随机外磁场对一维Blume-Capel模型动力学性质的影响

近几十年来,量子自旋系统的动力学性质引起了人们的广泛关注,随着研究的不断深入,随机自旋系统的性质受到了人们的重视. 利用递推关系式方法研究了高温极限下随机外磁场中自旋s=1的一维Blume-Capel模型的动力学性质, 通过计算自旋自关联函数和相应的谱密度,探讨了外场对系统动力学行为的影响.研究表明,在无晶格场的情况下, 当外场满足双模分布时,系统的动力学性质存在从中心峰值行为到集体模行为的交跨效应.当外场满足Gauss分布, 标准偏差较小时,系统也存在交跨效应;标准偏差足够大时,系统只表现为无序行为. 另外还研究了晶格场对系统动力学性质的影响,发现晶格场的存在减弱了系统的集体模行为.     

随机纵场对一维量子Ising模型动力学性质的影响

量子自旋系统的动力学性质是统计物理和凝聚态理论研究的热点问题.本文利用递推关系方法,通过计算系统的自旋关联函数及谱密度,研究了纵场对一维量子Ising模型动力学性质的影响.对于常数纵场的情况,发现当自旋耦合相互作用较弱时纵场能够引起不同动力学行为之间的交跨效应,且驱使系统出现了多种振动模式,但较强的自旋耦合相互作用会掩盖纵场的影响.对于随机纵场的情况,分别讨论了双模型随机纵场和高斯型随机纵场的影响,发现不同随机类型下的动力学结果有很大的差别,且高度依赖于随机分布中参数的选取,如双模分布的均值,高斯分布的均值和偏差等.尽管常数纵场和随机纵场下的动力学结果不同,但可以得到一个共同的结论:当纵场所占比重较大时,系统的中心峰值行为将得到保持.且此结论可以推广:系统哈密顿中非对易项的出现有利于中心峰值行为的保持.     

非马尔科夫环境中各向异性海森堡自旋链的几何量子失协

本文主要在非马尔科夫环境中,在时变磁场和Dzyaloshinski—Moriya相互作用下,以最大纠缠态■作为信道,研究了具有各种参数的两比特海森堡XYZ模型的几何量子失协.通过在非马尔科夫环境中的量子态扩散方法模拟了系统的几何量子失协随时间的演化关系.根据数值模拟结果显示:环境关联系数γ、自旋耦合系数J和J_z、余弦磁场强度B、以及自旋—轨道相互作用都能影响系统几何量子失协的性质.当环境关联系数γ较小时,几何量子失协呈现出明显的上升趋势,可见非马尔科夫环境对系统的几何量子失协表现出积极的作用.同时较大的自旋耦合系数J和余弦磁场强度B对系统的几何量子失协也有积极的影响.     

反对称Spin-1/2阻挫钻石链的基态和磁化行为研究

对含有次近邻节点自旋交换耦合的自旋-1/2伊辛-海森伯钻石链体系进行了研究,利用矩阵对角化和传递矩阵方法对基态磁相和宏观热力学量进行了严格求解,重点探讨了所有交换耦合均为反铁磁耦合时,体系节点伊辛自旋间次近邻相互作用的影响.研究结果表明次近邻节点伊辛自旋存在反铁磁耦合时会增强系统的阻挫效应,引入破坏平移对称性的经典亚铁磁相,使基态呈现出上上上下上上的自旋构型以及磁化曲线新颖的2/3磁化平台,丰富了体系的基态相图和宏观磁性行为.     

一维XY自旋链中双自旋比特的相干分布及其相变临界行为

本文在具有Dzyaloshinsky-Moriya(DM)相互作用的一维XY自旋链中,以双自旋比特系统为研究对象,基于Jensen-Shannon熵,研究了量子相干分布（局域相干和集体相干）及其相变临界行为。研究发现,通过改变自旋-自旋耦合作用和DM相互作用,可以有效调控局域相干与集体相干之间的相互转化。此外,局域相干和集体相干能够通过局部极值或发散的临界行为准确地表征自旋链的一阶和二阶量子相变。当一阶量子相变的探测受到自旋-自旋耦合作用和DM相互作用的干扰时,集体相干比局域相干具有更加稳定的识别效果。最后,长距离自旋对的总体相干和集体相干在表征二阶量子相变上都具有显著优势。     

次近邻相互作用对Heisenberg XX链纠缠影响的研究

研究了存在次近邻相互作用(耦合)时Heisenberg XX链的纠缠特性. 结果表明对近邻格点, 随着耦合常数J的变化, 次近邻相互作用的存在可能使其纠缠度增大或者减小; 而对次近邻格点, 引进次近邻相互作用却可以产生纠缠, 并且使其随着|J|的增大而增大. 近邻格点间纠缠存在的临界温度T_c随着J的增大而降低, 次近邻格点间纠缠存在的临界温度T_c随着|J|的增大而升高. 此外对纠缠W态的制备, 次近邻相互作用的存在还使得三量子位情形时W态产生的时刻改变, 而对于四量子位情形却没有影响.     

串型耦合双量子点处于自旋阻塞区时磁输运性质的研究

使用双杂质安德森模型的哈密顿量,从理论上研究了串型耦合双量子点系统处于自旋阻塞区时的磁输运性质,并用主方程近似方法求解了哈密顿量.结果表明,自旋轨道耦合作用导致的双量子点间的自旋反转隧穿能够解除系统的自旋阻塞.同时也研究了超精细相互作用导致的在量子点内自旋反转和双量子点之间的自旋关联对系统的磁输运性质的影响,取得了一些有价值的结果,并对相关的物理问题进行了讨论.     

自旋轨道耦合量子气体中的一些新进展

随着人造规范势和自旋轨道耦合在冷原子体系中的实现,对这类效应的研究成为了冷原子物理研究的热门方向之一.冷原子系统具有丰富的可操控性,因此不仅可以作为优秀的量子模拟平台来研究其他领域中有意义的模型和问题,还基于体系自身的特点衍生出了一系列新颖的问题和方向.本文将以综述的形式介绍具有自旋轨道耦合的超冷原子系统中的一些新研究进展,重点关注该体系中特有的物理要素,如耗散、新颖的相互作用形式、大自旋和长程相互作用对系统性质的影响.这些研究进展可以为理解自旋轨道耦合效应提供新的启示和思路.     

铁磁体中缺陷对自旋波的影响

本文提出了一个处理磁性杂质或其他缺陷在磁性晶体中对自旋波频谱的影响的一般理论方法,并特别着重讨论了局域模自旋波。以一维线性格子为例进行计算的结果显示出:一个代位磁性杂质,可能产生不只一个高于连续带顶的局域模。其产生的条件和其能级位置均通过J′S′/JS和J′/J表达出来,这里S′和S各为杂质原子和基质原子的自旋量子数,J′和J各为杂质与近邻之间和一般近邻之间的交换作用系数。高度集中的应变和间隙原子如致使邻近处的交换作用增大,也导致局域模的出现。我们也考虑了磁偶极矩相互作用的影响,证明其并不破坏这些局域模的     

N—N相互作用中自旋轨道耦合力的夸克模型

在夸克-夸克相互作用中考虑了单胶子交换的自旋轨道耦合力、由禁闭势导出的自旋轨道耦合力、由手征场σ交换提供的自旋轨道耦合力以及由双胶子交换提供的自旋轨道耦合力,从而可以统一地解释重子谱中的自旋轨道耦合效应及N-N散射中的各个分波的相移.     

Magnetization relaxation of uniaxial anisotropic ferromagnetic particles with linear reaction dynamics driven by DC/AC magnetic field

The response of uniaxial anisotropic ferromagnetic particles with linear reaction dynamics subjected to alternating current(AC) or direct current(DC) bias magnetic field is evaluated by the reaction–diffusion equation for the probability distribution function of the molecular concentration in the spherical coordinate system. The magnetization function and the probability distribution function of the magnetic particles in the reaction system are derived by using the Legendre polynomials and Laplace transform. We discuss the characteristics of magnetization and probability distribution of the magnetic particles with different anisotropic parameters driven by a DC and AC magnetic fields, respectively. It is shown that both the magnetization and the probability distribution decrease with time increasing due to the reaction process. The uniformity of the probability distribution and the amplitude of the magnetization are both affected by the anisotropic parameters.Meanwhile, the difference between the case with linear reaction dynamics and the non-reaction case is discussed.     

A study of void size growth in nonequilibrium stochastic systems of point defects

We study properties of voids growth dynamics in a stochastic system of point defects insolids under nonequilibrium conditions (sustained irradiation). It is shown thatfluctuations of defect production rate (external noise) increase the critical void radiuscomparing to a deterministic system. An automodel regime of void size growth in astochastic system is studied in detail. Considering a homogeneous system, it is found thatexternal noise does not change the universality of the void size distribution function;the mean void size evolves according to classical nucleation theory. The noise increasesthe mean void size and spreads the void size distribution. Studying dynamics of spatiallyextended systems it was shown that vacancies remaining in a matrix phase are able toorganize into vacancy enriched domains due to an instability caused by an elastic latticedeformation. It is shown that dynamics of voids growth is defined by void sinks strengthwith void size growth exponent varying from 1/3 up to 1/2.     

Quench dynamics of ultracold atoms in one-dimensional optical lattices with artificial gauge fields

We study the quench dynamics of noninteracting ultracold atoms loaded in one-dimensional(1D) optical lattices with artificial gauge fields, which are modeled by lattices with complex hopping coefficients. After suddenly changing the hopping coefficient, time evolutions of the density distribution, momentum distribution, and mass current at the center are studied for both finite uniform systems and trapped systems. Effects of filling factor, system size, statistics, harmonic trap, and phase difference in hopping are identified, and some interesting phenomena show up. For example, for a finite uniform fermionic system shock and rarefaction wave plateaus are formed at two ends, whose wave fronts move linearly with speed equaling to the maximal absolute group velocity. While for a finite uniform bosonic system the whole density distribution moves linearly at the group velocity. Only in a finite uniform fermionic system there can be a constant quasisteady-state current, whose amplitude is decided by the phase difference and filling factor. The quench dynamics can be tested in ultracold atoms with minimal modifications of available experimental techniques, and it is a very interesting and fundamental example of the transport phenomena and the nonequilibrium dynamics.     

Evidence for the Gompertz curve in the income distribution of Brazil 1978–2005

This work presents an empirical study of the evolution of the personal income distribution in Brazil. Yearly samples available from 1978 to 2005 were studied and evidence was found that the complementary cumulative distribution of personal income for 99% of the economically less favorable population is well represented by a Gompertz curve of the form G(x) = exp [exp (A-Bx)], where x is the normalized individual income. The complementary cumulative distribution of the remaining 1% richest part of the population is well represented by a Pareto power law distribution P(x) = βx^-α. This result means that similarly to other countries, Brazil’s income distribution is characterized by a well defined two class system. The parameters A, B, α, β were determined by a mixture of boundary conditions, normalization and fitting methods for every year in the time span of this study. Since the Gompertz curve is characteristic of growth models, its presence here suggests that these patterns in income distribution could be a consequence of the growth dynamics of the underlying economic system. In addition, we found out that the percentage share of both the Gompertzian and Paretian components relative to the total income shows an approximate cycling pattern with periods of about 4 years and whose maximum and minimum peaks in each component alternate at about every 2 years. This finding suggests that the growth dynamics of Brazil’s economic system might possibly follow a Goodwin-type class model dynamics based on the application of the Lotka-Volterra equation to economic growth and cycle.     

Influence of noise on the behavior of oscillators near the synchronization boundary

Nonautonomous behavior of oscillators in the presence of noise is considered. The influence of noise on the dynamics of local zero Lyapunov exponents for nonautonomous dynamic systems that are near the synchronization boundary is considered. It is shown that the action of noise on a nonautonomous dynamic system that is near the synchronization boundary produces domains of synchronous motion in the series realization, which alternate with asynchronous domains. In accordance with this, the distribution of local zero Lyapunov exponents corresponding to laminar phases shift toward negative values. This effect is demonstrated with a discrete-time system (map of a circle onto itself) that is a reference model to describe the synchronization phenomenon and also with a reference system exhibiting chaotic dynamics (Ressler system).     

Possible test of the thermodynamic approach to granular media

We study the steady state distribution of the energy of the Sherrington-Kirkpatrick model driven by a tapping mechanism which mimics the mechanically driven dynamics of granular media. The dynamics consists of two phases: a zero temperature relaxation phase which leads the system to a metastable state, then a tapping which excites the system thus reactivating the relaxational dynamics. Numerically, we investigate whether the distribution of the energies of the blocked states obtained agrees with a simple canonical form of the Edwards measure. It is found that this canonical measure is in good agreement with the dynamically measured energy distribution. A possible experimental test of the Edwards measure based on the study here is proposed.     

Irregular dynamics in a one-dimensional Bose system

We study many-body quantum dynamics of delta-interacting bosons confined in a one-dimensional ring. Main attention is paid to the transition from the mean-field to the Tonks-Girardeau regime using an approach developed in the theory of interacting particles. We analyze, both analytically and numerically, how the Shannon entropy of the wave function and the momentum distribution depend on time for weak and strong interactions. We show that the transition from regular (quasiperiodic) to irregular ("chaotic") dynamics coincides with the onset of the Tonks-Girardeau regime. In the latter regime, the momentum distribution of the system reveals a statistical relaxation to a steady state distribution. The transition can be observed experimentally by studying the interference fringes obtained after releasing the trap and letting the boson system expand ballistically.     

The multiple quantum NMR dynamics in systems of equivalent spins with a dipolar ordered initial state

The multiple quantum (MQ) NMR dynamics in the system of equivalent spins with the dipolar ordered initial state is considered. The high symmetry of the Hamiltonian responsible for the MQ NMR dynamics (the MQ Hamiltonian) is used to develop analytic and numerical methods for the investigation of the MQ NMR dynamics in systems consisting of hundreds of spins from the “first principles.” We obtain the dependence of the intensities of the MQ NMR coherences on their orders (profiles of the MQ NMR coherences) for systems of 200–600 spins. It is shown that these profiles may be well approximated by exponential distribution functions. We also compare the MQ NMR dynamics in the systems of equivalent spins having two different initial states, the dipolar ordered state and the thermal equilibrium state in a strong external magnetic field.