Entropic phase separation in polymer-microemulsion networks

We study theoretically a model system of a transient network of microemulsion droplets connected by telechelic polymers and explain recent experimental findings. Despite the absence of any specific interactions between either the droplets or polymer chains, we predict that as the number of polymers per drop is increased, the system undergoes a first-order phase separation into a dense, highly connected phase, in equilibrium with dilute droplets, decorated by polymer loops. The phase transition is purely entropic and is driven by the interplay between the translational entropy of the drops and the configurational entropy of the polymer connections between them. Because it is dominated by entropic effects, the phase behavior of the system is extremely robust and is independent of the detailed properties of either polymers or drops.     

The effect of atomic motion and two-quanta JCM on the information entropy

We study the interaction between a moving two-level atom and a single-mode field. The coupled atom-cavity system with atomic center-of-mass motion included is modeled by considering the dependence of the atomic motion along z-axis. At exact resonance between the internal atomic transition and the cavity eigenfrequency, an exact solution of the system is obtained and periodically modulated Rabi oscillations and regular translational motion are observed. We focused on the dynamics of both field Wehrl entropy and Wehrl phase distribution. The influence of the atomic motion on the evolution of von Neumann entropy and Wehrl entropy is examined. The results show that the atomic motion and the field-mode structure play important roles in the evolution of the von Neumann entropy, Wehrl entropy and Wehrl PD.     

Quantum phase transition and entanglement in Li atom system

By use of the exact diagonalization method, the quantum phase transition and entanglement in a 6-Li atom system are studied. It is found that entanglement appears before the quantum phase transition and disappears after it in this exactly solvable quantum system. The present results show that the von Neumann entropy, as a measure of entanglement, may reveal the quantum phase transition in this model.     

The Schouten et al. model applied to polymer mixture solutions

The lattice-gas model of Schouten, ten Seldam and Trappeniers is extended to mixtures of polymers. This model allows for three options on each lattice site: a molecule of type one, a molecule of type two or a hole. It describes a binary gas-liquid system in the molecular-field approximation. Following Flory, the chain length r is introduced as additional parameter in order to express the fact that the molecules occupy several lattice sites. In this paper we introduce two Flory chain length parameters (leading to the model of Tompa) in order to describe polymer mixtures in the presence of their vapor phase. The model can also be applied to a ternary mixture consisting of two polymers in a common solvent.The conditions under which the critical line becomes self-crossing are given and explicit expressions are evaluated for the coordinates of this point. The critical line is displayed for a number of such cases in the pressure-temperature projection.     

Entropy and phase diagram of valence transition

The electronic and lattice entropies associated with the valence transition are estimated. The electronic entropy in metallic phase is evaluated based on the model which includes the mixing between ?-level and d-band states, and the d-band superimposes the hybridized ?-level. The quasiharmonic approximation together with the Debye approximation are used to calculate the lattice entropy. For the first order transition occurring at low temperature the entropy of semiconducting phase is found to be lower than that of metallic phase. The reverse situation is obtained for high transition temperature. This explains the experimental fact that the slope of the phase boundary of valence transition in SmS changes its sign with temperature. The specific heat calculated in this model shows a broad maximum at low temperature.     

Entanglement and the phase transition in single-mode superradiance

We consider the entanglement properties of the quantum phase transition in the single-mode superradiance model, involving the interaction of a boson mode and an ensemble of atoms. For an infinite size system, the atom-field entanglement diverges logarithmically with the correlation length exponent. Using a continuous variable representation, we compare this to the divergence of the entropy in conformal field theories and derive an exact expression for the scaled concurrence and the cusplike nonanalyticity of the momentum squeezing.     

Directional ordering of fluctuations in a two-dimensional compass model

In the Mott insulating phase of the transition metal oxides, the effective orbital-orbital interaction is directional both in orbital space and in real space. We discuss a classical realization of directional coupling in two dimensions. Despite extensive degeneracy of the ground state, the model exhibits partial orbital ordering in the form of directional ordering of fluctuations at low temperatures stabilized by an entropy gap. Transition to the disordered phase is shown to be in the Ising universality class through exact mapping and multicanonical Monte Carlo simulations.     

Entanglement scaling in the one-dimensional Hubbard model at criticality

We derive exact expressions for the local entanglement entropy epsilon in the ground state of the one-dimensional Hubbard model at a quantum phase transition driven by a change in magnetic field h or chemical potential mu. The leading divergences of delta epsilon/delta h and delta epsilon/delta mu are shown to be directly related to those of the zero-temperature spin and charge susceptibilities. Logarithmic corrections to scaling signal a change in the number of local states accessible to the system as it undergoes the transition.     

Entanglement, fidelity, and quantum phase transition in antiferromagnetic-ferromagnetic alternating Heisenberg chain

The fidelity and entanglement entropy in an antiferromagnetic-ferromagnetic alternating Heisenberg chain are investigated by using the method of density-matrix renormalization-group. The effects of anisotropy on fidelity and entanglement entropy are investigated. The relations between fidelity, entanglement entropy and quantum phase transition are analyzed. It is found that the quantum phase transition point can be well characterized by both the ground-state entropy and fidelity for large system.     

软凝聚态物质物理学

软物质是指其某种物理性质在小的外力作用下能产生很大变化的凝聚态物,典型的例子包括液晶、高分子体系、胶体、微乳液等。软物质的结构和性质主要不是由内能,而是由熵来决定,较通俗地介绍了软物质的概念,仔细分析了熵在软物质中所起的作用,同时详细介绍了聚合物体系、胶体及生物膜等几种典型的软物质,通过硫化橡胶和无管虹吸等十分有趣的例子。说明了聚合物对流变性质的影响;通过分析硬球胶体的相变及相分离等行为说明了熵力的概念;仔细分仔了电稳定胶体的相互作用,并介绍了DLVO理论以及近年来发现的对这一理论的偏离,特别是约束条件下同号带电胶球的长程吸引相互作用及其对此现象的一些解释;对生物膜也作了初步介绍。人们对软物质的研究和理解目前还处于一个非常原始的阶段,深入研究和理解软物质的各种性质必将促进人类对自然和人类自身的认识。     

Quantum entanglement and fidelity of the two-site Holstein model

We study the polaronic crossover properties in the two-site Holstein model by the quantum entanglement and the fidelity. Based on the exact wave function obtained by the extended bosonic coherent states, the linear entropy and the fidelity are evaluated. The maximum of the entanglement between the electron and surrounding phonons is observed in a small polaron regime. A smooth change (no singularity) of the ground-state fidelity as well as its susceptibility appears in the intermediate coupling regime, which confirms a crossover rather than a phase transition in this system by this quantum information tool. In addition, the results for some quantities in the weak-coupling and strong-coupling limit are given analytically.     

Entanglement entropy, decoherence, and quantum phase transitions of a dissipative two-level system

The concept of entanglement entropy appears in multiple contexts, from black hole physics to quantum information theory, where it measures the entanglement of quantum states. We investigate the entanglement entropy in a simple model, the spin-boson model, which describes a qubit (two-level system) interacting with a collection of harmonic oscillators that models the environment responsible for decoherence and dissipation. The entanglement entropy allows to make a precise unification between entanglement of the spin with its environment, decoherence, and quantum phase transitions. We derive exact analytical results which are confirmed by Numerical Renormalization Group arguments both for an ohmic and a subohmic bosonic bath. The entanglement entropy obeys universal scalings. We make comparisons with entanglement properties in the quantum Ising model and in the Dicke model. We also emphasize the possibility of measuring this entropy using charge qubits subject to electromagnetic noise; such measurements would provide an empirical proof of the existence of entanglement entropy.     

气泡成核过程的格子Boltzmann方法模拟

利用精确差分格子Boltzmann模型探讨水在特定温度下的亚稳态及不稳定平衡态, 获得等温相变过程中形成气泡和液滴的条件, 模型预测结果与理论解符合良好. 在该等温模型的基础上耦合能量方程, 通过调节流体-壁面相互作用力获得不同的气泡与固壁间接触角, 从而建立了一种新的描述气液相变的格子Boltzmann理论模型. 利用该新模型模拟不同流体-壁面相互作用力下凹坑气泡成核过程, 再现了气泡成核过程中的三阶段特性; 探讨了接触角、曲率半径及气泡体积随气泡成核过程的变化关系, 获得了与文献结果定性符合的曲率-气泡体积关系曲线. 关键词： 格子Boltzmann方法 气泡成核过程 气液相变 接触角     

Semi-flexible compact polymers on fractal lattices

Semi-flexible compact polymers modeled by Hamiltonian walks with bending rigidity are studied on 3 and 4-simplex fractal lattices. Hamiltonian walks are weighted according to the number of bends in the walk, and total weights are obtained by an exact recursive treatment. Asymptotic form of the partition function, with temperature dependent scaling parameters, as well as the corresponding critical exponents, is determined. Various thermodynamic quantities are calculated numerically and presented graphically, and the possibility of phase transition between a compact molten globule and an ordered ‘crystal’ state is investigated. No phase transition is found on either of these two lattices, meaning that fractal geometry here prevents any kind of orientational order.     

The miscibility of polyethersulfone with phenoxy resin

Polyethersulfone has been shown to be miscible with phenoxy resin. Cast films were found to be clear and show a single, composition-dependent glass transition-Blends were found to phase separate on heating with a lower critical solution temperature around 185°C. The heat of mixing of low molecular weight analogs was small and negative, suggesting a favorable interaction between the polymers. Phase diagrams were simulated using the equation-of-state theory of Flory and co-workers and were found to agree closely with the observed behavior. The water vapor sorption of the blends was investigated, and this also suggested a weak favorable interaction between the polymers.     

Entanglement and criticality in quantum impurity systems

We investigate the entanglement between a spin and its environment in impurity systems which exhibit a second-order quantum phase transition separating a delocalized and a localized phase for the spin. As an application, we employ the spin-boson model, describing a two-level system (spin) coupled to a sub-Ohmic bosonic bath with power-law spectral density, J(omega) proportional to omega(s) and 0 < s < 1. Combining Wilson's numerical renormalization group method and hyperscaling relations, we demonstrate that the entanglement between the spin and its environment is always enhanced at the quantum phase transition resulting in a visible cusp (maximum) in the entropy of entanglement. We formulate a correspondence between criticality and impurity entanglement entropy, and the relevance of these ideas to nanosystems is outlined.     

Model for folding and aggregation in RNA secondary structures

We study the statistical mechanics of RNA secondary structures designed to have an attraction between two different types of structures as a model system for heteropolymer aggregation. The competition between the branching entropy of the secondary structure and the energy gained by pairing drives the RNA to undergo a "temperature independent" second order phase transition from a molten to an aggregated phase. The aggregated phase thus obtained has a macroscopically large number of contacts between different RNAs. The partition function scaling exponent for this phase is theta approximately 1/2 and the crossover exponent of the phase transition is nu approximately 5/3. The relevance of these calculations to the aggregation of biological molecules is discussed.     

Quantum entanglement in the Sachdev–Ye–Kitaev model and its generalizations

Entanglement is one of the most important concepts in quantum physics. We review recent progress in understanding the quantum entanglement in many-body systems using large-N solvable models: the Sachdev–Ye–Kitaev (SYK) model and its generalizations. We present the study of entanglement entropy in the original SYK model using three different approaches: the exact diagonalization, the eigenstate thermalization hypothesis, and the pathintegral representation. For coupled SYK models, the entanglement entropy shows linear growth and saturation at the thermal value. The saturation is related to replica wormholes in gravity. Finally, we consider the steady-state entanglement entropy of quantum many-body systems under repeated measurements. The traditional symmetry breaking in the enlarged replica space leads to the measurement-induced entanglement phase transition.     

Space-time phase transitions in driven kinetically constrained lattice models

Kinetically constrained models (KCMs) have been widely used to study and understand the origin of glassy dynamics. These models show an ergodic-nonergodic first-order phase transition between phases of distinct dynamical “activity”. We introduce driven variants of two popular KCMs, the FA model and the (2)-TLG, as models for driven supercooled liquids. By classifying trajectories through their entropy production we prove that driven KCMs display an analogous first-order space-time transition between dynamical phases of finite and vanishing entropy production. We discuss how trajectories with rare values of entropy production can be realized as typical trajectories of a mapped system with modified forces.