which changes into a first-order transition at tricritical points. In the temperature-density region which normally corresponds to the two-phase co-existence region the hydrogen density is inhomogeneous and varies on a macroscopic scale.

The peculiar nature of the a-a' phase transition is due to the long-range character of the elastic interaction, which ultimately results from the requirement of coherency of the host crystal. We argue that coherent metal-hydrogen systems offer examples of real systems where the classical theory of phase transitions applies.     

二维声学系统中的拓扑相变及边界传输

拓扑声学的发现产生了一种可以有效抑制反射的声学边界传输态,不仅让人们重新认识了声传输现象,也扩展了声学新原理功能器件的研究领域。文章将介绍二维系统中拓扑声学的基本概念以及一些拓扑非平庸体系。主要关注利用背景流体构造的声类量子霍尔效应,利用共振耦合环形波导构造Floquet拓扑绝缘体,声类量子自旋霍尔效应以及声谷霍尔效应等。     

Field-theoretical treatment of the liquid-vapor phase transition in nuclear systems

The liquid-vapor phase transition in hot nuclear matter is investigated in a field-theoretical approach employing euclidean-space (imaginary time) path-integral techniques. This approach allows us to study the nucleation due to both quantum and thermodynamic fluctuations. The bubbles of the new phase appear as instanton solutions of the euclidean-space field equations. The critical bubble sizes and associated transition probabilities are calculated. We examine the temperature and density values for which a phase transition may develop in hot nuclear matter produced in the course of a heavy-ion reaction.     

Cluster structure and the first-order phase transition in dipolar systems

The European Physical Journal E - The Monte Carlo technique is used to simulate a 3D dipolar hard-sphere system. The spatial and magnetic structure of clusters formed by magnetic dipolar...     

Superconductor-normal metal quantum phase transition in dissipative and non-equilibrium systems

Abstract

In physical systems, coupling to the environment gives rise to dissipation and decoherence. For nanoscopic materials, this may be a determining factor of their physical behaviour. However, even for macroscopic many-body systems, if the strength of this coupling is sufficiently strong, their ground-state properties and phase diagram may be severely modified. Also dissipation is essential to allow a system in the presence of a time-dependent perturbation to attain a steady, time-independent state. In this case, the non-equilibrium phase diagram depends on the intensity of the perturbation and on the strength of the coupling of the system to the outside world. In this paper, we investigate the effects of both dissipation and time-dependent external sources in the phase diagram of a many-body system at zero and finite temperatures. For concreteness, we consider the specific case of a superconducting layer under the action of an electric field and coupled to a metallic substrate. The former arises from a time dependent vector potential minimally coupled to the electrons in the layer. We introduce a Keldysh approach that allows to obtain the time dependence of the superconducting order parameter in an adiabatic regime. We study the phase diagram of this system as a function of the electric field, the coupling to the metallic substrate and temperature.