Random-growth urban model with geographical fitness

This paper formulates a random-growth urban model with a notion of geographical fitness. Using techniques of complex-network theory, we study our system as a type of preferential-attachment model with fitness, and we analyze its macro behavior to clarify the properties of the city-size distributions it predicts. First, restricting the geographical fitness to take positive values and using a continuum approach, we show that the city-size distributions predicted by our model asymptotically approach Pareto distributions with coefficients greater than unity. Then, allowing the geographical fitness to take negative values, we perform local coefficient analysis to show that the predicted city-size distributions can deviate from Pareto distributions, as is often observed in actual city-size distributions. As a result, the model we propose can generate a generic class of city-size distributions, including but not limited to Pareto distributions. For applications to city-population projections, our simple model requires randomness only when new cities are created, not during their subsequent growth. This property leads to smooth trajectories of city population growth, in contrast to other models using Gibrat’s law. In addition, a discrete form of our dynamical equations can be used to estimate past city populations based on present-day data; this fact allows quantitative assessment of the performance of our model. Further study is needed to determine appropriate formulas for the geographical fitness.     

用户需求行为对互联网动力学整体特性的影响

由Internet构成的复杂网络的动力学特性主要受到用户需求行为的影响,具备时域的统计规律性. 通过对区域群体用户需求行为的时域实验统计分析,发现用户对Web网站的访问频度及其生成的二分网络的入度分布也呈现幂律分布和集聚现象,其幂指数介于1.7到1.8之间. 建立了虚拟资源网络VRN和物理拓扑网络PTN双层模型,分析了双层模型映射机理,并对网络用户需求行为进行建模. 虚拟资源网络VRN对物理拓扑网络PTN映射过程的不同机理,模拟了Internet资源网络到物理网络的不同影响模式. 幂律分布的用户需求特性会 关键词： 复杂网络 无标度拓扑 用户需求 相变     

Power-law statistics for avalanches in a martensitic transformation

We devise a two-dimensional model that mimics the recently observed power-law distributions for the amplitudes and durations of the acoustic emission signals observed during martensitic transformation [Vives et al., Phys. Rev. Lett. 72, 1694 (1994)]. We include a threshold mechanism, long-range interaction between the transformed domains, inertial effects, and dissipation arising due to the motion of the interface. The model exhibits thermal hysteresis and, more importantly, it shows that the energy is released in the form of avalanches with power-law distributions for their amplitudes and durations. Computer simulations also reveal morphological features similar to those observed in real systems.     

Study of multiparticle spikes in central 4.5A GeV/c C-Cu collisions

An analysis of local fluctuations, or spikes, is performed for charged particles produced in central C-Cu collisions at 4.5 GeV/c/nucleon. The distributions of spike centers and the maximum density distributions are investigated for different narrow pseudorapidity windows to search for multiparticle dynamical correlations. Two peaks over statistical background are observed in the spike-center distributions with the structure similar to that expected from the coherent gluon radiation model and recently found in hadronic interactions. The dynamical contribution to maximum density fluctuations is obtained to be hidden by statistical correlations, although behavior of the distributions shows qualitative agreement with that from the one-dimensional intermittency model. The observed features of the two different approaches, coherent vs. stochastic, to the formation of the local dynamical fluctuations are discussed.     

高能核-核碰撞中粒子的(赝)快度分布与重叠柱模型

应用重叠柱模型描述了高能核-核碰撞中带电粒子的快度(或赝快度)分布. 对目前加速器上的固定靶实验而言, 观察到了相同相对强度的纵向流, 两个完全重叠的热化柱能够描述实验数据. 在更高能量范围(4A TeV以上), 观察到了更强的纵向流, 这时需要两个部分重叠的热化柱来描述实验数据. 用重叠柱模型计算得到的(赝)快度分布与1A GeV到100A TeV能区的实验结果符合.     

Formation of dense groups of particles in intermediate-energy nucleus-nucleus collisions

The formation of dense groups (fluctuations) of particles produced in the central CCu and MgMg collisions at the projectile momenta of, respectively, 4.5 and 4.3 GeV/c per nucleon is analyzed. The distributions of the maximum densities and of the centers of charged-particle fluctuations in pseudorapidity space are studied in searches for dynamical multiparticle correlations. The distributions of the centers show two peaks above the statistical background with a structure similar to that which is expected in the model of coherent gluon emission and which was observed in hadronic interactions. The charge independence of the distributions in question and an azimuthal isotropy of events involving pseudorapidity fluctuations are observed. The distributions of events with respect to the maximum density of fluctuations are governed primarily by the statistical contribution, although the behavior of the distributions in CCu collisions is in qualitative agreement with the prediction of the one-dimensional intermittency model. It is found that the resulting distributions are of a non-Poisson character both in CCu and in MgMg collisions. The results of this study indicate that, in describing local dynamical fluctuations in multiparticle production processes, the coherent and the stochastic approach supplement each other. The procedure employed in the analysis described here makes it possible to draw a direct comparison of the present results with the results of similar investigations of different reactions.     

Power-law connections: From Zipf to Heaps and beyond

In this paper we explore the asymptotic statistics of a general model of rank distributions in the large-ensemble limit; the construction of the general model is motivated by recent empirical studies of rank distributions. Applying Lorenzian, oligarchic, and Heapsian asymptotic analyses we establish a comprehensive set of closed-form results linking together rank distributions, probability distributions, oligarchy sizes, and innovation rates. In particular, the general results reveal the fundamental underlying connections between Zipf’s law, Pareto’s law, and Heaps’ law—three elemental empirical power-laws that are ubiquitously observed in the sciences.     

Semiotic dynamics in online social communities

A distributed classification paradigm known as collaborative tagging has been successfully deployed in large-scale web applications designed to manage and share diverse online resources. Users of these applications organize resources by associating with them freely chosen text labels, or tags. Here we regard tags as basic dynamical entities and study the semiotic dynamics underlying collaborative tagging. We collect data from a popular system and focus on tags associated with a given resource. We find that the frequencies of tags obey to a generalized Zipf’s law and show that a Yule–Simon process with memory can be used to explain the observed frequency distributions in terms of a simple model of user behavior     

Molecular dynamics driven by ultrashort laser pulses: Interference effects due to rescattering

A time-dependent Schrödinger equation is integrated numerically to investigate the dynamics of a model molecular system driven by a high-intensity ultrashort laser pulse. Two-dimensional photoelectron momentum distributions are analyzed. Highly nonmonotonic electron angular distributions are obtained that cannot be explained by diffraction in the double-well potential of a molecular ion. The nonmonotonicity is also demonstrated for atomic ionization and is attributed to the interference that occurs between components of an electron wave packet after its rescattering from the parent ion. An analytical model explaining the observed effects is developed.     

Reaction kinetic model of height selection in heteroepitaxial growth of quantum dots

A reaction kinetic model is proposed for height selection of heteroepitaxially growing nanometer-thick quantum dots. The model describes the growth by a set of rate equations for the combined size and height distributions of the dots. In addition to nucleation and growth, the model includes a coarse-grained conversion rate incorporating kinetics of height changes. With suitably chosen rate coefficients the model reproduces qualitatively the experimentally observed height-selected size distributions and their evolution. The results support the view that the height selection and the form of the size distribution both result from the oscillating energy barrier for the transformation of dots of different heights, and this transformation barrier is considerably larger in magnitude than oscillations in the electronic energy due to quantum well states in the dot.     

Characterizing char particle fragmentation during pulverized coal combustion

A particle population balance model was developed to predict the oxidation characteristics of an ensemble of char particles exposed to an environment in which their overall burning rates are controlled by the combined effects of oxygen diffusion through particle pores and chemical reactions (the zone II burning regime). The model allows for changes in particle size due to burning at the external surface, changes in particle apparent density due to internal burning at pore walls, and changes in the sizes and apparent densities of particles due to percolation type fragmentation. In percolation type fragmentation, fragments of all sizes less than that of the fragmenting particle are produced. The model follows the conversion of particles burning in a gaseous environment of specified temperature and oxygen content. The extent of conversion and particle size, apparent density, and temperature distributions are predicted in time.Experiments were performed in an entrained flow reactor to obtain the size and apparent density data needed to adjust model parameters. Pulverized Wyodak coal particles were injected into the reactor and char samples were extracted at selected residence times. The particle size distributions and apparent densities were measured for each sample extracted. The intrinsic chemical reactivity of the char to oxygen was also measured in experiments performed in a thermogravimetric analyzer. Data were used to adjust rate coefficients in a six-step reaction mechanism used to describe the oxidation process.Calculations made allowing for fragmentation with variations in the apparent densities of fragments yield the type of size, apparent density, and temperature distributions observed experimentally. These distributions broaden with increased char conversion in a manner that can only be predicted when fragmentation is accounted for with variations in fragment apparent density as well as size. The model also yields the type of ash size distributions observed experimentally.     

Monte Carlo simulation of electron avalanches and avalanche size distributions in methane

The simulation of electron avalanches and avalanche size distributions in methane is presented in this paper. A model for electron transport under the influence of a constant electric field based on the Monte Carlo method is described in detail. The model is verified and then used to simulate the avalanche development, to calculate the number of electrons in the avalanche (avalanche size), and to determine the avalanche size distribution. The simulated avalanche size distributions in methane are compared with the experimental results, and a good agreement is observed. The influence of inter‐electrode distance, pressure, and reduced electric field on the shape of the avalanche size distribution is discussed. The assumption from the literature that for a constant reduced electric field the shape of the reduced avalanche size distribution is independent of the mean size of the avalanche is confirmed for a wide range of experimental conditions. The simulations have shown that avalanche size distributions depend only on the reduced electric field, confirming the similarity principle.     

Scaling in the distribution of intertrade durations of Chinese stocks

The distribution of intertrade durations, defined as the waiting times between two consecutive transactions, is investigated based upon the limit order book data of 23 liquid Chinese stocks listed on the Shenzhen Stock Exchange in the whole year 2003. A scaling pattern is observed in the distributions of intertrade durations, where the empirical density functions of the normalized intertrade durations of all 23 stocks collapse onto a single curve. The scaling pattern is also observed in the intertrade duration distributions for filled and partially filled trades and in the conditional distributions. The ensemble distributions for all stocks are modeled by the Weibull and the Tsallis q-exponential distributions. Maximum likelihood estimation shows that the Weibull distribution outperforms the q-exponential for not-too-large intertrade durations which account for more than 98.5% of the data. Alternatively, nonlinear least-squares estimation selects the q-exponential as a better model, in which the optimization is conducted on the distance between empirical and theoretical values of the logarithmic probability densities. The distribution of intertrade durations is Weibull followed by a power-law tail with an asymptotic tail exponent close to 3.     

Distribution of backscattered intensity in the distorted Born approximation: Application to C-band SAR images of woodland

Abstract

The intensity distribution in synthetic aperture radar (SAR) images of woodland is known to depend upon imaging conditions. Whilst phenomenological models can be used to match observed backscatter distributions, a physical model is needed to explain their origins. Images of woodland obtained during airborne SAR trials are analysed and shown to exhibit non-exponential intensity distributions. Expressions are derived for the moments of the intensity distribution using discrete scattering models based on the Born and distorted Born approximations. The predictions of the Born approximation are such that, at all but extremely high resolutions, the intensity statistics reflect only fluctuations in the number of discrete scatterers in resolution cells. In the distorted Born approximation it is revealed that, even at modest resolutions, fluctuations in both number and cross section of objects can influence intensity distributions. This is shown to be a direct consequence of the incorporation of attenuation effects in the distorted Born model. The theory is applied to scattering from a model woodland canopy and shown to yield intensity moments in close agreement with observations. The consequences of the model for other scattering situations are discussed.     

Cluster growth poised on the edge of break-up: Size distributions with small exponent power-laws

In many naturally occurring growth processes, cluster size distributions of power-law form n(s)∝s^−τ with small exponents 0<τ<1 are observed. We suggest here that such distributions emerge naturally from cluster growth, where size dependent aggregation is counterbalanced by size dependent break-up. The model used in the study is a simple reaction kinetic model including only monomer-cluster processes. It is shown that under such conditions power-law size distributions with small exponents are obtained. Therefore, the results suggest that the ubiquity of small exponent power-law distributions is related to the growth process, where aggregation driven cluster growth is poised on the edge of cluster break-up.     

Au+Au重离子碰撞中5~200 GeV碰撞能量下的温度涨落与比热(英文)

使用多相输运（AMPT）模型来研究相对论重离子碰撞中强子物质的比热（C_V）与对撞能量的关系以及温度的高阶涨落,并将之与文献[PhysRevC.94.044901]实验数据的比热结果进行了比较。对经历相变的系统,比热（C_V）作为表征系统状态方程的热力学量,其值预期在临界点发散。而温度的高阶涨落对相变敏感,比热（CV）和温度的高阶涨落都是适于探测QCD相变和临界点的敏感探针。通过逐个事例的平均横动量（<p_T>）来提取有效温度T_eff,再通过粒子的有效温度T_eff的分布提取出了相应粒子的热容。通过有效温度（T_eff）的分布的高阶矩来计算温度的高阶涨落。发现AMPT模型中比热和温度的高阶矩都随温度单调递减。同时还发现在低碰撞能量时,实验数据的比热结果有随能量增加而有一个急速下降,与AMPT模型的走势显著不同。AMPT模型中没有QCD临界点,提供了一个无临界点的参考背景。AMPT模型的计算结果可与实验结果比较作为实验上寻找QCD临界点的参考。     

Hillock sizes after wet etching in silicon

The formation of two- and three-dimensional hillocks is regularly observed in Si(1 1 1) steps and Si(1 0 0) during wet etching. Frequently the resulting morphology consists of hillocks scattered on a landscape of limited roughness. Recently we proposed a mean field model (MFM) in which the observed hillock-and-valley pattern is possible under steady state if hillock etching is slightly faster than valley etching. This condition implies that hillock size distributions must be an exponential decreasing function. In this work, we report a systematic study of hillock size distributions of experimental morphologies obtained under different etchant concentrations in Si(1 0 0). We found that experimental hillock size distributions are in agreement with those predicted by the MFM.     

16O+232Th垒下全熔合裂变

利用裂变碎片的折叠角分布,从实验上实现了全熔合裂变和转移跟随裂变两种成份的区分.在此基础上测量了质心系能量72.61至80.11MeV ¹⁶O+²³²Th全熔合裂变截面和碎片角分布.包含靶核静态形变效应的耦合道模型计算与实验激发曲线一致.然而,裂变统计理论无法解释实验上观察到的全熔合裂变碎片角分布.而鞍点模型与断点模型的理论预言有较明显的差别.     

Self-Organization of Topology and Weight Dynamics on Networks from Merging and Regeneration

We propose a model of weighted networks in which the structural evolution is coupled with weight dynamics. Based on a simple merging and regeneration process, the model gives powel-law distributions of degree, strength and weight, as observed in many real networks. It should be emphasized that, in our model, the nontrivial degree-strength correlation can be reproduced and in agreement with empirical data. Moreover, the size-growing evolution model is also presented to meet the properties of real-world systems.     

A superstatistical model of metastasis and cancer survival

We introduce a superstatistical model for the progression statistics of malignant cancer cells. The metastatic cascade is modeled as a complex nonequilibrium system with several macroscopic pathways and inverse-chi-square distributed parameters of the underlying Poisson processes. The predictions of the model are in excellent agreement with observed survival-time probability distributions of breast cancer patients.