一个含崩塌概率的一维沙堆模型

在一维沙堆模型中加入了崩塌概率,并用元胞自动机的方法进行计算机模拟,发现该模型具有自组织临界性,其临界指数α=1.50±0.02,并且还发现只有当崩塌概率处于0.05~0.98时,系统才体现出自组织临界性.另外,根据该模型的结果,解释了一维米粒堆实验中出现的自组织临界现象.     

二维格气模型中动力学相变与自组织临界现象

应用数值方法研究了一种具有封闭边界条件的Manna沙堆模型——格气模型.通过模拟，发现 系统存在一个临界密度ρ_c≈0.684…，得到了相应的临界指数α≈0.452±0.02 ，并且还发现只有在特定的粒子数密度范围，系统才能体现出自组织临界性. 关键词： 沙堆模型 格气模型 自组织临界现象     

一维 Frenkel-Kontorova(FK)模型原子链的相变研究

文章基于一维Frenkel-Kontorova模型,研究了边界原子的初始速度对原子链运动状态的影响,数值模拟结果表明:当v1时,原子处于"振荡区",当v₁2时,原子处于"混沌区",当v>v₂时,原子处于 "均匀区". 同时我们发现临界速度v₁和v₂随原子数目和垫底势高度的变化而变化. 关键词： Frenkel-Kontorova(FK)模型 相变     

一维元胞自动机随机交通流模型的理论分析与计算机实验

在一维局部作用元胞自动机(CA)交通流模型中，引入刹车噪声与产生、消失概率，得到一个完全随机的CA交通流模型．利用平衡自旋理论对该模型的研究表明，当P_in＝P_out≠0时，初始密度分布不影响系统的最终状态，求得ρ_ｔ＝0.5；当满足条件P_in＋P_b＝1与P_in＝P_out时，得到简单的“线性”模型，该模型在条件｜1－2P_in｜＝1下，表现出长程相关，导致严重交通“阻塞”的出现．理论结果与计算机实验一致． 关键词：     

外延Si1-xGex薄层中浅杂质的光热电离光谱

在Si_1-xGe_x合金外延层中利用光热电离光谱方法观察到了Si_0.92Ge_0.08合金中浅施主磷的2p_±及3p_±态的分立谱线，并根据有效质量近似理论估算了Si_0.92Ge_0.08合金中磷施主的电离能为45.51meV．与硅相比，由于合金的无序效应，Si_1-xGe_x合金的分立谱线发生明显展宽 关键词：     

等离子体屏蔽效应对原子能级和振子强度的影响

利用Debye模型,研究了等离子体屏蔽效应对热等离子体中原子能级和振子强度的影响.通过在MCDF模型中引入等离子体屏蔽效应,计算了MnXXII-BrXXII等11个类Be离子在等离子体环境下2s²—［2s_1/2,2p_1/2］₁和2s²—［2s_1/2,2p_3/2］₁跃迁的能级和振子强度.计算结果表明,等离子体屏蔽效应使得类Be离子2s²—［2s_1/2,2p_1/2］₁跃迁的激发能量增大,从而导致谱线蓝移现象;并且随着屏蔽效应的不断增强,蓝移的程度会逐渐加大.屏蔽效应对于2s²—［2s_1/2,2p_3/2］₁跃迁的振子强度也有类似的影响. 关键词： Debye 等离子体屏蔽 跃迁能级 振子强度     

用多步激发技术测定锶的(5p3/2ns)1及(5p3/2nd)3自电离谱

用分步激发方法及偏振光技术,测定了锶的(5p_3/2ns)₁(n=13-21)及(5p_3/2 nd)₃(n=13-24)各自电离谱,在低于5p_1/2电离限的(5p_3/2ns)₁态的实电子激发谱中,出现对应于(5p_1/2ns)₁及(5p_1/2nd)₁的畸变峰,畸变峰也出现于(5p_3/2nd)₃的实电子激发谱中。这说明(5p_jns)(5p_jnd)自电离系列间存在相互作用。 关键词：     

π凝聚态中的弱过程对中子星振动的阻尼效应

本文主要讨论了中子星内部发生从正常中子物态到π凝聚物态相变所产生的星体振动的阻尼,由弱作用反应u(p₁)→u(p₂)+e^-+v_e和u(p₁)+e^-→u(p₂)+v_e算出振动阻尼时标,并考虑了中子星内部的热演化。 关键词：     

Ba原子6pnd(J=1, 3)自电离光谱的实验研究

采用三台可调谐激光实施孤立实激发,分三步将处于基态的Ba原子激发到6p_1/2nd(J=1,3)和6p_3/2nd(J=1,3)自电离态上,获得了分别从6snd¹D₂(n=7—15)和6snd³D₂(n=7—12) 激发而得到的6p_1/2nd(J=1,3)和6p_3/2nd (J=1,3)自电离光谱,重点对主量子数n较低的自电离态进行了实验研究. 通过光谱的线形拟合得到了上述能级的位置和宽度等数据,进而获得了量子亏损和约化宽度等信息. 通过对不同系列的自电离光谱的分析和比较,详细讨论了这些自电离态的光谱特征及其复杂光谱结构的成因. 关键词： 孤立实激发 组态相互作用 自电离态     

Sr(5p1/2nd)3自电离谱的测定

用分步激发技术测得了Sr(5p_1/2nd)₃系列(n=11—24)自电离能级的位置及其有效量子数ν_1/2。对其与(5p_3/2nd)₃,系列间可能存在的组态相互作用,进行了初步探讨。 关键词：     

On the magnetically dilute Heisenberg and Ising ferromagnetics

The dependence of Curie point, θ _c, on magnetic concentration, p, is investigated, for the randomly dilute ferromagnetic Heisenberg spin 1/2 problem, by considering successive approximations to 1/χ=0, where χ is the susceptibility. This extends to the next approximation work previously reported by the authors. The simple cubic and body-centred cubic lattices are dealt with in detail. It is concluded that the behaviour of θ _c(p) for small p is not yet fully established: it is possible that the critical concentration p _c, below which there is no ferromagnetic behaviour, is greater for the Heisenberg model than for the corresponding Ising model (where it is the analogous site percolation problem critical probability).     

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     

Numerical analysis of In0.53Ga0.47As/InP single photon avalanche diodes

A rigorous theoretical model for In 0.53 Ga 0.47 As/InP single photon avalanche diode is utilized to investigate the dependences of single photon quantum efficiency and dark count probability on structure and operation condition.In the model,low field impact ionizations in charge and absorption layers are allowed,while avalanche breakdown can occur only in the multiplication layer.The origin of dark counts is discussed and the results indicate that the dominant mechanism that gives rise to dark counts depends on both device structure and operating condition.When the multiplication layer is thicker than a critical thickness or the temperature is higher than a critical value,generation-recombination in the absorption layer is the dominative mechanism;otherwise band-to-band tunneling in the multiplication layer dominates the dark counts.The thicknesses of charge and multiplication layers greatly affect the dark count and the peak single photon quantum efficiency and increasing the multiplication layer width may reduce the dark count probability and increase the peak single photon quantum efficiency.However,when the multiplication layer width exceeds 1 μm,the peak single photon quantum efficiency increases slowly and it is finally saturated at the quantum efficiency of the single photon avalanche diodes.     

Universality classes in the random-storage sandpile model

The avalanche statistics in a stochastic sandpile model where toppling takes place with a probability p is investigated. The limiting case p=1 corresponds to the Bak-Tang-Wiesenfeld (BTW) model with a deterministic toppling rule. Based on the moment analysis of the distribution of avalanche sizes we conclude that for 0相似文献   

Growth and addition in a herding model

A model of herding is introduced which is exceptionally simple, incorporating only two phenomena, growth and addition. At each time step either (i) with probability p the system grows through the introduction of a new agent or (ii) with probability q = 1 - p a free agent already in the system is added at random to a group of size k with rate A_k. Two versions of the model, A _k = k and A _k = 1, are solved and in both versions we find two different types of behaviour. When p > 1/2 all the moments of the distribution of group sizes are linear in time for large time and the group distribution is power-law. When p < 1/2 the system runs out of free agents in a finite time. Received 12 February 2002 Published online 9 July 2002     

非晶态Fe13Ni67.2P4.5B15.3合金的临界现象

本文报道非晶态Fe₁₃Ni_67.2P_4.5B_15.3合金的磁化强度与温度和磁场关系的测量结果。在居里温度附近样品的磁特性符合二级相变规律,得到临界指数β=0.39±0.02,γ=1.56±0.06,δ=5.20±0.1,样品的居里温度T_c=(180.4±0.2)K。在实验误差范围内,临界指数β,γ,δ满足γ=β(δ-1)关系,在168—192K温度范围,实验数据满足二级相变的磁状态方程。当T>270K时,样品顺磁磁化率服从居里-外斯定律,由居里-外斯常数c计算出有效顺磁磁矩P_eff=3.19 μ_B。 关键词：     

Long-range connections, quantum magnets and dilute contact processes

In this article, we briefly review the critical behaviour of a long-range percolation model in which any two sites are connected with a probability that falls off algebraically with the distance. The results of this percolation transition are used to describe the quantum phase transitions in a dilute transverse Ising model at the percolation threshold p_c of the long-range connected lattice. In the similar spirit, we propose a new model of a contact process defined on the same long-range diluted lattice and explore the transitions at p_c. The long-range nature of the percolation transition allows us to evaluate some critical exponents exactly in both the above models. Moreover, mean field theory is valid for a wide region of parameter space. In either case, the strength of Griffiths McCoy singularities are tunable as the range parameter is varied.     

Variational calculations on the energy levels of graphene quantum antidots

The critical properties of the two-dimensional Ising and Blume-Capel model on directedsmall-world lattices with quenched connectivity disorder are investigated. The disordered system is simulated by applying the Monte Carlo method with heat bath update algorithm and histogram re-weighting techniques. The critical temperature, as well as the critical exponents are obtained. For both models the critical parameters have been obtained for several values of the rewiring probability p. It is found that these disorder systems do not belong to the same universality class as two-dimensional ferromagnetic model on regular lattices. In particular, the Blume-Capel model, with zero crystal field interaction, on a directedsmall-world lattice presents a second-order phase transition for p < p _c , and a first-order phase transition for p > p _c , where p _c  ≈ 0.25. The critical exponents for p < p _c are different from those of the same model on a regular lattice, but are identical to the exponents of the Ising model on directedsmall-world lattice.     

Critical Thresholds and the Limit Distribution in the Bak-Sneppen Model

One of the key problems related to the Bak-Sneppen evolution model is to compute the limit distribution of the fitnesses in the stationary regime, as the size of the system tends to infinity. Simulations in [3, 1, 4] suggest that the one-dimensional limit marginal distribution is uniform on (p_c, 1), for some p_c 0.667. In this paper we define three critical thresholds related to avalanche characteristics. We prove that if these critical thresholds are the same and equal to some p_c (we can only prove that two of them are the same) then the limit distribution is the product of uniform distributions on (p_c, 1), and moreover p_c<0.75. Our proofs are based on a self-similar graphical representation of the avalanches.