光学双稳态和混沌运动中的临界现象

本文以具有反馈时间延迟的非线性系统弛豫方程为出发点,讨论了双稳态临界点的临界现象。发现双稳区边沿的临界慢化与混沌运动中切分叉点附近阵发混沌时间间隔发散具有一致性,临界指数为1/2。在双稳区(尖顶突变模型)的尖顶处,与双稳区边沿临界点不同,它与混沌运动中倍周期分叉点及劈分叉点的临界慢化具有一致性,临界指数为1。上述结果具有普适性。以液晶混合光学双稳系统为例,进行了计算机实验,其结果与分析相一致。 关键词：     

基于临界慢化现象的气候突变前兆信号的初步研究

近年来,临界慢化现象在揭示复杂动力系统是否趋于临界性灾变方面展示了重要潜力.基于临界慢化的理念,研究了气候突变的早期预警信号.针对20世纪70年代末80年代初的气候突变,对全国月平均温度资料和太平洋年代际振荡(PDO)指数进行分析,分别计算了表征临界慢化现象的方差和自相关系数.结果表明,气候发生突变前,全国月平均温度和PDO指数都存在明显的临界慢化现象,这表明临界慢化现象可能是气候突变发生前的一个早期信号.将临界慢化理论用于气候突变前兆信号的研究,对深入认识气候突变和捕捉气候突变前兆信号都具有重要的现实意义和科学价值.     

试用临界慢化原理探讨气候突变

近年来, 临界慢化现象在揭示复杂动力系统是否趋向于临界性灾变方面展示了重要潜力. 本文基于临界慢化的理念, 着重研究不同噪声影响下, 气候突变前兆信号出现的时间; 对含有不同噪声的气温观测资料、太平洋年代际振荡指数以及阿留申低压强度资料进行处理, 研究突变的前兆信号. 以20世纪70年代末80年代初这次气候突变为例, 分别计算了表征临界慢化现象的方差和自相关系数. 结果表明, 临界慢化现象出现在具有不同信噪比的资料中, 这表明临界慢化现象可能是气候突变发生前的一个前兆信号, 并且噪声对气候突变前兆信号的检测结果影响较小, 从而论证了临界慢化现象检测突变前兆信号的可靠性, 为该方法在实际观测资料中的广泛应用提供了实验基础.     

中国不同区域气候突变的前兆信号

近年来, 临界慢化现象在揭示复杂动力系统是否趋向于临界性突变方面展示了重要潜力. 在此基于临界慢化的理念, 对含有不同噪声的中国一级气象地理区划的11大区域的气温观测资料进行处理, 研究气候突变的前兆信号. 首先利用Mann-Kendall (M-K)方法准确的找出各个区域的突变位置, 然后计算了表征临界慢化现象的自相关系数以此寻找气候突变前兆信号, 并研究了不同噪声对气候突变前兆信号的影响. 结果表明, 针对不同的信噪比、不同区域的气温资料在气候突变发生前5–10年均出现了自 相关系数增大的临界慢化现象, 临界慢化现象可能是气候突变发生前的一个前兆信号; 噪声对气候突变前兆信号的检测结果影响较小, 论证了临界慢化现象检测突变前兆信号的可靠性, 为该方法在实际观测资料中的应用提供了实验基础. 关键词： 气候突变 临界慢化 M-K方法 前兆信号     

正电子慢化体的研究和进展

正电子湮没技术是一种研究材料的微观缺陷和相变的灵敏工具,在通常的正电子谱仪中,正电子能量为MeV量级,在样品中注入深度比较学（－100μm）,主要研究材料体内的平均缺陷密度,慢正电子束方法把正电子的能量降低为keV量级（而且可以调节）,注入比较浅（－μm）,所以是研究表面缺陷的探测手段,正电子慢化体是产生慢正电子的关键设备,对其研究有重要意义,文章综述了慢化体研究的历史和现状,从物理概念出发介绍使正电子慢化的四种可能方法和当今慢化体的五种几何排列方法,其中应用最广泛的是钨慢化体和百叶窗式的排列方式,效率最高的是惰性气体固体慢化体,而加电场慢化体是有待开发的高效慢化体。     

PGNAA煤质在线测量中同时提高C、O和其它元素测量精度的慢化体模拟

提出两种慢化体设计方案,第一种方案直接在原有慢化体中引入孔道,第二种方案在慢化体中增加一层铅之后再引入孔道.利用蒙特卡罗方法对两种方案进行研究.计算结果表明,采用第一种方案可以有效地提升C、O元素的测量精度,但同时会降低H、Si等元素的测量精度;采用第二种方案可提高C元素和O元素的测量精度,同时也可以提高H、Si等其它元素的测量精度.     

PGNAA煤质在线测量中同时提高C、O和其它元素测量精度的慢化体模拟（英文）

提出两种慢化体设计方案,第一种方案直接在原有慢化体中引入孔道,第二种方案在慢化体中增加一层铅之后再引入孔道.利用蒙特卡罗方法对两种方案进行研究.计算结果表明,采用第一种方案可以有效地提升C、O元素的测量精度,但同时会降低H、Si等元素的测量精度;采用第二种方案可提高C元素和O元素的测量精度,同时也可以提高H、Si等其它元素的测量精度.     

散裂中子源耦合慢化器的中子学研究

利用蒙特－卡罗方法研究了散裂中子源中耦合慢化器的中子学特性。给出了冷中子与热中子慢化器的中子能谱。甲烷慢化器提供了性能非常好的冷中子，在低功率的散裂中子源中得到了应用。计算了慢中子引出的角分布。由于较低的氢密度，液态氢的漫化能力低于水与液态甲烷，但是可以通过增加预慢化器来弥补这一问题。2cm厚的水预慢化器层可以大约减少热量在低温慢化器中的热量沉积33％而不破坏中子特性。     

非线性动力系统在分岔点处的临界慢化行为

Lyapunov指数是定量描述非线性动力系统轨道稳定性的主要方法之一，同时也是分析系统分岔行为的常用手段.实际应用中，人们通常只关心Lyapunov指数的正负，并以此来判断系统轨道是否稳定，而对于Lyapunov指数为零，即动力学分岔点处系统的行为特征讨论甚少.本文以几类经典的非线性动力系统为例，针对系统在分岔点处的轨道稳定性进行理论和数值分析.研究发现，不同系统在分岔点处其微扰后的轨道均以幂律，而非指数的形式收敛，呈现出经典物理系统在相变临界点处的慢化行为.通过理论分析，我们解析得到分岔点处计算临界指数的一般公式，并通过数值模拟对理论公式的准确性进行了验证.临界慢化是物理系统在相变点处的普遍现象，文中关于非线性系统在分岔点处临界慢化行为的发现将加深人们对于动力学分岔本质的认识，同时也是对现有教材中关于Lyapunov指数相关知识的有益补充.     

临界点附近气—液相变过程的实验研究

本文利用动态数字散斑干涉技术对氟利昂—１３在临界点附近的气液相变过程进行了实验研究,详细介绍了该实验的装置、原理,并给出了氟利昂—１３气液相变过程在临界点附近的数字散斑干涉图象及计算结果。获得了氟利昂—１３全场的密度分布及其变化规律,还获得了气液两相界面厚度及其随温度的变化和临界乳光现象。本文的工作为深入研究气液相变过程在临界点附近的微观机理提供了实验手段和条件。     

Does the onset of epileptic seizure start from a bifurcation point?

Detection of epileptic seizures is a major challenge of these days. There are lots of papers which pay their attention to this subject. Recently, some dynamical disease with attacks such as epilepsy are considered as a system in which critical slowing down can be seen before their attacks (seizure). Although there are not many researches on the prediction of seizures using this phenomenon. Recently [P. Milanowski, P. Suffczynski, Int. J. Neural Syst. 26, 1650053 (2016)] have investigated the application of critical slowing down indicators and surprisingly they found that only in 8% of nearby 300 epileptic patients have the evidence of critical slowing down before seizures. The main goal of this paper is finding the answer of the important question “can we trust that epileptic seizures are bifurcations in the neural system”. In order to find the answer, different studies on the prediction of seizure are investigated and we prove that features which are used in those papers are critical slowing down indicators although they are not aware of it. So we present some reasons for the occurrence of critical slowing down before the seizure. We hope that this study will be a motivation of future studies on the application of critical slowing down indicators for predicting epileptic seizures.     

Critical slowing down in optical bistability

The role of critical slowing down in optical bistability is discussed, taking fully into account the non-linear effects. In particular, we show that the critical slowing down may lead to interesting novel applications, such as compact optical delay lines and electric field to time converters.     

A study of the early warning signals of abrupt change in the Pacific decadal oscillation

In recent years, the phenomenon of a critical slowing down has demonstrated its major potential in discovering whether a complex dynamic system tends to abruptly change at critical points. This research on the Pacific decadal oscillation(PDO) index has been made on the basis of the critical slowing down principle in order to analyze its early warning signal of abrupt change. The chaotic characteristics of the PDO index sequence at different times are determined by using the largest Lyapunov exponent(LLE). The relationship between the regional sea surface temperature(SST) background field and the early warning signal of the PDO abrupt change is further studied through calculating the variance of the SST in the PDO region and the spatial distribution of the autocorrelation coefficient, thereby providing the experimental foundation for the extensive application of the method of the critical slowing down phenomenon. Our results show that the phenomenon of critical slowing down, such as the increase of the variance and autocorrelation coefficient, will continue for six years before the abrupt change of the PDO index. This phenomenon of the critical slowing down can be regarded as one of the early warning signals of an abrupt change. Through calculating the LLE of the PDO index during different times, it is also found that the strongest chaotic characteristics of the system occurred between 1971 and 1975 in the early stages of an abrupt change(1976), and the system was at the stage of a critical slowing down, which proves the reliability of the early warning signal of abrupt change discovered in 1970 from the mechanism. In addition, the variance of the SST,along with the spatial distribution of the autocorrelation coefficient in the corresponding PDO region, also demonstrates the corresponding relationship between the change of the background field of the SST and the change of the PDO.     

三维随机点阵Ising模型的集团Monte Carlo方法模拟

为了解决Monte Carlo模拟中的临界慢化问题,应用S-W方法,对一维随机点阵的模型进行了研究。找到该系统的相变点为β_c=0.075±0.001,与此同时,在相变点附近研究了SW方法的动力学性质,计算得动力学临界指数z=0.74±0.03,结果表明,SW方法对克服三维随机点阵Ising模型的临界慢化是非常有效的。 关键词：     

Fluctuation,relaxation, and extensivity of macrovariables in nonequilibrium systems

The extensive property of a macrovariable is proved for a quantal system whose Hamiltonian depends on time and for a stochastic system whose temporal evolution operator depends on time. These generalized situations are concerned with bulk-contact open systems. The extensive property, fluctuation, and nonlinear relaxation are investigated explicitly by calculating rigorously generating functions in exactly soluble models such as the linear stochastic model and linearXY model. The relation between the nonlinear critical slowing down and linear critical slowing down is also discussed.     

Equilibration and relaxation times at the chiral phase transition including reheating

We investigate the relaxational dynamics of the order parameter of chiral symmetry breaking, the sigma mean-field, with a heat bath consisting of quarks and antiquarks. A semiclassical stochastic Langevin equation of motion is obtained from the linear sigma model with constituent quarks. The equilibration of the system is studied for a first order phase transition and a critical point, where a different behavior is found. At the first order phase transition we observe the phase coexistence and at a critical point the phenomenon of critical slowing down with large relaxation times. We go beyond existing Langevin studies and include reheating of the heat bath by determining the energy dissipation during the relaxational process. The energy of the entire system is conserved. In a critical point scenario we again observe critical slowing down.     

On the critical dynamics of a disordered system

The dynamic critical exponent is calculated for a disordered time-dependent Ginzburg-Landau model with n isotropically coupled spin components per site. For n<4, in 3.99 dimensions, the critical slowing down is enhanced.     

Early warning signals of abrupt temperature change in different regions of China over the past 50 years

In this paper, the early warning signals of abrupt temperature change in different regions of China are investigated. Seven regions are divided on the basis of different climate temperature patterns, obtained through the rotated empirical orthogonal function, and the signal-to-noise temperature ratios for each region are then calculated. Based on the concept of critical slowing down, the temperature data that contain noise in the different regions of China are preprocessed to study the early warning signals of abrupt climate change. First, the Mann-Kendall method is used to identify the instant of abrupt climate change in the temperature data. Second, autocorrelation coefficients that can identify critical slowing down are calculated. The results show that the critical slowing down phenomenon appeared in temperature data about 5-10 years before abrupt climate change occurred, which indicates that the critical slowing down phenomenon is a possible early warning signal for abrupt climate change, and that noise has less influence on the detection results of the early warning signals. Accordingly, this demonstrates that the model is reliable in identifying the early warning signals of abrupt climate change based on detecting the critical slowing down phenomenon, which provides an experimental basis for the actual application of the method.