Dynamic mechanism of the temperature dependence of the Portevin-Le Châtelier effect

The influence of temperature on the dynamic and static characteristics of unstable plastic flow due to the Portevin-Le Chatelier effect is investigated experimentally and by numerical modeling. It is concluded that the distinctive features of the complex macroscopic behavior of the plastic instability are determined by collective processes associated with the long-range interaction of dislocations and lead to dynamic effects of mesocopic scale. Fiz. Tverd. Tela (St. Petersburg) 40, 487–492 (March 1998)     

Self-organizing dynamics of the human brain: Critical instabilities and Sil'nikov chaos

Using a sensorimotor coordination task in conjunction with an array of SQUIDs (Superconducting QUantum Interference Devices) we demonstrate critical instabilities in human brain activity patterns. Analysis of the dominant spatial pattern of the brain and its time-varying amplitude displays a task-dependent geometry characteristic of Sil'nikov-like chaos, which changes qualitatively at the transition. (c) 1995 American Institute of Physics.     

Recurrence analysis of the Portevin-Le Chatelier effect

Tensile tests were carried out by deforming polycrystalline samples of Al-2.5%Mg alloy at room temperature in a wide range of strain rates where the Portevin-Le Chatelier (PLC) effect was observed. The experimental stress-time series data have been analyzed using the recurrence analysis technique based on the Recurrence Plot (RP) and the Recurrence Quantification Analysis (RQA) to study the change in the dynamical behavior of the PLC effect with the imposed strain rate. Our study revealed that the RQA is able to detect the unique crossover phenomenon in the PLC dynamics.     

退火铝合金中Portevin-Le Chatelier效应的数值模拟研究

通过综合考虑沉淀动力学和动态应变时效机理，建立了一套具有明确物理内涵的唯象本构模型.并在对不同加载应变率下的单轴拉伸实验的数值模拟中，得到了3种类型的Portevin-Le Chatelier效应应力曲线.计算结果与实验数据的较好吻合，验证了该模型的有效性. 关键词： Portevin-Le Chatelier效应 动态应变时效 数值模拟     

Hard-loop dynamics of non-Abelian plasma instabilities

Non-Abelian plasma instabilities may be responsible for the fast apparent quark-gluon thermalization in relativistic heavy-ion collisions if their exponential growth is not hindered by nonlinearities. We study numerically the real-time evolution of instabilities in an anisotropic non-Abelian plasma with an SU(2) gauge group in the hard-loop approximation. We find exponential growth of non-Abelian plasma instabilities both in the linear and in the strongly nonlinear regimes, with only a brief phase of subexponential behavior in between.     

固溶处理对Al-Cu合金中Portevin-Le Chatelier效应空域行为的影响

研究了在定加载速率拉伸条件下，固溶处理对Al-Cu合金材料中PLC(Portevin-Le Chatelier)效应空域行为的影响.在较低拉伸速率时，变形初期PLC变形带在试件宽度方向中央附近某点处“成核”.随着变形的继续，出现多带共存的现象.且带宽较小，带与拉伸轴向的夹角较大.在较大拉伸速率条件下，PLC变形带先连续传播，再随机出现. 关键词： PLC效应 动态应变时效 固溶处理     

Critical scattering and pionic instabilities in heavy ion collisions

We investigate the effect of collective instabilities in heavy ion collisions. The focus is on critical scattering phenomena associated with pionic instabilities. The decay rate Γ of excited many-body systems is calculated in RPA. Γ is shown to give the rate of spontaneous “phonon” pair production. We express Γ as a sum of a collective, Γ_col, and a scattering, Γ_scat, rate. Γ_col is the pair production rate of phonons in unstable states. In the case of pionic instabilities, Γ_col is the condensation rate of π⁺π^? and π⁰π⁰ pairs into unstable states. Γ_scat is the pair production rate of phonons in the particle-hole excitation region and gives the two-body scattering rate in the medium. An effective (density-dependent) two-body cross section is obtained. The difference between critical scattering of external particles in a system near equilibrium and that of constituents of systems far from equilibrium is investigated. A model calculation suggests the existence of pionic instabilities in heavy ion collisions. Growth rates of unstable modes and the effective cross sections displaying critical scattering are calculated. Finally, we estimate Γ_scat and Γ_col.     

Scaling behavior of the Portevin-Le Chatelier effect in an Al-2.5%Mg alloy

The scaling behavior of the Portevin-Le Chatelier (PLC) effect was studied by deforming an Al-2.5%Mg alloy for a wide range of strain rates. To reveal the exact scaling nature, the time series data of true stress versus time, obtained during deformation, were analyzed by two complementary methods: the finite variance scaling method and the diffusion entropy analysis. From these analyses we could establish that, in the entire span of strain rates, the PLC effect showed the Levy-walk property.     

红外测温法研究Al-Mg合金中的Portevin-Le Chatelier效应

试件塑性变形过程伴随着机械能向热能的转化.利用红外测温法,通过分析红外热像仪采集的温度场图像,系统研究了Al-Mg合金中的Portevin-Le Chatelier （PLC）效应.在不同应变率下,实验得到了三类锯齿形应力-应变曲线,分析了相应情况下试件温度变化曲线的异同及其原因,探讨了三种类型PLC变形带的空间传播特性.研究发现,试件表面的温升随着应变率的增加而增加;PLC带的倾角转向发生在试件的两端或者带外的温度最高处. 关键词： Portevin-Le Chatelier效应 红外测温 Al-Mg合金     

Investigation of Portevin-Le Chatelier effect in 5456 Al-based alloy using digital image correlation

A variety of experimental methods have been proposed for Portevin-Le Chatelier (PLC) effect. They mainly focused on the in-plane deformation. In order to achieve the high-accuracy measurement, three-dimensional digital image correlation (3D-DIC) was employed in this work to investigate the PLC effect in 5456 Al-based alloy. The temporal and spatial evolutions of deformation in the full field of specimen surface were observed. The large deformation of localized necking was determined experimentally. The distributions of out-of-plane displacement over the loading procedure were also obtained. Furthermore, a comparison of measurement accuracy between two-dimensional digital image correlation (2D-DIC) and 3D-DIC was also performed. Due to the theoretical restriction, the measurement accuracy of 2D-DIC decreases with the increase of deformation. A maximum discrepancy of about 20% with 3D-DIC was observed in this work. Therefore, 3D-DIC is actually more essential for the high-accuracy investigation of PLC effect.     

Critical behavior of the Chayes-Machta-Swendsen-Wang dynamics

We study the dynamic critical behavior of the Chayes-Machta dynamics for the Fortuin-Kasteleyn random-cluster model, which generalizes the Swendsen-Wang dynamics for the q-state Potts model to noninteger q, in two and three spatial dimensions, by Monte Carlo simulation. We show that the Li-Sokal bound z >or= alpha/nu is close to but probably not sharp in d = 2 and is far from sharp in d = 3, for all q. The conjecture z >or= beta/nu is false (for some values of q) in both d = 2 and d = 3.     

Critical dynamics of magnets

We review our current understanding of the critical dynamics of magnets above and below the transition temperature with focus on the effects due to the dipole-dipole interaction present in all real magnets. Significant progress in our understanding of real ferromagnets in the vicinity of the critical point has been made in the last decade through improved experimental techniques and theoretical advances in taking into account realistic spin-spin interactions. We start our review with a discussion of the theoretical results for the critical dynamics based on recent renormalization group, mode coupling and spin-wave theories. A detailed comparison is made of the theory with experimental results obtained by different measuring techniques, such as neutron scattering, hyperfine interaction, muon spin resonance, electron spin resonance, and magnetic relaxation, in various materials. Furthermore we discuss the effects of dipolar interaction on the critical dynamics of three-dimensional isotropic antiferromagnets and uniaxial ferromagnets. Special attention is also paid to a discussion of the consequences of dipolar anisotropies on the existence of magnetic order and the spin-wave spectrum in two-dimensional ferromagnets and antiferromagnets. We close our review with a formulation of critical dynamics in terms of nonlinear Langevin equations.     

Critical dynamics in isotropic ferromagnets

Modern high resolution inelastic neutron scattering methods make very long wavelength fluctuations accessible to dynamical investigation. The new results on Fe and EuO show that: (a) the $\text{q}{}^{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ power law is well borne out at T = T_c; (b) strong zero-wavenumber relaxation due to spin non-conserving forces can make the predicted hydrodynamic spin diffusion at T >T_c unobservable; (c) this zero-wavenumber relaxation is the only appreciable deviation from the $\text{z =}\text{5}\text{2}$ exchange scaling behaviour; (d) there is no evidence for the predicted cross-over to dipolar dynamics a finite q's.     

Critical dynamics of superconductors in the charged regime

We investigate the finite temperature critical dynamics of three-dimensional superconductors in the charged regime, described by a transverse gauge field coupling to the superconducting order parameter. Assuming relaxational dynamics for both the order parameter and the gauge fields, within a dynamical renormalization group scheme, we find a new dynamic universality class characterized by a finite fixed point ratio between the transport coefficients associated with the order parameter and gauge fields, respectively. We find signatures of this universality class in various measurable physical quantities, and in the existence of a universal amplitude ratio formed by a combination of physical quantities.     

Critical dynamics in the presence of relaxing defects

The influence of defects coupling linearly to the order parameter on the critical behaviour of structural phase transitions is studied. A continuum model for the statics and dynamics is introduced and is investigated by the renormalization group theory. If the defects are slow a central peak is found, the characteristic width of which is determined by the defects. Concerning the critical behaviour the dynamical model belongs to the same universality class as the one studied previously by Grinstein, MA, and Mazenko. The concentration dependence of the central peak response is discussed inside and outside the critical region.     

Critical dynamics of the one-dimensional BEG model

We apply domain wall motion arguments to study the critical relaxation of the 1-d BEG model with single spin flip dynamics. Non-universal critical dynamical behaviour is found for the two order parameter dynamical exponents. These are shown to depend both on the rates chosen and on the system's couplings.     

Critical dynamics of the random spherical model

The spherical model with random infinite-range interactions which shows a spin-glass property is investigated rigorously by the kinetic model. The uniform magnetization as well as the order parameter shows a critical slowing down. The static nonlinear susceptibility is found to diverge at the freezing temperature.     

Observation of the decay dynamics and instabilities of megagauss field structures in laser-produced plasmas

Monoenergetic proton radiography was used to make the first measurements of the long-time-scale dynamics and evolution of megagauss laser-plasma-generated magnetic field structures. While a 1-ns 10(14) W/cm2 laser beam is on, the field structure expands in tandem with a hemispherical plasma bubble, maintaining a rigorous 2D cylindrical symmetry. With the laser off, the bubble continues to expand as the field decays; however, the outer field structure becomes distinctly asymmetric, indicating instability. Similarly, localized asymmetry growth in the bubble interior indicates another kind of instability. 2D LASNEX hydrosimulations qualitatively match the cylindrically averaged post-laser plasma evolution but even then it underpredicts the field dissipation rate and of course completely misses the 3D asymmetry growth.     

Critical dynamics of dipolar ferromagnets

The dynamical scaling functions for ferromagnets with dipolar interactions are computed by mode coupling theory above the critical temperature T_c. On the basis of this theory we explain apparently conflicting features of neutron scattering experiments on EuO, EuS and Fe. The position of the crossover from isotropic to dipolar critical dynamics is determined and further experiments are proposed.