A general isomorphism approach to thermodynamic and transport properties of binary fluid mixtures near critical points

A general isomorphism approach to critical phenomena in binary fluid mixtures that may exhibit complex critical-line behavior is developed by relating the two relevant scaling fields to linear combinations of three physical field variables. These physical field variables are related to the temperature and chemical potentials of the two components. The proposed approach includes crossover from vapor-liquid critical behavior to liquid-liquid critical behavior and incorporates also the critical behavior near other special points on critical loci. It is shown that the key factor which determines the apparent behavior of the thermodynamic and transport properties of near-critical mixtures is the shape of the critical locus. The number of system-dependent coefficients that determine the asymptotic critical behavior is elucidated. The choice of zero-points of entropy and energy in binary mixtures is also discussed. The approach provides a powerful tool for predicting thermodynamic and transport properties of fluid mixtures in the critical region.     

The critical magnetic field of anisotropic two-band magnetic superconductors

The upper and lower critical fields, and the critical field ratio of an anisotropic two-band magnetic superconductor in the Ginzburg–Landau (GL) scenario is derived analytically. The temperature-dependent upper critical field is investigated and applied to Fe-based superconductors. We find that a very high value of zero-temperature upper critical field in Fe-based superconductors can be found in the negative differential susceptibility region. The temperature-dependent upper critical field is presented in two formulas, in the empirical view and in the GL two-band view, which agrees with the experimental results.     

Effect of longitudinal applied magnetic field on the self-pinched critical current in intense electron beam diode

The effect of applied longitudinal magnetic field on the self-pinched critical current in the intense electron beam diode is discussed. The self-pinched critical current is derived and its validity is tested by numerical simulations. The results shows that an applied longitudinal magnetic field tends to increase the self-pinched critical current. Without the effect of anode plasma, the maximal diode current approximately equals the self-pinched critical current with the longitudinal magnetic field applied; when self-pinched occurs, the diode current approaches the self-pinched critical current.     

Crossover Critical Behavior in the Three-Dimensional Ising Model

The character of critical behavior in physical systems depends on the range of interactions. In the limit of infinite range of the interactions, systems will exhibit mean-field critical behavior, i.e., critical behavior not affected by fluctuations of the order parameter. If the interaction range is finite, the critical behavior asymptotically close to the critical point is determined by fluctuations and the actual critical behavior depends on the particular universality class. A variety of systems, including fluids and anisotropic ferromagnets, belongs to the three-dimensional Ising universality class. Recent numerical studies of Ising models with different interaction ranges have revealed a spectacular crossover between the asymptotic fluctuation-induced critical behavior and mean-field-type critical behavior. In this work, we compare these numerical results with a crossover Landau model based on renormalization-group matching. For this purpose we consider an application of the crossover Landau model to the three-dimensional Ising model without fitting to any adjustable parameters. The crossover behavior of the critical susceptibility and of the order parameter is analyzed over a broad range (ten orders) of the scaled distance to the critical temperature. The dependence of the coupling constant on the interaction range, governing the crossover critical behavior, is discussed.     

Critical behavior of light in mode-locked lasers

Light is shown to exhibit critical and tricritical behavior in passively mode-locked lasers with externally injected pulses. It is a first and unique example of critical phenomena in a one-dimensional many-body light-mode system. The phase diagrams consist of regimes with continuous wave, driven parapulses, spontaneous pulses via mode condensation, and heterogeneous pulses, separated by phase transition lines that terminate with critical or tricritical points. Enhanced non-Gaussian fluctuations and collective dynamics are present at the critical and tricritical points, showing a mode system analog of the critical opalescence phenomenon. The critical exponents are calculated and shown to comply with the mean field theory, which is rigorous in the light system.     

Critical phenomena in a disc-percolation model and their application to relativistic heavy ion collisions

By studying the critical phenomena in continuum-percolation of discs, we find a new approach to locate the critical point, i.e.using the inflection point of P_∞ as an evaluation of the percolation threshold.The susceptibility, defined as the derivative of P_∞, possesses a finite-size scaling property, where the scaling exponent is the reciprocal of ν, the critical exponent of the correlation length.A possible application of this approach to the study of the critical phenomena in relativistic heavy ion collisions is discussed.The critical point for deconfinement can be extracted by the inflection point of P_(QGP)-the probability for the event with QGP formation.The finite-size scaling of its derivative can give the critical exponent ν, which is a rare case that can provide an experimental measure of a critical exponent in heavy ion collisions.     

Heat capacity of a Cr2O3 antiferromagnet near the critical temperature

The heat capacity of a Cr₂O₃ antiferromagnet near the critical temperature is precisely measured by ac calorimetry. The critical behavior of the heat capacity is examined. The regularities of variations in the universal critical parameters near the critical point are determined, and their values are calculated. A crossover from the Heisenberg (n=3) to the Ising (n=1) critical behavior is revealed.     

Spontaneous-search method and short-time dynamics: applications to the Domany-Kinzel cellular automaton

The one-dimensional Domany-Kinzel cellular automaton is investigated by two numerical approaches: (i) the spontaneous-search method, which is a method appropriated for a search of criticality; (ii) short-time dynamics. Both critical frontiers of the system are investigated, namely, the one separating the frozen and active phases, as well as the critical line determined by damage spreading between two cellular automata, that splits the active phase into the nonchaotic and chaotic phases. The efficiency of the spontaneous-search method is established herein through a precise estimate of both critical frontiers, and in addition to that, it is shown that this method may also be used in the determination of the critical exponent ν_⊥. Using the critical frontiers obtained, other exponents are estimated through short-time dynamics. It is verified that the critical exponents of both critical frontiers fall in the universality class of directed percolation.     

高温超导磁体临界电流影响因素的研究

临界电流值是描述Bi2223高温超导带材性能的一个基本参数,在一定的温度条件下,Bi2223高温超导带材的临界电流是带材所在位置磁场大小和磁场方向的函数,其短样的临界电流值可以通过四引线法测量,单根超导带材的自场很小,磁场对临界电流的影响可以忽略.高温超导磁体的临界电流被定义成引发该磁体失超的最小电流,高温超导磁体的自场比单根超导带材的自场要大得多,磁体各个位置的磁场大小和方向各不相同,很难用理论的方法准确计算磁体的临界电流.对于高温超导磁体而言,除了磁场的影响因素以外,绕制磁体所用的超导带材自身的均匀性也是影响其临界电流的一个重要因素.本文对这两个因素进行探讨,并着重讨论高温超导带材自身的均匀性对临界电流大小的影响,本文的结论可以为高温超导磁体的设计、磁体绕制时带材的选择、磁体运行时安全工作电流的确定提供帮助.     

Computer simulation of the critical behavior of 3D disordered ising model

The critical behavior of the disordered ferromagnetic Ising model is studied numerically by the Monte Carlo method in a wide range of variation of concentration of nonmagnetic impurity atoms. The temperature dependences of correlation length and magnetic susceptibility are determined for samples with various spin concentrations and various linear sizes. The finite-size scaling technique is used for obtaining scaling functions for these quantities, which exhibit a universal behavior in the critical region; the critical temperatures and static critical exponents are also determined using scaling corrections. On the basis of variation of the scaling functions and values of critical exponents upon a change in the concentration, the conclusion is drawn concerning the existence of two universal classes of the critical behavior of the diluted Ising model with different characteristics for weakly and strongly disordered systems.     

Thermodynamic properties of the superconductivity in quasi-two-dimensional Dirac electronic systems

The thermodynamic properties of superconducting Dirac electronic systems is analyzed in the vicinity of quantum critical point. The system is characterized by a quantum critical point at zero doping, such that the critical temperature vanishes below some finite value of interaction strength. It is found that the specific heat jump of the system largely deviates from the conventional BCS theory value in the vicinity of quantum critical point. We investigated the region of applicability of the mean-field theory using the Ginzburg-Landau functional.     

Critical behavior of disordered systems with a free surface

The behavior of homogeneous and disordered systems with a free boundary is described on the basis of group theory in the two-loop approximation directly in three-dimensional space. The effect of the free boundary on the regime of the bulk critical behavior is revealed. It is shown that the boundedness of the system slightly affects the regime of the bulk critical behavior in the case of the ordinary transition, whereas this effect is more noticeable in the case of the special transition. Surface critical phenomena are described for homogeneous and disordered systems, and the critical exponents are calculated in the two-loop approximation. It is shown that the effect of impurities is insignificant in the special phase transition, whereas it is more noticeable in the ordinary phase transition. The derived critical exponents are compared with the computer-simulation results.     

Simulation of the Critical Adsorption of Semi-Flexible Polymers

The critical adsorption of semi-flexible pol.ymer chains on attractive surfaces is studied using Monte Carlo simulations.The results reveal that the critical adsorption point of a free polymer chain is the same as that of an end-grafted one.For the end-grafted polymer,we find that the finite-size scaling relation and the maximum fluctuation of adsorbed monomers are equivalent in estimating the critical adsorption point.The effect of chain stiffness on the critical adsorption is also investigated.The surface attraction strength for the critical adsorption of semi-flexible polymer chain decreases exponentially with an increase in the chain stiffness;In other words,lower adsorption energy is needed to adsorb a stiffer polymer chain.The result is explained from the viewpoint of the free energy profile for the adsorption.     

Boundary entropy of one-dimensional quantum systems at low temperature

The boundary beta function generates the renormalization group acting on the universality classes of one-dimensional quantum systems with boundary which are critical in the bulk but not critical at the boundary. We prove a gradient formula for the boundary beta function, expressing it as the gradient of the boundary entropy s at fixed nonzero temperature. The gradient formula implies that s decreases under renormalization, except at critical points (where it stays constant). At a critical point, the number exp((s) is the "ground-state degeneracy," g, of Affleck and Ludwig, so we have proved their long-standing conjecture that g decreases under renormalization, from critical point to critical point. The gradient formula also implies that s decreases with temperature, except at critical points, where it is independent of temperature. It remains open whether the boundary entropy is always bounded below.     

On the critical behavior of the Ising model with mixed two- and three-spin interactions

A study is made of a two-dimensional Ising model with staggered three-spin interactions in one direction and two-spin interactions in the other. The phase diagram of the model and its critical behavior are explored by conventional finite-size scaling and by exploiting relations between mass gap amplitudes and critical exponents predicted by conformal invariance. The model is found to exhibit a line of continuously varying critical exponents, which bifurcates into two Ising critical lines. This similarity of the model with the Ashkin-Teller model leads to a conjecture for the exact critical indices along the nonuniversal critical curve. Earlier contradictions about the universality class of the uniform (isotropic) case of the model are clarified.     

Critical behaviour of random compressible magnets

The critical properties of a compressible random magnet are studied using renormalization group methods. Then-component orderparameter is coupled to quenched disorder and to the elastic fluctuations of the anisotropic solid. It is shown, that the critical behaviour of a compressible random magnet is in general the same as that of a random magnet on a rigid lattice. However, if the specific heat exponent of the ideal magnet is positive and the disorder is sufficiently weak, a macroscopic instability may prevent the system in reaching the critical point. The resulting first-order transition may be preceded by pseudocritical behaviour characteristic to pure compressible magnets. The effect of random magnetic fields on the critical properties of compressible magnets is also discussed.     

Multicritical behavior at the kosterlitz-thouless critical point

A multicritical critical point for the two dimensional planar model is analyzed by studying an exactly soluable limit of a related model—the generalized Villain model. The statistical mechanics of this model is written in terms of vortex and symmetry breaking excitations. In these terms, the problem reduces to a kind of two dimensional problem with interacting electric charges and magnetic monopoles. In this form, the problem is manifestly self-dual. The multicritical behavior is exhibited in a three-dimensional phase space in which the axes are the coupling strength of a “square” symmetry breaking which favors four possible directions for the planar model vectors. The analysis of this multicritical point shows that it is the intersection of at least six critical lines—each with continuously varying critical indices. Two of these lines are described by the exactly soluable gaussian model. The other four are isomorphic to one another, and each one has—as a point on the line—a critical point of the Ashkin-Teller model. We argue that each of these lines might be in an equivalent universality class to the line of critical points which occurs in the Baxter and Ashkin-Teller models. We make a suggestion about which point on these critical lines might be in the same universality class as our multicritical point. Correlation functions at the intersection point are calculated and used to develop an expansion of critical indices about this point. This expansion gives a potential method for calculating the critical behavior along the critical lines of the model.     

基于声发射传感器的高温超导线圈临界电流的研究

用经验失超判据Ec=1μV/cm可以对高温超导线圈在直流情况下的临界电流进行检测,但对于高温超导线圈交流时的临界电流还没有一个标准.利用声发射传感器对高温超导线圈的交流临界电流进行了初步研究,并把结果与直流临界电流进行了比较,结果表明在低于直流临界电流时,声发射传感器获得了明显改变的声信号,即高温超导线圈的交流临界电流...     

非局域程度对空间暗孤子相互作用的影响

对1+1维非局域自散焦介质中暗孤子的相互作用进行了研究. 数值模拟结果表明:在非局域自散焦介质中,存在着暗孤子相互作用的临界点. 在临界点上,吸引力等于排斥力;在临界点外,暗孤子间相互作用可能表现为相互吸引也可能表现为相互排斥. 关键词： 非局域自散焦 相互作用 暗孤子