Rheological properties vs Local Dynamics in model disordered materials at Low Temperature

We study the rheological response at low temperature of a sheared model disordered material as a function of the bond rigidity. We find that the flow curves follow a Herschel-Bulkley law, whatever is the bond rigidity, with an exponent close to 0.5. Interestingly, the apparent viscosity can be related to a single relevant time scale $t_{rel}$, suggesting a strong connection between the local dynamics and the global mechanical behaviour. We propose a model based on the competition between the nucleation and the avalanche-like propagation of spatial strain heterogeneities. This model can explain the Herschel-Bulkley exponent on the basis of the size dependence of the heterogeneities on the shear rate.Comment: 9 pages, 7 figure

Dynamic exponent in Extremal models of Pinning

The depinning transition of a front moving in a time-independent random potential is studied. The temporal development of the overall roughness w(L,t) of an initially flat front, $w(t)\propto t^\beta$, is the classical means to have access to the dynamic exponent. However, in the case of front propagation in quenched disorder via extremal dynamics, we show that the initial increase in front roughness implies an extra dependence over the system size which comes from the fact that the activity is essentially localized in a narrow region of space. We propose an analytic expression for the $\beta$ exponent and confirm this for different models (crack front propagation, Edwards-Wilkinson model in a quenched noise, ...).Comment: RevTex, 3 figures .ep

Transverse and longitudinal vibrations in amorphous silicon

We show that harmonic vibrations in amorphous silicon can be decomposed to transverse and longitudinal components in all frequency range even in the absence of the well defined wave vector ${\bf q}$. For this purpose we define the transverse component of the eigenvector with given $\omega$ as a component, which does not change the volumes of Voronoi cells around atoms. The longitudinal component is the remaining orthogonal component. We have found the longitudinal and transverse components of the vibrational density of states for numerical model of amorphous silicon. The vibrations are mostly transverse below 7 THz and above 15 THz. In the frequency interval in between the vibrations have a longitudinal nature. Just this sudden transformation of vibrations at 7 THz from almost transverse to almost longitudinal ones explains the prominent peak in the diffusivity of the amorphous silicon just above 7 THz.Comment: 6 pages, 3 figure

Raman spectroscopy of femtosecond multi-pulse irradiation of vitreous silica: experiment and simulation

We report an experimental and numerical study of femtosecond multi-pulse laser-induced densification in vitreous silica (v-SiO2) and its signature in Raman spectra. We compare the experimental findings to recently developed molecular dynamics (MD) approach accounting for bond-breaking due to laser irradiation, together with a dynamical matrix approach and bond polarizability model based on first-principle calculations for the estimation of Raman spectra. We observe two stages of the laser-induced densification and Raman spectrum evolution: growth during several hundreds of pulses followed by further saturation. At the medium-range, the network connectivity change in v-SiO2 is expressed in reduction of major ring fractions leading to more compacted structure. With the help of Sen & Thorpe model, we also study the short-range order transformation and derive the interbonding Si-O-Si angle change from the Raman measurements. Experimental findings are in excellent agreement with our MD simulations, and, hence, support bond-breaking mechanism of laser-induced densification. Thus, our modeling explains well the laser-induced changes both in the short-range order caused by the appearance of Si-coordination defects and medium-range order connected to evolution of the ring distribution. Finally, our findings disclose similarities between sheared-, permanently-densified- and laser-induced-glass and suggest interesting future experiment in order to clarify the impact of the thermo-mechanical history on glasses under shear, cold- and hot-compression, and laser-induced densification

The non dissipative damping of the Rabi oscillations as a "which-path" information

Rabi oscillations may be viewed as an interference phenomenon due to a coherent superposition of different quantum paths, like in the Young's two-slit experiment. The inclusion of the atomic external variables causes a non dissipative damping of the Rabi oscillations. More generally, the atomic translational dynamics induces damping in the correlation functions which describe non classical behaviors of the field and internal atomic variables, leading to the separability of these two subsystems. We discuss on the possibility of interpreting this intrinsic decoherence as a "which-way" information effect and we apply to this case a quantitative analysis of the complementarity relation as introduced by Englert [Phys. Rev. Lett. \textbf{77}, 2154 (1996)].Comment: 5 pages, 2 figure

Particle displacements in the elastic deformation of amorphous materials: local fluctuations vs. non-affine field

We study the local disorder in the deformation of amorphous materials by decomposing the particle displacements into a continuous, inhomogeneous field and the corresponding fluctuations. We compare these fields to the commonly used non-affine displacements in an elastically deformed 2D Lennard-Jones glass. Unlike the non-affine field, the fluctuations are very localized, and exhibit a much smaller (and system size independent) correlation length, on the order of a particle diameter, supporting the applicability of the notion of local "defects" to such materials. We propose a scalar "noise" field to characterize the fluctuations, as an additional field for extended continuum models, e.g., to describe the localized irreversible events observed during plastic deformation.Comment: Minor corrections to match the published versio

Antiviral RNA Interference against Orsay Virus Is neither Systemic nor Transgenerational in Caenorhabditis elegans.

UNLABELLED: Antiviral RNA-mediated silencing (RNA interference [RNAi]) acts as a powerful innate immunity defense in plants, invertebrates, and mammals. In Caenorhabditis elegans, RNAi is systemic; i.e., RNAi silencing signals can move between cells and tissues. Furthermore, RNAi effects can be inherited transgenerationally and may last for many generations. Neither the biological relevance of systemic RNAi nor transgenerational RNAi is currently understood. Here we examined the role of both pathways in the protection of C. elegans from viral infection. We studied the Orsay virus, a positive-strand RNA virus related to Nodaviridae and the first and only virus known to infect C. elegans. Immunity to Orsay virus infection requires the RNAi pathway. Surprisingly, we found that genes required for systemic or transgenerational RNAi did not have a role in antiviral defense. Furthermore, we found that Orsay virus infection did not elicit a systemic RNAi response even when a target for RNAi was provided by using transgenes. Finally, we show that viral siRNAs, the effectors of RNAi, are not inherited to a level that provides any significant resistance to viral infection in the next generation. We conclude that systemic or transgenerational RNAi does not play a role in the defense against natural Orsay virus infection. Furthermore, our data suggest that there is a qualitative difference between experimental RNAi and antiviral RNAi. Our data are consistent with a model of systemic and transgenerational RNAi that requires a nuclear or germ line component that is lacking in almost all RNA virus infections. IMPORTANCE: Since its discovery in Caenorhabditis elegans, RNAi has proven a valuable scientific tool in many organisms. In C. elegans, exogenous RNAi spreads throughout the organism and can be passed between generations; however, there has been controversy as to the endogenous role(s) that the RNAi pathway plays. One endogenous role for which spreading both within the infected organism and between generations would be advantageous is a role in viral defense. In plants, antiviral RNAi is systemic and the spread of RNAi between cells provides protection against subsequent viral infection. Here we investigated this by using the only naturally occurring virus known to infect C. elegans, Orsay virus, and surprisingly found that, in contrast to the exogenous RNAi pathway, the antiviral RNAi response targeted against this virus does not spread systemically throughout the organism and cannot be passed between generations. These results suggest that there are differences between the two pathways that remain to be discovered

Risque-Pays et Investissements Directs Etrangers au Benin

The purpose of this paper is to analyze the impact of country risk on the entry of foreign direct investment into Benin For this purpose at least one variable of each country risk component political risk economic risk and financial risk has been taken into account in the model From the Hendry-based Error Correction Model ECM over the period from 1980 to 2015 the results from the econometric estimation suggest that FDI inflow is positively explained by the rate of economic growth and the rate while foreign debt and corruption do not favor the growth of FDI in Benin For the country to become a pole of attractiveness of FDI it is imperative to clean up the institutional framework in the field of political right to undertake economic reforms and to work for a real financial independence to ensure its solvency and self-financin

Collective Behavior of Asperities in Dry Friction at Small Velocities

We investigate a simple model of dry friction based on extremal dynamics of asperities. At small velocities, correlations develop between the asperities, whose range becomes infinite in the limit of infinitely slow driving, where the system is self-organized critical. This collective phenomenon leads to effective aging of the asperities and results in velocity dependence of the friction force in the form $F\sim 1- \exp(-1/v)$.Comment: 7 pages, 8 figures, revtex, submitted to Phys. Rev.