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How popular is your paper? An empirical study of the citation distribution   总被引:39,自引:0,他引:39  
Numerical data for the distribution of citations are examined for: (i) papers published in 1981 in journals which are catalogued by the Institute for Scientific Information (783,339 papers) and (ii) 20 years of publications in Physical Review D, vols. 11-50 (24,296 papers). A Zipf plot of the number of citations to a given paper versus its citation rank appears to be consistent with a power-law dependence for leading rank papers, with exponent close to -1/2. This, in turn, suggests that the number of papers with x citations, N(x), has a large-x power law decay , with . Received: 12 May 1998 / Accepted: 12 May 1998  相似文献
Economic small-world behavior in weighted networks   总被引:24,自引:0,他引:24  
The small-world phenomenon has been already the subject of a huge variety of papers, showing its appeareance in a variety of systems. However, some big holes still remain to be filled, as the commonly adopted mathematical formulation is valid only for topological networks. In this paper we propose a generalization of the theory of small worlds based on two leading concepts, efficiency and cost, and valid also for weighted networks. Efficiency measures how well information propagates over the network, and cost measures how expensive it is to build a network. The combination of these factors leads us to introduce the concept of economic small worlds, that formalizes the idea of networks that are “cheap” to build, and nevertheless efficient in propagating information, both at global and local scale. In this way we provide an adequate tool to quantitatively analyze the behaviour of complex networks in the real world. Various complex systems are studied, ranging from the realm of neural networks, to social sciences, to communication and transportation networks. In each case, economic small worlds are found. Moreover, using the economic small-world framework, the construction principles of these networks can be quantitatively analyzed and compared, giving good insights on how efficiency and economy principles combine up to shape all these systems. Received 6 November 2002 / Received in final form 24 January 2003 Published online 1st April 2003 RID="a" ID="a"e-mail: latora@ct.infn.it  相似文献
Inverse cubic law for the distribution of stock price variations   总被引:10,自引:0,他引:10  
The probability distribution of stock price changes is studied by analyzing a database (the Trades and Quotes Database) documenting every trade for all stocks in three major US stock markets, for the two year period January 1994 - December 1995. A sample of 40 million data points is extracted, which is substantially larger than studied hitherto. We find an asymptotic power-law behavior for the cumulative distribution with an exponent , well outside the Lévy regime . Received: 23 April 1998 / Revised and Accepted: 24 April 1998  相似文献
斯特林型高频脉冲管制冷机的现状与样机研制   总被引:10,自引:0,他引:10  
首先介绍了国际上高频脉冲管制冷机的研究现状及未来发展趋势 ,表明高效高频脉冲管制冷机的产品和样机在国际上已日趋成熟。然后简要介绍了作者近几年在德国研制的高频脉冲管制冷机样机的性能和特点。作者获得的性能和国际上最好性能的比较表明 ,作者获得的性能结果已处于世界先进行列  相似文献
Hierarchical structure in financial markets   总被引:10,自引:0,他引:10  
I find a hierarchical arrangement of stocks traded in a financial market by investigating the daily time series of the logarithm of stock price. The topological space is a subdominant ultrametric space associated with a graph connecting the stocks of the portfolio analyzed. The graph is obtained starting from the matrix of correlation coefficient computed between all pairs of stocks of the portfolio by considering the synchronous time evolution of the difference of the logarithm of daily stock price. The hierarchical tree of the subdominant ultrametric space associated with the graph provides a meaningful economic taxonomy. Received 24 March 1999 and Received in final form 28 June 1999  相似文献
Thin multilayer films (Ge/Sb/Ge/Sb/Si substrate) have been irradiated with single nanosecond laser pulses (=193 nm). Real-Time Reflectivity (RTR) measurements have been used to follow the transformation in situ and cross-sectional transmission electron microscope analysis was used to study both the microstructure and the composition profile before and after irradiation. Melting and mixing are both found to nucleate at preferential sites in the upper Ge/Sb interface. During this process the film surface topography changes in a way not previously seen, and rippling of the film is observed due to lateral mass flow induced in the Sb layer underneath the surface, most probably arising from volume changes upon melting. For the highest irradiation energy densities, melting of the whole multilayer configuration takes place, the ripples are no longer observed, and following cooling and solidification, a mixed amorphous GeSb film is formed.  相似文献
90°反射式相位延迟器的设计   总被引:5,自引:2,他引:3  
铜蒸气激光反射镜在非正入射的时候 ,两个不同的偏振态之间会产生不同的相移。通过优化设计 ,在4 90~ 5 30nm之间p、s波获得了 90°的相移 ,同时也使反射率在 99 998%以上。Ag层的厚度对于相移不敏感 ,并且当其厚度大于一定值的时候 ,对反射率没有影响。根据误差分析 ,制备薄膜时其沉积速率精度控制在± 1%以下 ,在光谱范围内能达到± 15 2 8°的相移误差 ,相移均在 5 0 4nm处附近存在有一个收敛值。折射率的变化控制在± 1%以下 ,在光谱范围内能达到± 12 77°的相移误差。最外一层厚度变化± 1% ,其相移变化达到± 5 5°,2~ 5层和 9~ 16层对相移的影响也在 0 .5°之上 ,其余各层对相移影响非常的小。使用时的入射角控制在± 1°时 ,在光谱范围内能达到± 2 .86°的相移误差。在 5 30nm附近的波段对入射角不敏感。  相似文献
90°束转动环形非稳腔模场数值分析   总被引:4,自引:0,他引:4  
用矩阵方法导出了UR90(unstableringresonatorwith900167beamrotation)环形非稳腔的衍射积分方程,获得了腔的等价菲涅耳数Neq的关系公式,分析了反向模的抑制方法并给出反向模抑制镜的曲率半径,通过数值计算的方法阐明了腔模本征值与放大因子M的变化关系,当腔的放大因子M=1.2时,得到了腔的模场分布曲线。  相似文献
本文研究了用VA-90气态原子化装置与AA-670原子吸收分光光度计联用测定食品,饮料中的微量砷,结果表明:测定方法简单,快速,线性范围宽。本法的检出限为2ng/L^-1,回收率87.5%-115%,变异系数为5.4%-6.1%。  相似文献
Chow R  Blindt R  Chivers R  Povey M 《Ultrasonics》2003,41(8):595-604
Two investigations of the sonocrystallisation of ice are reported for solutions in the concentration range of 0-45 wt.% sucrose. The first, carried out at 20 kHz using a commercial sonicator shows clear evidence of the stimulation of primary nucleation, by increasing the temperature at which nucleation takes place. The nucleation temperature is also increased as the ultrasonic power output level or duty cycles are increased. The second uses a novel measurement cell which permits the direct visualisation of the crystallisation and cavitation with a light microscope while different levels of alternating pressures at a frequency of 67 kHz are applied to the sample, whose heating and cooling is carefully controlled and monitored. This provides confirmation of the fragmentation of ice dendrites growing in a sucrose solution to produce smaller new ice crystals (secondary nucleation). These observations show that the primary and secondary nucleation of ice are both possible during the sonocrystallisation of ice.  相似文献
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