量子细胞神经网络的超混沌特性研究

研究了由量子点细胞自动机构成的量子细胞神经网络的非线性动力学特性.以量子点细胞的极化率和量子相位作为状态变量，对3个细胞耦合的量子细胞神经网络进行了理论分析和计 算机仿真研究.结果表明，该网络系统呈现复杂的混沌动力学行为，混沌振荡产生非常容易. 由数值计算得到的两个最大正Lyapunov指数证实了该系统具有超混沌特性. 关键词： 量子点细胞自动机 极化率 量子细胞神经网络 超混沌     

磁性量子元胞自动机拐角结构的理论模拟和实验

磁性量子元胞自动机拐角结构是一种同时含有铁磁耦合和反铁磁耦合的电路. 理论模拟了外加时钟场作用下铸铁磁性材料拐角结构的信号传递. 采用微磁仿真给出了信号传递的磁化演化图, 结果证实了铸铁纳磁体能够实现含有两种冗长耦合结构的稳定转换. 实验制备了相应的拐角结构, 扫描电子显微图和磁力显微图结果显示了成功的电路图案和正确的两种耦合方式信号传递.     

耦合双量子点的磁序与电子结构

耦合双量子点中电子间静电相互作用对电子在系统中隧穿效应具有重要影响.考虑电子隧穿,交换及Hubbard关联作用后,在单态近似下,本文求解了耦合双量子点二电子问题,讨论了铁磁基态及Mott局域化态出现的条件。这里的结果表明电子的隧穿不利于铁磁基态的形成,Hubbard关联越强越有利于Mott局域化态的形成,并且在交换作用下Mott局域化态会转变为铁磁基态. 此外,外场可以诱导磁序相的改变。     

Temperature dependence of spin-transfer-induced switching of nanomagnets

We measure the temperature, magnetic-field, and current dependence for the switching of nanomagnets by a spin-polarized current. Depending on current bias, switching can occur between either two static magnetic states or a static state and a current-driven precessional mode. In both cases, the switching is thermally activated and governed by the sample temperature, not a higher effective magnetic temperature. The activation barriers for switching between static states depend linearly on current, with a weaker dependence for dynamic to static switching.     

Imaging magnetic responses of nanomagnets by XPEEM

The Spin-resolved Photoelectron Emission Microscope (SPEEM) is a permanently installed set-up at Helmholtz-Zentrum Berlin (HZB). Due to its specific contrast it is mainly used for magnetic imaging and micro-spectroscopy with quantitative analysis. A crucial point in magnetic imaging is the application of magnetic fields. Many experiments require observation of magnetic responses or the preparation of a certain magnetic state during the measurement. We present a dedicated magnetic sample holder combining magnetic field during imaging with additional temperature control. This set-up enables SPEEM to measure magnetization curves of individual Fe nanocubes (18 nm)³ in size. If additionally alternating magnetic fields are applied we can image the local magnetic AC susceptibility (χ_AC) as a function of temperature. The latter is ideally suited to visualize local variations of the Curie temperature (T_C) in nano- and microstructures.     

Probabilistic signatures of spatiotemporal intermittency in the coupled sine circle map lattice

The phase diagram of the coupled sine circle map system exhibits a variety of interesting phenomena including spreading regions with spatiotemporal intermittency, non-spreading regions with spatial intermittency, and coherent structures termed solitons. A spreading to non-spreading transition is seen in the system. A cellular automaton version of the coupled system maps the spreading to non-spreading transition to a transition from a probabilistic to a deterministic cellular automaton. The solitonic sector of the system shows spatiotemporal intermittency with soliton creation, propagation and absorption. A probabilistic cellular automaton mapping is set up for this sector which can identify each one of these phenomena.      

宏微观耦合模拟熔池不同区域中枝晶竞争生长过程

针对熔化焊过程建立了宏微观耦合模型,模拟了熔池内不同区域凝固过程中随机取向枝晶的竞争生长过程. 通过宏观三维有限元模型计算熔池中瞬态的传热传质过程,利用双线性插值算法将凝固参数传递给微观组织模型. 采用元胞自动机法模拟随机取向的枝晶在熔池凝固条件下的竞争生长过程. 模拟结果表明,所建立的微观模型能够精确模拟任意生长取向的枝晶. 凝固条件中最大温度梯度方向对枝晶竞争过程有明显选择作用,生长方向与最大温度梯度方向相同或接近的枝晶在竞争中具有更大优势. 焊缝中的晶粒组织由枝晶簇发展形成,晶粒组织的形貌演变取决于相邻枝晶簇之间的竞争过程,具有择优取向的枝晶簇会逐渐排挤非择优取向的枝晶簇并最终将其阻挡在凝固组织内部,宏观晶粒的取向与其内部枝晶簇的生长方向并不一定相同. 熔池中心线附近区域在焊接过程中具有更小的温度梯度、更大的凝固速率以及更大的局部冷却速率,凝固后可以获得更加细小的焊缝枝晶组织. 枝晶间距的模拟结果与相应凝固条件下的试验数据符合较好. 关键词： 焊接熔池 枝晶形貌 竞争生长 元胞自动机     

Optimal design of nanomagnets for targeted hyperthermia

We have numerically simulated the dynamic response of single-domain nanomagnets to alternating magnetic fields and discuss nanometer-sized thermal seeds suitable for selectively targeted magnetic hyperthermia. When we assign priority to the easy delivery and biological safety of these fields, the maximum heating power is obtained by the irradiation of a weak field at a medium frequency (∼1 MHz) on almost spherical magnetite nanomagnets with a diameter of approximately 20 nm. On the other hand, the irradiation of the amplified field at a low frequency (∼100 kHz) is more effective if the neural stimulation is allowable.     

Effect of resistance feedback on spin torque-induced switching of nanomagnets

In large magnetoresistance devices spin torque-induced changes in resistance can produce GHz current and voltage oscillations which can affect magnetization reversal. In addition, capacitive shunting in large resistance devices can further reduce the current, adversely affecting spin torque switching. Here, we simultaneously solve the Landau-Lifshitz-Gilbert equation with spin torque and the transmission line telegrapher's equations to study the effects of resistance feedback and capacitance on magnetization reversal of both spin valves and magnetic tunnel junctions. While for spin valves parallel (P) to anti-parallel (AP) switching is adversely affected by the resistance feedback due to saturation of the spin torque, in low resistance magnetic tunnel junctions P-AP switching is enhanced. We study the effect of resistance feedback on the switching time of magnetic tunnel junctions, and show that magnetization switching is only affected by capacitive shunting in the pF range.     

基于动态地场和元胞自动机的自行车流建模

针对现有自行车流模型对车辆间作用考虑不足的问题,在原多车道元胞自动机模型的基础上引入动态地场,在模型中纳入了排斥力,并充分考虑了前车速度对后车行驶的影响,体现了自行车的灵活性等特征,提出了换道成本的概念并建立了新的基于比较交通环境的换道规则. 数值仿真结果表明：1）该模型能更准确地描述自行车流,反映自行车流的特征,得到的数据符合实际;2）单位车道宽度的通行能力随着车道数的增加有小幅下降;3）激进的驾驶风格使得换道次数大幅增加,通行能力下降. 关键词： 自行车流 元胞自动机 动态地场 仿真     

互注入垂直腔表面发射激光器的多次偏振转换特性研究

基于垂直腔表面发射激光器(VCSELs)的自旋反转模型, 研究了互注入VCSELs系统中参数连续变化所引起的多次偏振转换(PS)特性. 研究结果表明: 连续改变互注入强度ξ、两个互注入VCSELs中一个激光器的激射频率以及同时连续改变两个激光器的激射频率, 均可产生多次的PS. 详细分析了上述三种参数连续变化所引起的多次PS的特性, 并讨论了耦合延时时间对多次PS特性的影响.     

Magnetization behavior of nanomagnets for patterned media application

Bit patterned media (BPM) which utilize each magnetic nanostructured dot as one recorded bit has attracted much interest as a promising candidate for future high-density magnetic recording. In this study, the magnetization reversal behaviors of nanostructured L1₀-FePt, Co/Pt multilayer (ML), and CoPt/Ru dots are investigated. For Co/Pt and CoPt/Ru nanodots, the bi-stable state is maintained in a very wide size range up to several hundred nm, and the magnetization reversal is dominated by the nucleation of a small reversed nucleus with the dimension of domain wall width. On the other hand, the critical size for the bi-stability of L1₀-FePt is about 60 nm, and its magnetization reversal proceeds via domain wall displacement even for such a small dot size. These reversal behaviors, depending on the magnetic materials, might be attributed to the difference in structural inhomogeneity, such as defects. In addition to the magnetic properties, the structural uniformity of the material could be crucial for the BPM application.     

基于教室人群疏散实验的行人流建模和模拟

基于教室人群疏散实验, 从中归纳出疏散过程中行人的基本运动特征. 将桌椅分别视为不可穿越和可穿越的静态障碍物, 而行人则被当成可移动的障碍物, 这将导致背景场随人群的运动而动态更新, 因此可以更好地反映前方拥挤程度对后面人群路径选择行为的影响. 采用基于动态背景场的元胞自动机模型研究了不同桌椅排列和出口宽度的教室人群疏散过程, 给出了疏散时间的空间分布以及平均和最大疏散时间, 再现了实验中人群疏散的基本特征. 数值模拟结果表明, 疏散时间取决于桌椅的排列方式和教室出口的宽度. 对于同一种排列, 出口越小则疏散时间越长; 对于给定的出口宽度, 通常随着过道数的增加, 疏散时间随之减少; 当过道数增加且过道宽度不足以两人并行, 从两侧进入过道的行人会发生冲突, 使疏散效率有所降低; 靠近出口一侧墙壁设置过道有利于人群的疏散. 文中进一步分析了模拟与实验结果存在差异的原因.     

Modeling fatigue of ascending stair evacuation with modified fine discrete floor field cellular automata

Fatigue even increases the complexity of the pedestrian dynamics which is regarded as a kind of nonlinear system, and might have a significant negative impact on the crowd evacuation. However, it has never been investigated completely and properly. Thus, the fine discrete floor field cellular automata model is modified by integrating the fatigue function to explore the influence of fatigue on the crowd ascending evacuation. The simulation fits well with the empirical data and the observations quantitatively and qualitatively, indicating the model captures the main features of evacuation considering fatigue impact. As a prediction, without merging streams, compared with the case of walking in constant speed, when fatigue is considered, it takes 71.4% longer for all persons to enter the stairs and 87.2% longer to evacuate. With merging streams, fatigue has little impact on the inflow, while it makes the total evacuation time 84.2% longer.     

高速桥梁瓶颈模型属性的研究

针对现实中桥梁形成的交通流瓶颈，建立了一个基于元胞自动机交通流桥梁瓶颈模型. 在周期性边界条件下，将系统的状态分成六“相”，利用平均场理论和决定性NS模型的结果，得到每条车道上的密度关系. 分析的理论值与计算机数值模拟结果符合得很好. 关键词： 元胞自动机模型 桥梁瓶颈 决定性NS模型 选择概率     

用CA方法模拟界面能各向异性对胞晶生长形态的影响

用元胞自动机（CellularAutomaton,CA）模型研究了界面能各向异性对二维定向凝固胞晶的生长形态的影响,建立了判定胞晶生长达到稳态的判据．结果显示,当界面能各向异性强度非常小时,胞晶尖端很容易分岔,胞晶形态不容易稳定．而当界面能各向异性强度足够大时,容易形成稳定的胞晶形态,同时界面能各向异性强度会显著影响稳定胞晶的形态,界面能各向异性越强,稳态胞晶间距越小,胞晶尖端半径越小,尖端半径与胞晶间距的比值越小,固液界面前沿的浓度与过冷度越小．     

Periodic behavior of cellular automata

I propose an explanation of the observation of a globally synchronized behavior of deterministic cellular automata and coupled map lattices, together with local fluctuations.     

Magnetic elements for switching magnetization magnetic force microscopy tips

Using combination of micromagnetic calculations and magnetic force microscopy (MFM) imaging we find optimal parameters for novel magnetic tips suitable for switching magnetization MFM. Switching magnetization MFM is based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within the technique the sum of the scanned data with reversed tip magnetization depicts local atomic forces, while their difference maps the local magnetic forces. Here we propose the design and calculate the magnetic properties of tips suitable for this scanning probe technique. We find that for best performance the spin-polarized tips must exhibit low magnetic moment, low switching fields, and single-domain state at remanence. The switching field of such tips is calculated and optimum shape of the Permalloy elements for the tips is found. We show excellent correspondence between calculated and experimental results for Py elements.     

通道中行人-机动车相互作用机理的建模和模拟

本文研究了通道中行人与车辆同向或反向运动时的人车相互作用.车辆运动的描述采用细化的确定性元胞自动机模型,而行人流则采用考虑背景场的格子气模型.车辆及其影响区被视为一种可移动的障碍物,形成动态变化的背景场,可以更好地反映人车之间的相互作用.通过数值模拟得到典型参数下的行人流基本图以及平均车速随行人密度的变化曲线.人车反向时行人流基本图中存在两个临界密度,其间的行人流量-密度曲线呈线性分布,曲线斜率k主要依赖于车辆宽度和行人预判时间,而平均车速近似为k,即反向车辆形成的移动瓶颈和行人拥堵向上游传播的速度是一致的.文中进一步考察了行人预判时间、车辆宽度及限速对人车混合交通流的影响.人车同向时,这三个参数的影响都不明显.人车反向时,当车辆宽度较小,即使在很高密度下,车辆仍可以前行,而更大的行人预判时间也有助于车辆的运动.     

Dynamics of Adsorbate Islands with Nanoscale Resolution

Surface catalytic processes produce, under certain conditions, small clusters of adsorbed atoms or groups, called islands which, after they have been formed, move as individual entities. Here we consider the catalytic reduction of NO with hydrogen on platinum. (i) Using video field ion microscopy, we observe the dynamic motion of small hydroxyl islands on the Pt(001) plane; despite changes in their morphology, the islands dimensions are confined to values corresponding to 10 to 30 Pt atoms suggesting cooperative effects to be in operation. (ii) We construct an automaton (or lattice Monte-Carlo) model on the basis of a set of elementary processes governing the microscopic dynamics. The agreement between the simulation results and the experimental observations suggests a possible mechanism for the formation and dynamics of hydroxyl islands.