二维自旋系统aging现象的数值模拟研究

应用动力学蒙特卡罗模拟方法，对二维Ising模型和二维XY模型的aging现象展开数值研究.系统在零外场条件下从高温无序相淬火到临界温度T_c时，通过测量自关联函数A(t,t′)，观察到二维Ising模型和二维XY模型的aging现象，获得更精确的动力学临界指数λ_c和z的值.特别是对二维XY模型，当初始态为完全无序态时，模拟结果证实存在关于标度行为的对数修正. 关键词： 蒙特卡罗模拟 相变和临界行为 aging现象     

通道宽度和初始流量对颗粒稀疏流-密集流转变临界开口的影响

通过实验研究斜面上二维颗粒流，当出口尺寸减小到临界值Dc时，发生稀疏流到密集流的突变.发现临界尺寸Dc与初始流量和通道宽度有关，通道宽度一定的情况下，临界开口尺寸Dc近似随初始流量Q0的平方根增大.在初始流量Q0一定时，临界开口尺寸Dc随通道宽度W近似线性增大.给出了这些关系的表达式，理论计算与实验观测结果一致.同时，也讨论了通道宽度影响临界开口尺寸的原因. 关键词： 颗粒物质 颗粒流     

颗粒速度在颗粒流稀疏流-密集流转变中的作用

用实验和计算模拟的方法研究了颗粒流中的颗粒速度与颗粒流特性的关系.实验研究发现当入口流量固定时，在出口上方高速运动的颗粒会使颗粒流由稀疏流向密集流转变的临界出口尺寸变小.当颗粒流转变为密集流后，颗粒速度的作用被出口上方的颗粒堆积区所消耗，最终变得与颗粒速度无关.二维分子动力学模拟计算得到了与实验相同的结论.通过二维分子动力学模拟计算，还给出了不同颗粒速度下体系的密度和速率在空间的分布图.这些分布图显示随着颗粒到达出口上方的瞬间速度的不同，颗粒堆积区的密度和高度均会改变，并最终导致颗粒流流动状态的改变. 关键词： 颗粒流 颗粒气体 分子动力学模拟     

颗粒物质从稀疏流到密集流转变的普适规律

二维颗粒流实验研究发现，当初始稀疏颗粒流量固定，出口的尺寸减小到一临界值，或固定出口的尺寸，颗粒流量增大到一临界值时，都会发生流量的突然减小，从稀疏流转变为密集流．得到了临界流量与出口尺寸、颗粒尺寸及通道宽度之间的普适标度关系，揭示了“瓶颈效应”的物理本质．     

欠掺杂高温超导体中的涡旋电荷结构相变

利用平均场t-t′-U-V-V_c模型,通过自洽求解Bogoliubov-de Gennes方程,研究了高温超导体中涡旋结构的相变.发现增大原位排斥势U,自旋密度波、电荷密度波以及d波序参量由棋盘结构转变为条纹结构.模型哈密顿量中引入合适强度的长程库仑势后,欠掺杂高温超导体样品中也可以出现二维或者棋盘结构,结果与文献报道的扫描隧道显微镜实验结果一致. 关键词： 高温超导 涡旋结构 长程库仑势     

开口角度对二维颗粒流稀疏流-密集流转变的影响

用计算机模拟的方法研究了开口角度对二维颗粒流稀疏流—密集流转变的影响.在固定入口流量和固定颗粒数两种条件下,均发现当开口角度大于零时,开口角度的增大可以提高颗粒流由稀疏流向密集流转变的最大出口流量.在稀疏流状态下,出口流量与开口角度无关;而在密集流状态下,出口流量随开口角度的增大而增大.进一步的计算还发现增加开口角度可以提高颗粒流出开口的流动速度,且最大出口流量与颗粒的流动速度呈线性关系. 关键词： 颗粒物质 颗粒流 分子动力学模拟     

阴极电压脉冲占空比对钛合金微弧氧化膜特性的影响

利用自制多功能微弧氧化电源,在保持双极性电压脉冲幅度不变的条件下,研究了阴极电压脉冲占空比（d_c）对钛合金微弧氧化膜特性的影响.结果表明：阴、阳极的峰值电流随处理时间的变化分为几个不同阶段,各阶段的开始和结束时间与d_c密切相关.氧化膜主要由金红石和锐钛矿相TiO₂组成,金红石相TiO₂的相对含量在d_c=50%附近随d_c的 关键词： 微弧氧化 钛合金 占空比     

分子动力学模拟尺寸对纳米Cu颗粒等温晶化过程的影响

采用分子动力学方法和镶嵌原子势, 模拟了4000个Cu原子和13500个Cu原子(简称Cu₄₀₀₀和Cu₁₃₅₀₀)组成的纳米颗粒以及块体Cu的等温晶化过程. 通过对这些颗粒在晶化过程中结构和动力学行为的分析研究, 发现低温时, 不同尺寸的纳米Cu颗粒均出现多步晶化, 且晶化时间的分布曲线远比高温时范围大; 除了温度, 颗粒尺寸对晶化行为也有重要的影响, 尺寸越大, 晶化时间越长, 最终的晶化程度越高; 但是晶化时间随尺寸增大而增加的趋势不会一直持续, 发现存在一个临界尺寸r_c, 小于r_c时, 晶化时间随颗粒尺寸增大而增加, 大于r_c时,晶化时间随尺寸增大而减小.     

平面颗粒流的瓶颈效应及其与速度的关系

通过实验研究了二维传送带上圆片颗粒运动与开口大小及传送带速度的关系，发现当开口尺寸d固定时，存在临界速度vc，在v＜vc范围，流量Q随速度v线性增大，在vc处Q-v关系发生突变.改变颗粒运动速度v或改变开口尺寸d均有临界值，使颗粒流规律发生转变.这是由于开口尺寸或速度变化使颗粒间相互作用及颗粒流状态变化造成的. 关键词： 平面颗粒流 瓶颈效应     

Co(S1-xSex)2系统中的铁磁量子相变

针对Co(S_1-xSe_x)₂系统在x=0.11附近发生的铁磁金属到顺磁金属相变,制备了一系列不同Se替代浓度的多晶样品.通过对其结构和电阻率-温度ρ(T)关系的系统观测,结果发现,样品铁磁相变温度T_C随着Se替代浓度x值的增加,以(1-x)^1/2关系单调下降,其二级铁磁相变转变为一级相变 关键词： 量子相变 自旋量子涨落 1-xSe_x)₂')" href="#">Co(S_1-xSe_x)₂     

Frustration of antiferromagnetism in the t-t′-Hubbard model at weak coupling

The perfect-nesting instability towards antiferromagnetism of the Hubbard model is suppressed by next-nearest neighbor hopping t′. The asymptotic behavior of the critical coupling U_c(t′) at small t′ is calculated in dimensions d = 2,3, ∞ using Hartree theory; this yields the exact result at least in d > 2. The order of the transition is also determined. A region of stability of a metallic antiferromagnetic phase in d = 3 is identified.     

Jamming patterns in a two-dimensional hopper

We report experimental studies of jamming phenomenon of monodisperse metal disks falling through a two-dimensional hopper when the hopper opening is larger than three times the size of the disks. For each jamming event, the configuration of the arch formed at the hopper opening is studied. The cumulative distribution functionsf _d (X) for hoppers of opening sized are measured. (HereX is the horizontal component of the arch vector, which is defined as the displacement vector from the center of the first disk to the center of the last disk in the arch.) We found that the distribution off _d (X) can be collasped into a master curveG(X) = f _d (X)μ(d) that decays exponentially forX > 4. The scaling factorμ(d) is a decreasing function ofd and is approximately proportional to the jamming probability.     

Magnetic circular dichroism and absorption spectra of Cs2ZrCl6:Mo4+

Absorption and magnetic circular dichroism (M.C.D.) spectra of Mo⁴⁺ in Cs₂ZrCl₆ have been recorded under high resolution at liquid helium temperature over the range 20 000–38 000 cm^-1. All of the bands can be assigned to parity-allowed, ligand-to-metal charge-transfer transitions in both MoCl₆ ^2-(4d²) and an impurity ion which has been identified as MoOCl₅ ^2-(4d¹). The d¹ and d² systems are easily distinguished by the temperature dependence of their M.C.D. spectra, and detailed assignments are proposed for the latter. The absorption spectrum in the region 23 000–26 000 cm^-1 has recently been attributed to a d→d transition in the ion MoCl₆ ^-(4d¹) [1], but the M.C.D. spectrum proves this assignment to be incorrect, and we find no evidence for the presence of this singly-charged species. From the temperature dependence of the M.C.D. spectrum of MoCl₆ ^2-, we estimate the second-order spin-orbit splitting of the E and T ₂ components of the ³ T _1g ground state to be 27 ± 5 cm^-1 with E lowest. The electrostatic splitting of the ³ T _2u and ⁵ T _2u charge-transfer states arising from the t _1u (π + σ) →t _2g excitation is estimated to be close to 515 cm^-1.     

Factors influencing the density profiles of granular flux in a two-dimensional inclined channel

The two-dimensional dilute granular flow on a smooth incline bounded by rough sidewalls is investigated experimentally, and the transverse density profiles of granular flux (ζ = ρ v) across the channel are measured. The results show that the transverse density profiles of granular flux are symmetric about the channel center and that the density of granular flux near the boundary is clearly lower than that of the center. There is a critical width of channel W _c for the transition of the density of granular flux. The density of granular flux near the boundary decays with the increasing of inclination (sinθ) of the channel. Supported by the National Natural Science Foundation of China (Grant No. 10274071) and the Department of Education of Zhejiang Province (Grant Nos. 20050916 and y200804093)     

Time distribution and loss of scaling in granular flow

Two cellular automata models with directed mass flow and internal time scales are studied by numerical simulations. Relaxation rules are a combination of probabilistic critical height (probability of toppling p) and deterministic critical slope processes with internal correlation time t_c equal to the avalanche lifetime, in model A, and ,in model B. In both cases nonuniversal scaling properties of avalanche distributions are found for , where is related to directed percolation threshold in d=3. Distributions of avalanche durations for are studied in detail, exhibiting multifractal scaling behavior in model A, and finite size scaling behavior in model B, and scaling exponents are determined as a function of p. At a phase transition to noncritical steady state occurs. Due to difference in the relaxation mechanisms, avalanche statistics at approaches the parity conserving universality class in model A, and the mean-field universality class in model B. We also estimate roughness exponent at the transition. Received: 29 May 1998 / Revised: 8 September 1998 / Accepted: 10 September 1998     

Laser thermal response of a finite slab as a function of the laser pulse parameters

This paper deals with the problem of heating a finite slab using laser radiation in relation to the parameters characterizing the laser pulse, namely: q_max(W/m²), the maximum laser power density, t₀ the time interval required to reach q_max and t_d, the pulse time duration. The pulse shape q(t) is suggested in the form: q(t)=βq_max(t/t_d)(1-(t/t_d))exp-B(t-t₀/t_d), where β and B are parameters. Fitting with published experimental pulse [Ready JF. Effects due to absorption of laser radiation. J Appl Phys 1965;36:462–68] is made. Fourier series expansion technique is considered to solve the problem. The critical time required to initiate melting t_m is estimated for four metallic elements and five semiconductors, namely: Al, Cu, Ag, Au (aluminum, copper, silver, and gold), cadmium sulfide, germanium, silicon, alpha beryllium oxide, and silicon carbide. Five pulses with different characteristic parameters are considered.Computations revealed that the thermal response of the targets is highly affected by q_max and t_o, while the pulse time duration is less effective in determining the value of t_m. Moreover, it is revealed that the relation between t_m and the melting temperature for the same laser pulse is nonlinear for the considered targets under the indicated conditions.     

Electrical breakdown as an insulator-to-metal transition

The effect of dielectric strengthening under electrical breakdown in thin oxide films and other materials is discussed. The breakdown phenomenon is considered as an insulator-to-metal phase transition. The effect of strengthening is thus associated with the fact that no phase transition seems to be possible when the system size is decreased below a certain characteristic length d _c (it is a so-called ‘tachyon instability’). This dimension is estimated to be d _c?～?ξ, where ξ is the correlation length for metal–insulator transition.     

Nonlinear dynamics of a two-dimensional lattice

The dynamics of the nonlinear excitations in a two-dimensional (2D) φ⁴-diatomic lattice, with nonlinear on-site electron-phonon coupling at the polarizable ion site has been presented, without considering the self consistent phonon approximation. One of the major results obtained from our calculations is in the understanding of continuous structural phase transition, where we have obtained the minimum in soft mode frequency at a soft mode temperatureT _s (>T _c), not at critical temperatureT _c. This occurs due to the anisotropy of such 2D systems.     

Global nature of dilute-to-dense transition of granular flows in a 2D channel

The dilute-to-dense transition of granular flow of particle size d(0) is studied experimentally in a two-dimensional channel (width D) with confined exit (width d). Our results show that with fixed d and D there is a maximum inflow rate Q(c) above which the flow changes from dilute to dense and the outflow rate drops abruptly from Q(c) to a dense rate Q(d). A rescaled critical rate q(c) is found to be a function of a scaling variable lambda only: q(c) approximately F(lambda), where lambda identical with d/d(0) d/D-d. This form of lambda suggests that the dilute-to-dense transition is a global property of the flow, unlike the jamming transition which depends only on d/d(0). Furthermore, the transition is found to occur when the area fraction of particles near the exit exceeds a critical value which is close to 0.65+/-0.03.