Preferential Exchange: Strengthening Connections in Complex Networks

Many social, technological and biological interactions involve network relationships whose outcome intimately depends on the structure of the network and on the strengths of the connections. Yet, although much information is now available concerning the structure of many networks, the strengths are more difficult to measure. Here we show that, for one particular social network, notably the e-mail network, a suitable measure of the strength of the connections can be available. We also propose a simple mechanism, based on positive feedback and reciprocity, that can explain the observed behavior and that hints toward specific dynamics of formation and reinforcement of network connections. Network data from contexts different from social sciences indicate that power-law, and generally broad, distributions of the connection strength are ubiquitous, and the proposed mechanism has a wide range of applicability.Comment: 4 pages, 2 .eps figure

The Fermiac or Fermi’s Trolley

The Fermiac, known also as Fermi’s trolley or Monte Carlo trolley, is an analog computer used to determine the change in time of the neutron population in a nuclear device, via the Monte Carlo method. It was invented by Enrico Fermi and constructed by Percy King at Los Alamos in 1947, and used for about two years. A replica of the Fermiac was built at INFN mechanical workshops of Bologna in 2015, on behalf of the Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, thanks to the original drawings made available by Los Alamos National Laboratory (LANL). This reproduction of the Fermiac was put in use, and a simulation was developed

BCB Based Packaging for Low Actuation Voltage RF MEMS Devices

This paper outlines the issues related to RF MEMS packaging and low actuation voltage. An original approach is presented concerning the modeling of capacitive contacts using multiphysics simulation and advanced characterization. A similar approach is used concerning packaging development where multi-physics simulations are used to optimize the process. A devoted package architecture is proposed featuring very low loss at microwave range

Roughness and Finite Size Effect in the NYSE Stock-Price Fluctuations

We consider the roughness properties of NYSE (New York Stock Exchange) stock-price fluctuations. The statistical properties of the data are relatively homogeneous within the same day but the large jumps between different days prevent the extension of the analysis to large times. This leads to intrinsic finite size effects which alter the apparent Hurst (H) exponent. We show, by analytical methods, that finite size effects always lead to an enhancement of H. We then consider the effect of fat tails on the analysis of the roughness and show that the finite size effects are strongly enhanced by the fat tails. The non stationarity of the stock price dynamics also enhances the finite size effects which, in principle, can become important even in the asymptotic regime. We then compute the Hurst exponent for a set of stocks of the NYSE and argue that the interpretation of the value of H is highly ambiguous in view of the above results. Finally we propose an alternative determination of the roughness in terms of the fluctuations from moving averages with variable characteristic times. This permits to eliminate most of the previous problems and to characterize the roughness in useful way. In particular this approach corresponds to the automatic elimination of trends at any scale.Comment: 13 pages, 11 fugure

Exact Results for the Roughness of a Finite Size Random Walk

We consider the role of finite size effects on the value of the effective Hurst exponent H. This problem is motivated by the properties of the high frequency daily stock-prices. For a finite size random walk we derive some exact results based on Spitzer's identity. The conclusion is that finite size effects strongly enhance the value of H and the convergency to the asymptotic value (H=1/2) is rather slow. This result has a series of conceptual and practical implication which we discuss.Comment: 5 pages, 3 figure

Role of guanine nucleotides in the regulation of the Ras/cAMP pathway in Saccharomyces cerevisiae

AbstractThe CDC25 gene product is a guanine nucleotide exchange factor for Ras proteins in yeast. Recently it has been suggested that the intracellular levels of guanine nucleotides may influence the exchange reaction. To test this hypothesis we measured the levels of nucleotides in yeast cells under different growth conditions and the relative amount of Ras2-GTP. The intracellular GTP/GDP ratio was found to be very sensitive to growth conditions: the ratio is high, close to that of ATP/ADP during exponential growth, but it decreases rapidly before the beginning of stationary phase, and it drops further under starvation conditions. The addition of glucose to glucose-starved cells causes a fast increase of the GTP/GDP ratio. The relative amount of Ras2-GTP changes in a parallel way suggesting that there is a correlation with the cytosolic GTP/GDP ratio. In addition ‘in vitro’ mixed-nucleotide exchange experiments done on purified Ras2 protein demonstrated that the GTP and GDP concentrations influence the extent of Ras2-GTP loading giving further support to their possible regulatory role

From RF-Microsystem Technology to RF-Nanotechnology

The RF microsystem technology is believed to introduce a paradigm switch in the wireless revolution. Although only few companies are to date doing successful business with RF-MEMS, and on a case-by-case basis, important issues need yet to be addressed in order to maximize yield and performance stability and hence, outperform alternative competitive technologies (e.g. ferroelectric, SoS, SOI,…). Namely the behavior instability associated to: 1) internal stresses of the free standing thin layers (metal and/or dielectric) and 2) the mechanical contact degradation, be it ohmic or capacitive, which may occur due to low forces, on small areas, and while handling severe current densities.The investigation and understanding of these complex scenario, has been the core of theoretical and experimental investigations carried out in the framework of the research activity that will be presented here. The reported results encompass activities which go from coupled physics (multiphysics) modeling, to the development of experimental platforms intended to tackles the underlying physics of failure. Several original findings on RF-MEMS reliability in particular with respect to the major failure mechanisms such as dielectric charging, metal contact degradation and thermal induced phenomena have been obtained. The original use of advanced experimental setup (surface scanning microscopy, light interferometer profilometry) has allowed the definition of innovative methodology capable to isolate and separately tackle the different degradation phenomena under arbitrary working conditions. This has finally permitted on the one hand to shed some light on possible optimization (e.g. packaging) conditions, and on the other to explore the limits of microsystem technology down to the nanoscale. At nanoscale indeed many phenomena take place and can be exploited to either enhance conventional functionalities and performances (e.g. miniaturization, speed or frequency) or introduce new ones (e.g. ballistic transport). At nanoscale, moreover, many phenomena exhibit their most interesting properties in the RF spectrum (e.g. micromechanical resonances). Owing to the fact that today’s minimum manufacturable features have sizes comparable with the fundamental technological limits (e.g. surface roughness, metal grain size, …), the next generation of smart systems requires a switching paradigm on how new miniaturized components are conceived and fabricated. In fact endowed by superior electrical and mechanical performances, novel nanostructured materials (e.g. carbon based, as carbon nanotube (CNT) and graphene) may provide an answer to this endeavor. Extensively studied in the DC and in the optical range, the studies engaged in LAAS have been among the first to target microwave and millimiterwave transport properties in carbon-based material paving the way toward RF nanodevices. Preliminary modeling study performed on original test structures have highlighted the possibility to implement novel functionalities such as the coupling between the electromagnetic (RF) and microelectromechanical energy in vibrating CNT (toward the nanoradio) or the high speed detection based on ballistic transport in graphene three-terminal junction (TTJ). At the same time these study have contributed to identify the several challenges still laying ahead such as the development of adequate design and modeling tools (ballistic/diffusive, multiphysics and large scale factor) and practical implementation issues such as the effects of material quality and graphene-metal contact on the electrical transport. These subjects are the focus of presently on-going and future research activities and may represent a cornerstone of future wireless applications from microwave up to the THz range

DC and radio-frequency transmission characteristics of double-walled carbon nanotubes-based ink

In this paper, double-walled carbon nanotubes (DWNTs) network layers were patterned using inkjet transfer printing. The remarkable conductive characteristics of carbon nanotubes (CNTs) are considered as promising candidates for transmission line as well as microelectronic interconnects of an arbitrary pattern. In this work, the DWNTs were prepared by the catalytic chemical vapor deposition process, oxidized and dispersed in ethylene glycol solution. The DWNTs networks were deposited between electrodes contact and then characterized at DC through current-voltage measurements, low frequency, and high frequency by scattering parameters measurements from 40 MHz up to 40 GHz through a vector network analyzer. By varying the number of inkjet overwrites, the results confirm that the DC resistance of DWNTs networks can be varied according to their number and that furthermore the networks preserve ohmic characteristics up to 100 MHz. The microwave transmission parameters were obtained from the measured S-parameter data. An algorithm is developed to calculate the propagation constant "γ", attenuation constant "α" in order to show the frequency dependence of the equivalent resistance of DWNTs networks, which decreases with increasing frequency

Analysis of the Secondary Structure of the Catalytic Domain of Mouse Ras Exchange Factor CDC25Mm

The minimal active domain GEF domain. of the mouse Ras exchange factor CDC25Mm was purified to homogeneity from recombinant Escherichia coli culture. The 256 amino acids polypeptide shows high activity in vitro and forms a stable complex with H-ras p21 in absence of guanine nucleotides. Circular dichroism CD. spectra in the far UV region indicate that this domain is highly structured with a high content of a-helix 42%.. Near UV CD spectra evidenced good signal due to phenylalanine and tyrosine while a poor contribution was elicited by the three tryptophan residues contained in this domain. The tryptophan fluorescence signal was scarcely affected by denaturation of the protein or by formation of the binary complex with H-ras p21, suggesting that the Trp residues, which are well conserved in the GEF domain of several Ras-exchange factors, were exposed to the surface of the protein and they are not most probably directly involved in the interaction with Ras proteins. q1998 Elsevier Science B.