From natural hazards to outer space and to Plan9

International audienceAn information system for scholarly work on natural hazards calls for the design of a computer system for transmission of information over very long periods, and for traceability. An abstract analysis shows that these requirements are dual to the fundamental question of assisting the cognitive activity of a user using external memories, which reaches a very general scope. The solutions should be implemented at the operating system level, mainly the file system, and Plan9's file systems and other properties make it the soundest base for our work. We present our road-map for development

Complete pressure dependent phase diagrams for SrFe2As2 and BaFe2As2

The temperature dependent electrical resistivity of single crystalline SrFe2As2 and BaFe2As2 has been measured in a liquid medium, modified Bridgman anvil cell for pressures in excess of 75 kbar. These data allow for the determination of the pressure dependence of the higher temperature, structural / antiferromagnetic phase transitions as well as the lower temperature superconducting phase transition. For both compounds the ambient pressure, higher temperature structural / antiferromagnetic phase transition can be fully suppressed with a dome-like region of zero resistivity found to be centered about its critical pressure. Indeed, qualitatively, the temperature dependence of the resistivity curves closest to the critical pressures are the closest to linear, consistent with possible quantum criticality. For pressures significantly higher than the critical pressure the zero resistivity state is suppressed and the low temperature resistivity curves asymptotically approach a universal, low temperature manifold. These results are consistent with the hypothesis that correlations / fluctuations associated with the ambient-pressure, high-temperature, tetragonal phase have to be brought to low enough temperature to allow superconductivity, but if too fully suppressed can lead to the loss of the superconducting state

High shock release in ultrafast laser irradiated metals: Scenario for material ejection

We present one-dimensional numerical simulations describing the behavior of solid matter exposed to subpicosecond near infrared pulsed laser radiation. We point out to the role of strong isochoric heating as a mechanism for producing highly non-equilibrium thermodynamic states. In the case of metals, the conditions of material ejection from the surface are discussed in a hydrodynamic context, allowing correlation of the thermodynamic features with ablation mechanisms. A convenient synthetic representation of the thermodynamic processes is presented, emphasizing different competitive pathways of material ejection. Based on the study of the relaxation and cooling processes which constrain the system to follow original thermodynamic paths, we establish that the metal surface can exhibit several kinds of phase evolution which can result in phase explosion or fragmentation. An estimation of the amount of material exceeding the specific energy required for melting is reported for copper and aluminum and a theoretical value of the limit-size of the recast material after ultrashort laser irradiation is determined. Ablation by mechanical fragmentation is also analysed and compared to experimental data for aluminum subjected to high tensile pressures and ultrafast loading rates. Spallation is expected to occur at the rear surface of the aluminum foils and a comparison with simulation results can determine a spall strength value related to high strain rates

The UN in the lab

We consider two alternatives to inaction for governments combating terrorism, which we term Defense and Prevention. Defense consists of investing in resources that reduce the impact of an attack, and generates a negative externality to other governments, making their countries a more attractive objective for terrorists. In contrast, Prevention, which consists of investing in resources that reduce the ability of the terrorist organization to mount an attack, creates a positive externality by reducing the overall threat of terrorism for all. This interaction is captured using a simple 3×3 “Nested Prisoner’s Dilemma” game, with a single Nash equilibrium where both countries choose Defense. Due to the structure of this interaction, countries can benefit from coordination of policy choices, and international institutions (such as the UN) can be utilized to facilitate coordination by implementing agreements to share the burden of Prevention. We introduce an institution that implements a burden-sharing policy for Prevention, and investigate experimentally whether subjects coordinate on a cooperative strategy more frequently under different levels of cost sharing. In all treatments, burden sharing leaves the Prisoner’s Dilemma structure and Nash equilibrium of the game unchanged. We compare three levels of burden sharing to a baseline in a between-subjects design, and find that burden sharing generates a non-linear effect on the choice of the efficient Prevention strategy and overall performance. Only an institution supporting a high level of mandatory burden sharing generates a significant improvement in the use of the Prevention strategy

Similarities between structural distortions under pressure and chemical doping in superconducting BaFe2As2

The discovery of a new family of high Tc materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent, for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked, however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal to orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behavior under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2

Transient optical response of ultrafast nonequilibrium excited metals: Effects of electron-electron contribution to collisional absorption

Approaching energy coupling in laser-irradiated metals, we point out the role of electron-electron collision as an efficient control factor for ultrafast optical absorption. The high degree of laser-induced electron-ion nonequilibrium drives a complex absorption pattern with consequences on the transient optical properties. Consequently, high electronic temperatures determine largely the collision frequency and establish a transition between absorptive regimes in solid and plasma phases. In particular, taking into account umklapp electron-electron collisions, we performed hydrodynamic simulations of the laser-matter interaction to calculate laser energy deposition during the electron-ion nonequilibrium stage and subsequent matter transformation phases. We observe strong correlations between optical and thermodynamic properties according to the experimental situations. A suitable connection between solid and plasma regimes is chosen in accordance with models that describe the behavior in extreme, asymptotic regimes. The proposed approach describes as well situations encountered in pump-probe types of experiments, where the state of matter is probed after initial excitation. Comparison with experimental measurements shows simulation results which are sufficiently accurate to interpret the observed material behavior. A numerical probe is proposed to analyze the transient optical properties of matter exposed to ultrashort pulsed laser irradiation at moderate and high intensities. Various thermodynamic states are assigned to the observed optical variation. Qualitative indications of the amount of energy coupled in the irradiated targets are obtained. Keywords: ultrafast absorption ; umklapp electron-electron collision ; collisional absorption ; laser-matter interactio

Pressure versus concentration tuning of the superconductivity in Ba(Fe(1-x)Cox)2As2

In the iron arsenide compound BaFe2As2, superconductivity can be induced either by a variation of its chemical composition, e.g., by replacing Fe with Co, or by a reduction of the unit-cell volume through the application of hydrostatic pressure p. In contrast to chemical substitutions, pressure is expected to introduce no additional disorder into the lattice. We compare the two routes to superconductivity by measuring the p dependence of the superconducting transition temperature Tc of Ba(Fe(1-x)Cox)2As2 single crystals with different Co content x. We find that Tc(p) of underdoped and overdoped samples increases and decreases, respectively, tracking quantitatively the Tc(x) dependence. To clarify to which extent the superconductivity relies on distinct structural features we analyze the crystal structure as a function of x and compare the results with that of BaFe2As2 under pressure.Comment: 14 pages, 4 figures, to be published in JPSJ Vol. 79 No. 12. The copyright is held by The Physical Society of Japa

Market Power and Collusion on Interconnection Phone Market in Tunisia : What Lessons from International Experiences

We try in this paper to characterize the state of mobile phone market in Tunisia. Our study is based on a survey of foreign experience (Europe) in detecting collusive behavior and a comparison of the critical threshold of collusion between operators in developing countries like Tunisia. The market power is estimated based on the work of Parker Roller (1997) and the assumption of "Balanced Calling Pattern". We use then the model of Friedman (1971) to compare the critical threshold of collusion. We show that the "conduct parameter" measuring the intensity of competition is not null during the period 1993-2011. Results show also that collusion is easier on the Tunisian market that on the Algerian, Jordanian, or Moroccan one