RECERCAT - Física Fonamental
http://www.recercat.cat:80/handle/2072/48815
Tue, 23 Sep 2014 18:47:46 GMT2014-09-23T18:47:46ZThe Channel Imagehttp://www.recercat.cat:80/bitstream/id/34146/
http://www.recercat.cat:80/handle/2072/48815
Nature of the Epidemic Threshold for the Susceptible-Infected-Susceptible Dynamics in Networks
http://www.recercat.cat:80/handle/2072/227863
Nature of the Epidemic Threshold for the Susceptible-Infected-Susceptible Dynamics in Networks
Boguñá, Marián; Castellano, Claudio; Pastor-Satorras, R. (Romualdo), 1967-
We develop an analytical approach to the susceptible-infected-susceptible epidemic model that allows us to unravel the true origin of the absence of an epidemic threshold in heterogeneous networks. We find that a delicate balance between the number of high degree nodes in the network and the topological distance between them dictates the existence or absence of such a threshold. In particular, small-world random networks with a degree distribution decaying slower than an exponential have a vanishing epidemic threshold in the thermodynamic limit.
http://www.recercat.cat:80/handle/2072/227863Shifted loops and coercivity from field imprinted high energy barriers in ferritin and ferrihydrite nanoparticles
http://www.recercat.cat:80/handle/2072/238226
Shifted loops and coercivity from field imprinted high energy barriers in ferritin and ferrihydrite nanoparticles
Silva, N. J. O.; Amaral, V. S.; Urtizberea, A.; Bustamante, R.; Millán, A.; Palacio, F.; Kampert, E.; Zeitler, U.; Brion, Sophie de; Iglesias, Òscar; Labarta, Amílcar
We show that the coercive field in ferritin and ferrihydrite depends on the maximum magnetic field in a hysteresis loop and that coercivity and loop shifts depend both on the maximum and cooling fields. In the case of ferritin, we show that the time dependence of the magnetization also depends on the maximum and previous cooling fields. This behavior is associated to changes in the intraparticle energy barriers imprinted by these fields. Accordingly, the dependence of the coercive and loop-shift fields with the maximum field in ferritin and ferrihydrite can be described within the frame of a uniform-rotation model considering a dependence of the energy barrier with the maximum and the cooling fields.
http://www.recercat.cat:80/handle/2072/238226First-passage and escape problems in the Feller process
http://www.recercat.cat:80/handle/2072/238910
First-passage and escape problems in the Feller process
Masoliver, Jaume, 1951-; Perelló, Josep, 1974-
The Feller process is an one-dimensional diffusion process with linear drift and state-dependent diffusion coefficient vanishing at the origin. The process is positive definite and it is this property along with its linear character that have made Feller process a convenient candidate for the modeling of a number of phenomena ranging from single-neuron firing to volatility of financial assets. While general properties of the process have long been well known, less known are properties related to level crossing such as the first-passage and the escape problems. In this work we thoroughly address these questions.
http://www.recercat.cat:80/handle/2072/238910Magnetic properties of an Fe/Cu granular multilayer
http://www.recercat.cat:80/handle/2072/202651
Magnetic properties of an Fe/Cu granular multilayer
Rubistein, Mark; Tejada Palacios, Javier; Zhang, Xixiang
A Cu/Fe granular film, formed from a multilayer film and composed of particles of Fe imbedded in Cu, has had several of its important properties investigated. Measurements include ferromagentic resonance, magnetoresistance, Mössbauer effect, magnetic viscosity, and magnetization. The two‐phase behavior of these immiscible alloy systems, and the effect of multilayering on the shape of the magnetic precipitates, can explain some of these properties. An explanation of the ferromagnetic resonance line shape is proffered. An extraordinary macroscopic quantum tunneling effect is found to govern the magnetic relaxation at the lowest temperatures.
http://www.recercat.cat:80/handle/2072/202651Excitation modes of vortices in submicron magnetic disks
http://www.recercat.cat:80/handle/2072/225214
Excitation modes of vortices in submicron magnetic disks
Zarzuela, R.; Chudnovsky, Eugene M., 1948-; Tejada Palacios, Javier
Classical and quantum theory of spin waves in the vortex state of a mesoscopic submicron magnetic disk have been developed with account of the finite mass density of the vortex. Oscillations of the vortex core resemble oscillations of a charged string in a potential well in the presence of the magnetic field. A conventional gyrotropic frequency appears as a gap in the spectrum of spin waves of the vortex. The mass of the vortex has been computed, and the result agrees with experimental findings. The finite vortex mass generates a high-frequency branch of spin waves. The effects of an external magnetic field and dissipation have been addressed.
http://www.recercat.cat:80/handle/2072/225214Coarse-graining and thermodynamics in far-from-equilibrium systems
http://www.recercat.cat:80/handle/2072/226990
Coarse-graining and thermodynamics in far-from-equilibrium systems
Rubí Capaceti, José Miguel; Pérez Madrid, Agustín
Lying at the core of statistical physics is the need to reduce the number of degrees of freedom in a system. Coarse-graining is a frequently-used procedure to bridge molecular modeling with experiments. In equilibrium systems, this task can be readily performed; however in systems outside equilibrium, a possible lack of equilibration of the eliminated degrees of freedom may lead to incomplete or even misleading descriptions. Here, we present some examples showing how an improper coarse-graining procedure may result in linear approaches to nonlinear processes, miscalculations of activation rates and violations of the fluctuation-dissipation theorem.
http://www.recercat.cat:80/handle/2072/226990Force Spectroscopy with Dual-Trap Optical Tweezers: Molecular Stiffness Measurements and Coupled Fluctuations Analysis
http://www.recercat.cat:80/handle/2072/227928
Force Spectroscopy with Dual-Trap Optical Tweezers: Molecular Stiffness Measurements and Coupled Fluctuations Analysis
Ribezzi-Crivellari, M.; Ritort Farran, Fèlix
ABSTRACT Dual-trap optical tweezers are often used in high-resolution measurements in single-molecule biophysics. Such measurements can be hindered by the presence of extraneous noise sources, the most prominent of which is the coupling of fluctuations along different spatial directions, which may affect any optical tweezers setup. In this article, we analyze, both from the theoretical and the experimental points of view, the most common source for these couplings in dual-trap optical-tweezers setups: the misalignment of traps and tether. We give criteria to distinguish different kinds of misalignment, to estimate their quantitative relevance and to include them in the data analysis. The experimental data is obtained in a, to our knowledge, novel dual-trap optical-tweezers setup that directly measures forces. In the case in which misalignment is negligible, we provide a method to measure the stiffness of traps and tether based on variance analysis. This method can be seen as a calibration technique valid beyond the linear trap region. Our analysis is then employed to measure the persistence length of dsDNA tethers of three different lengths spanning two orders of magnitude. The effective persistence length of such tethers is shown to decrease with the contour length, in accordance with previous studies.
http://www.recercat.cat:80/handle/2072/227928Local thermodynamics and the generalized Gibbs-Duhem equation in systems with long-range interactions
http://www.recercat.cat:80/handle/2072/227864
Local thermodynamics and the generalized Gibbs-Duhem equation in systems with long-range interactions
Latella, Iván; Pérez Madrid, Agustín
The local thermodynamics of a system with long-range interactions in d dimensions is studied using the mean-field approximation. Long-range interactions are introduced through pair interaction potentials that decay as a power law in the interparticle distance. We compute the local entropy, Helmholtz free energy, and grand potential per particle in the microcanonical, canonical, and grand canonical ensembles, respectively. From the local entropy per particle we obtain the local equation of state of the system by using the condition of local thermodynamic equilibrium. This local equation of state has the form of the ideal gas equation of state, but with the density depending on the potential characterizing long-range interactions. By volume integration of the relation between the different thermodynamic potentials at the local level, we find the corresponding equation satisfied by the potentials at the global level. It is shown that the potential energy enters as a thermodynamic variable that modifies the global thermodynamic potentials. As a result, we find a generalized Gibbs-Duhem equation that relates the potential energy to the temperature, pressure, and chemical potential. For the marginal case where the power of the decaying interaction potential is equal to the dimension of the space, the usual Gibbs-Duhem equation is recovered. As examples of the application of this equation, we consider spatially uniform interaction potentials and the self-gravitating gas. We also point out a close relationship with the thermodynamics of small systems.
http://www.recercat.cat:80/handle/2072/227864Cluster glasses of ultrasoft particles
http://www.recercat.cat:80/handle/2072/238909
Cluster glasses of ultrasoft particles
Coslovich, Daniele; Bernabei, Marco; Moreno, Ángel J.
We present molecular dynamics (MD) simulations results for dense fluids of ultrasoft, fully penetrable particles. These are a binary mixture and a polydisperse system of particles interacting via the generalized exponential model, which is known to yield cluster crystal phases for the corresponding monodisperse systems. Because of the dispersity in the particle size, the systems investigated in this work do not crystallize and form disordered cluster phases. The clusteringtransition appears as a smooth crossover to a regime in which particles are mostly located in clusters, isolated particles being infrequent. The analysis of the internal cluster structure reveals microsegregation of the big and small particles, with a strong homo-coordination in the binary mixture. Upon further lowering the temperature below the clusteringtransition, the motion of the clusters" centers-of-mass slows down dramatically, giving way to a cluster glass transition. In the cluster glass, the diffusivities remain finite and display an activated temperature dependence, indicating that relaxation in the cluster glass occurs via particle hopping in a nearly arrested matrix of clusters. Finally we discuss the influence of the microscopic dynamics on the transport properties by comparing the MD results with Monte Carlo simulations.
http://www.recercat.cat:80/handle/2072/238909Exit times in non-Markovian drifting continuous-time random-walk processes
http://www.recercat.cat:80/handle/2072/207941
Exit times in non-Markovian drifting continuous-time random-walk processes
Montero Torralbo, Miquel; Villarroel, Javier
By appealing to renewal theory we determine the equations that the mean exit time of a continuous-time random walk with drift satisfies both when the present coincides with a jump instant or when it does not. Particular attention is paid to the corrections ensuing from the non-Markovian nature of the process. We show that when drift and jumps have the same sign the relevant integral equations can be solved in closed form. The case when holding times have the classical Erlang distribution is considered in detail.
http://www.recercat.cat:80/handle/2072/207941Interplay between surface anisotropy and dipolar interactions in an assembly of nanomagnets
http://www.recercat.cat:80/handle/2072/227858
Interplay between surface anisotropy and dipolar interactions in an assembly of nanomagnets
Sabsabi, Z.; Vernay, F.; Iglesias, Òscar; Kachkachi, H.
We study the interplay between the effects of surface anisotropy and dipolar interactions in monodisperse assemblies of nanomagnets with oriented anisotropy. We derive asymptotic formulas for the assembly magnetization, taking into account temperature, applied field, core and surface anisotropy, and dipolar interparticle interactions. We find that the interplay between surface anisotropy and dipolar interactions is well described by the analytical expression of the assembly magnetization derived here: the overall sign of the product of the two parameters governing the surface and the dipolar contributions determines whether intrinsic and collective terms compete or have synergistic effects on the magnetization. This is illustrated by the magnetization curves of γ-Fe2O3 nanoparticle assemblies in the low concentration limit.
http://www.recercat.cat:80/handle/2072/227858Dynamic arrest in polymer melts: competition between packing and intramolecular barriers
http://www.recercat.cat:80/handle/2072/224146
Dynamic arrest in polymer melts: competition between packing and intramolecular barriers
Bernabei, Marco; Moreno, Ángel J.; Colmenero, Juan
We present molecular dynamics simulations of a simple model for polymer melts with intramolecular barriers. We investigate structural relaxation as a function of the barrier strength. Dynamic correlators can be consistently analyzed within the framework of the mode coupling theory of the glass transition. Control parameters are tuned in order to induce a competition between general packing effects and polymer-specific intramolecular barriers as mechanisms for dynamic arrest. This competition yields unusually large values of the so-called mode coupling theory exponent parameter and rationalizes qualitatively different observations for simple bead-spring and realistic polymers. The systematic study of the effect of intramolecular barriers presented here also establishes a fundamental difference between the nature of the glass transition in polymers and in simple glass formers.
http://www.recercat.cat:80/handle/2072/224146The role of intramolecular barriers on the glass transition of polymers: computer simulations versus mode coupling theory
http://www.recercat.cat:80/handle/2072/224145
The role of intramolecular barriers on the glass transition of polymers: computer simulations versus mode coupling theory
Bernabei, Marco; Moreno, Ángel J.; Colmenero, Juan
We present computer simulations of a simple bead-spring model for polymer melts with intramolecular barriers. By systematically tuning the strength of the barriers, we investigate their role on the glass transition. Dynamic observables are analyzed within the framework of the mode coupling theory (MCT). Critical nonergodicity parameters, critical temperatures, and dynamic exponents are obtained from consistent fits of simulation data to MCT asymptotic laws. The so-obtained MCT λ-exponent increases from standard values for fully flexible chains to values close to the upper limit for stiff chains. In analogy with systems exhibiting higher-order MCT transitions, we suggest that the observed large λ-values arise form the interplay between two distinct mechanisms for dynamic arrest: general packing effects and polymer-specific intramolecular barriers. We compare simulation results with numerical solutions of the MCT equations for polymer systems, within the polymer reference interaction site model (PRISM) for static correlations. We verify that the approximations introduced by the PRISM are fulfilled by simulations, with the same quality for all the range of investigated barrier strength. The numerical solutions reproduce the qualitative trends of simulations for the dependence of the nonergodicity parameters and critical temperatures on the barrier strength. In particular, the increase in the barrier strength at fixed density increases the localization length and the critical temperature. However the qualitative agreement between theory and simulation breaks in the limit of stiff chains. We discuss the possible origin of this feature.
http://www.recercat.cat:80/handle/2072/224145From caging to rouse dynamics in polymer melts with intramolecular barriers: a critical test of the mode coupling theory
http://www.recercat.cat:80/handle/2072/238781
From caging to rouse dynamics in polymer melts with intramolecular barriers: a critical test of the mode coupling theory
Bernabei, Marco; Moreno, Ángel J.; Zaccarelli, Emanuela; Sciortino, Francesco; Colmenero, Juan
By means of computer simulations and solution of the equations of the mode coupling theory (MCT),we investigate the role of the intramolecular barriers on several dynamic aspects of nonentangled polymers. The investigated dynamic range extends from the caging regime characteristic of glass-formers to the relaxation of the chain Rouse modes. We review our recent work on this question,provide new results, and critically discuss the limitations of the theory. Solutions of the MCT for the structural relaxation reproduce qualitative trends of simulations for weak and moderate barriers. However, a progressive discrepancy is revealed as the limit of stiff chains is approached. This dis-agreement does not seem related with dynamic heterogeneities, which indeed are not enhanced by increasing barrier strength. It is not connected either with the breakdown of the convolution approximation for three-point static correlations, which retains its validity for stiff chains. These findings suggest the need of an improvement of the MCT equations for polymer melts. Concerning the relaxation of the chain degrees of freedom, MCT provides a microscopic basis for time scales from chain reorientation down to the caging regime. It rationalizes, from first principles, the observed deviations from the Rouse model on increasing the barrier strength. These include anomalous scaling of relaxation times, long-time plateaux, and nonmonotonous wavelength dependence of the mode correlators.
http://www.recercat.cat:80/handle/2072/238781Friedman's Excess energy and the McMillan-Mayer theory of solutions:Thermodynamics
http://www.recercat.cat:80/handle/2072/226987
Friedman's Excess energy and the McMillan-Mayer theory of solutions:Thermodynamics
Gómez Estévez, Juan Luis
In his version of the theory of multicomponent systems, Friedman used the analogy which exists between the virial expansion for the osmotic pressure obtained from the McMillan-Mayer (MM) theory of solutions in the grand canonical ensemble and the virial expansion for the pressure of a real gas. For the calculation of the thermodynamic properties of the solution, Friedman proposed a definition for the"excess free energy" that is a reminder of the ancient idea for the"osmotic work". However, the precise meaning to be attached to his free energy is, within other reasons, not well defined because in osmotic equilibrium the solution is not a closed system and for a given process the total amount of solvent in the solution varies. In this paper, an analysis based on thermodynamics is presented in order to obtain the exact and precise definition for Friedman"s excess free energy and its use in the comparison with the experimental data.
http://www.recercat.cat:80/handle/2072/226987Cherenkov mesons as in-medium quark energy loss
http://www.recercat.cat:80/handle/2072/238337
Cherenkov mesons as in-medium quark energy loss
Casalderrey-Solana, Jorge; Fernández, Daniel (Fernández Moreno); Mateos, David (Mateos Solé)
We recently showed that a heavy quark moving sufficiently fast through a quark-gluon plasma may lose energy by Cherenkov-radiating mesons [1]. Here we review our previous holographic calculation of the energy loss in N=4 Super Yang-Mills and extend it to longitudinal vector mesons and scalar mesons. We also discuss phenomenological implications for heavy-ion collision experiments. Although the Cherenkov energy loss is an O(1/Nc) effect, a ballpark estimate yields a value of dE/dx for Nc=3 which is comparable to that of other mechanisms.
http://www.recercat.cat:80/handle/2072/238337Counter-propagating dual-trap optical tweezers based on linear momentum conservation
http://www.recercat.cat:80/handle/2072/225127
Counter-propagating dual-trap optical tweezers based on linear momentum conservation
Ribezzi-Crivellari, M.; Huguet, J. M.; Ritort Farran, Fèlix
We present a dual-trap optical tweezers setup which directly measures forces using linear momentum conservation. The setup uses a counter-propagating geometry, which allows momentum measurement on each beam separately. The experimental advantages of this setup include low drift due to all-optical manipulation, and a robust calibration (independent of the features of the trapped object or buffer medium) due to the force measurement method. Although this design does not attain the high-resolution of some co-propagating setups, we show that it can be used to perform different single molecule measurements: fluctuation-based molecular stiffness characterization at different forces and hopping experiments on molecular hairpins. Remarkably, in our setup it is possible to manipulate very short tethers (such as molecular hairpins with short handles) down to the limit where beads are almost in contact. The setup is used to illustrate a novel method for measuring the stiffness of optical traps and tethers on the basis of equilibrium force fluctuations, i.e., without the need of measuring the force vs molecular extension curve. This method is of general interest for dual trap optical tweezers setups and can be extended to setups which do not directly measure forces.
http://www.recercat.cat:80/handle/2072/225127Thermodynamic stability of nanosized multicomponent bubbles/droplets: The square gradient theory and the capillary approach
http://www.recercat.cat:80/handle/2072/224828
Thermodynamic stability of nanosized multicomponent bubbles/droplets: The square gradient theory and the capillary approach
Wilhelmsen, Øivind; Bedeaux, Dick; Kjelstrup, Signe; Reguera, D. (David)
Formation of nanosized droplets/bubbles from a metastable bulk phase is connected to many unresolved scientific questions. We analyze the properties and stability of multicomponent droplets and bubbles in the canonical ensemble, and compare with single-component systems. The bubbles/droplets are described on the mesoscopic level by square gradient theory. Furthermore, we compare the results to a capillary model which gives a macroscopic description. Remarkably, the solutions of the square gradient model, representing bubbles and droplets, are accurately reproduced by the capillary model except in the vicinity of the spinodals. The solutions of the square gradient model form closed loops, which shows the inherent symmetry and connected nature of bubbles and droplets. A thermodynamic stability analysis is carried out, where the second variation of the square gradient description is compared to the eigenvalues of the Hessian matrix in the capillary description. The analysis shows that it is impossible to stabilize arbitrarily small bubbles or droplets in closed systems and gives insight into metastable regions close to the minimum bubble/droplet radii. Despite the large difference in complexity, the square gradient and the capillary model predict the same finite threshold sizes and very similar stability limits for bubbles and droplets, both for single-component and two-component systems.
http://www.recercat.cat:80/handle/2072/224828All-sky search algorithms for monochromatic signals in bar gravitational wave detector data
http://www.recercat.cat:80/handle/2072/219867
All-sky search algorithms for monochromatic signals in bar gravitational wave detector data
Lobo Gutiérrez, José Alberto; Montero Torralbo, Miquel
In this paper we design and develop several filtering strategies for the analysis of data generated by a resonant bar gravitational wave (GW) antenna, with the goal of assessing the presence (or absence) therein of long-duration monochromatic GW signals, as well as the eventual amplitude and frequency of the signals, within the sensitivity band of the detector. Such signals are most likely generated in the fast rotation of slightly asymmetric spinning stars. We develop practical procedures, together with a study of their statistical properties, which will provide us with useful information on the performance of each technique. The selection of candidate events will then be established according to threshold-crossing probabilities, based on the Neyman-Pearson criterion. In particular, it will be shown that our approach, based on phase estimation, presents a better signal-to-noise ratio than does pure spectral analysis, the most common approach.
http://www.recercat.cat:80/handle/2072/219867Glassy magnetic phase driven by short range charge and magnetic ordering in nanocrystalline La1/3Sr2/3FeO3-δ: Magnetization, Mössbauer, and polarized neutron studies
http://www.recercat.cat:80/handle/2072/221759
Glassy magnetic phase driven by short range charge and magnetic ordering in nanocrystalline La1/3Sr2/3FeO3-δ: Magnetization, Mössbauer, and polarized neutron studies
Sabyasachi, Sk.; Patra, M.; Majumdar, S.; Giri, S.; Das, S.; Amaral, V. S.; Iglesias, Òscar; Borghols, W.; Chatterji, T.
The charge ordered La1/3Sr2/3FeO3−δ (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, M¨ossbauer, and polarized neutron studies. A complex scenario of short-range charge and magnetic ordering is realized from the polarized neutron studies in nanocrystalline specimen. This short-range ordering does not involve any change in spin state and modification in the charge disproportion between Fe3+ and Fe5+ compared to bulk counterpart as evident in the M¨ossbauer results. The refinement of magnetic diffraction peaks provides magnetic moments of Fe3+ and Fe5+ are about 3.15 μB and 1.57 μB for bulk, and 2.7 μB and 0.53 μB for nanocrystalline specimen, respectively. The destabilization of charge ordering leads to magnetic phase separation, giving rise to the robust exchange bias (EB) effect. Strikingly, EB field at 5 K attains a value as high as 4.4 kOe for average size ∼70 nm, which is zero for the bulk counterpart. A strong frequency dependence of ac susceptibility reveals cluster-glass-like transition around ∼65 K, below which EB appears. Overall results propose that finite-size effect directs the complex glassy magnetic behavior driven by unconventional short-range charge and magnetic ordering, and magnetic phase separation appears in nanocrystalline LSFO.
http://www.recercat.cat:80/handle/2072/221759Holographic isotropization linearized
http://www.recercat.cat:80/handle/2072/238353
Holographic isotropization linearized
Heller, Michal P.; Mateos, David (Mateos Solé); Van der Schee, Wilke; Triana, Miquel
The holographic isotropization of a highly anisotropic, homogeneous, strongly coupled, non-Abelian plasma was simplified in ref. [1] by linearizing Einstein"s equations around the final, equilibrium state. This approximation reproduces the expectation value of the boundary stress tensor with a 20% accuracy. Here we elaborate on these results and extend them to observables that are directly sensitive to the bulk interior, focusing for simplicity on the entropy production on the event horizon. We also consider next-to-leading-order corrections and show that the leading terms alone provide a better description of the isotropization process for the states that are furthest from equilibrium.
http://www.recercat.cat:80/handle/2072/238353Effective Electrodiffusion equation for non uniform nanochannels
http://www.recercat.cat:80/handle/2072/222312
Effective Electrodiffusion equation for non uniform nanochannels
Marini-Bettolo-Marconi, Umberto; Melchionna, Simonne; Pagonabarraga Mora, Ignacio
We derive a one dimensional formulation of the Planck-Nernst-Poisson equation to describe the dynamics of of a symmetric binary electrolyte in channels whose section is of nanometric section and varies along the axial direction. The approach is in the spirit of the Fick-Jacobs di fusion equation and leads to a system of coupled equations for the partial densities which depends on the charge sitting at the walls in a non trivial fashion. We consider two kinds of non uniformities, those due to the spatial variation of charge distribution and those due to the shape variation of the pore and report one and three-dimensional solutions of the electrokinetic equations.
http://www.recercat.cat:80/handle/2072/222312From Full stopping to transparency in a holographic model of heavy ion collisions
http://www.recercat.cat:80/handle/2072/226988
From Full stopping to transparency in a holographic model of heavy ion collisions
Casalderrey-Solana, Jorge; Heller, Michal P.; Mateos, David (Mateos Solé); Van der Schee, Wilke
We numerically simulate planar shock wave collisions in anti-de Sitter space as a model for heavy ion collisions of large nuclei. We uncover a crossover between two different dynamical regimes as a function of the collision energy. At low energies the shocks first stop and then explode in a manner approximately described by hydrodynamics, in close similarity with the Landau model. At high energies the receding fragments move outwards at the speed of light, with a region of negative energy density and negative longitudinal pressure trailing behind them. The rapidity distribution of the energy density at late times around midrapidity is not approximately boost invariant but Gaussian, albeit with a width that increases with the collision energy.
http://www.recercat.cat:80/handle/2072/226988A New Mechanism of Quark Energy Loss
http://www.recercat.cat:80/handle/2072/207938
A New Mechanism of Quark Energy Loss
Casalderrey-Solana, Jorge; Fernández, Daniel (Fernández Moreno); Mateos, David (Mateos Solé)
We show that a heavy quark moving sufficiently fast through a quark-gluon plasma may lose energy by Cherenkov-radiating mesons. We demonstrate that this takes place in all strongly coupled, large-Nc plasmas with a gravity dual. The energy loss is exactly calculable in these models despite being an O(1/Nc)-effect. We discuss phenomenological implications for heavy-ion collision experiments.
http://www.recercat.cat:80/handle/2072/207938Goya's artwork imaging with Terahertz waves
http://www.recercat.cat:80/handle/2072/223429
Goya's artwork imaging with Terahertz waves
Seco-Martorell, C.; López-Domínguez, V.; Arauz-Garofalo, Gianluca; Redo-Sanchez, A.; Tejada Palacios, Javier
In this paper we use a Terahertz (THz) time-domain system to image and analyze the structure of an artwork attributed to the Spanish artist Goya painted in 1771. The THz images show features that cannot be seen with optical inspection and complement data obtained with X-ray imaging that provide evidence of its authenticity, which is validated by other independent studies. For instance, a feature with a strong resemblance with one of Goya"s known signatures is seen in the THz images. In particular, this paper demonstrates the potential of THz imaging as a complementary technique along with X-ray for the verification and authentication of artwork pieces through the detection of features that remain hidden to optical inspection.
http://www.recercat.cat:80/handle/2072/223429Influence of the feeding mechanism on deposits of square particles
http://www.recercat.cat:80/handle/2072/227859
Influence of the feeding mechanism on deposits of square particles
Acevedo, M.; Hidalgo, R. C.; Zuriguel, I.; Maza, D.; Pagonabarraga Mora, Ignacio
In a previous paper [Hidalgo et al., Phys. Rev. Lett. 103, 118001 (2009)] it was shown that square particles deposited in a silo tend to align with a diagonal parallel to the gravity, giving rise to a deposit with very particular properties. Here we explore, both experimentally and numerically, the effect on these properties of the filling mechanism. In particular, we modify the volume fraction of the initial configuration from which the grains are deposited. Starting from a very dilute case, increasing the volume fraction results in an enhancement of the disorder in the final deposit characterized by a decrease of the final packing fraction and a reduction of the number of particles oriented with their diagonal in the direction of gravity. However, for very high initial volume fractions, the final packing fraction increases again. This result implies that two deposits with the same final packing fraction can be obtained from very different initial conditions. The structural properties of such deposits are analyzed, revealing that, although the final volume fraction is the same, their micromechanical properties notably differ.
http://www.recercat.cat:80/handle/2072/227859Renewal equations for option pricing
http://www.recercat.cat:80/handle/2072/207942
Renewal equations for option pricing
Montero Torralbo, Miquel
In this paper we will develop a methodology for obtaining pricing expressions for financial instruments whose underlying asset can be described through a simple continuous-time random walk (CTRW) market model. Our approach is very natural to the issue because it is based in the use of renewal equations, and therefore it enhances the potential use of CTRW techniques in finance. We solve these equations for typical contract specifications, in a particular but exemplifying case. We also show how a formal general solution can be found for more exotic derivatives, and we compare prices for alternative models of the underlying. Finally, we recover the celebrated results for the Wiener process under certain limits.
http://www.recercat.cat:80/handle/2072/207942'Making Science Understandable'. Divulgació científica i mediació editorial: el cas de The Wisdom of the Body de Walter B. Cannon (1871-1945)
http://www.recercat.cat:80/handle/2072/221425
'Making Science Understandable'. Divulgació científica i mediació editorial: el cas de The Wisdom of the Body de Walter B. Cannon (1871-1945)
Martínez Vidal, Àlvar; Sallent Del Colombo, Emma
The present paper aims at an historical reconstruction, in the framework of the publishing industry in the years between the two World Wars, of the role played by the publisher William W. Norton in the genesis, published in 1932, and new edition in 1938, of Walter B. Cannon"s book The Wisdom of the Body. With the analysis of this case study, we aimed at contributing to the current criticism of the «dominant view», which tries, in an uncritical manner, that scientific popularization follows an ineluctable, continuous and linear evolution.
http://www.recercat.cat:80/handle/2072/221425Monotonic continuous-time random walks with drift and stochastic reset events
http://www.recercat.cat:80/handle/2072/207939
Monotonic continuous-time random walks with drift and stochastic reset events
Montero Torralbo, Miquel; Villarroel, Javier
In this paper we consider a stochastic process that may experience random reset events which suddenly bring the system to the starting value and analyze the relevant statistical magnitudes. We focus our attention on monotonic continuous-time random walks with a constant drift: The process increases between the reset events, either by the effect of the random jumps, or by the action of the deterministic drift. As a result of all these combined factors interesting properties emerge, like the existence (for any drift strength) of a stationary transition probability density function, or the faculty of the model to reproduce power-law-like behavior. General formulas for two extreme statistics, the survival probability, and the mean exit time, are also derived. To corroborate in an independent way the results of the paper, Monte Carlo methods were used. These numerical estimations are in full agreement with the analytical predictions.
http://www.recercat.cat:80/handle/2072/207939Elastic properties and secondary structure formation of single-stranded DNA at monovalent and divalent salt conditions
http://www.recercat.cat:80/handle/2072/239422
Elastic properties and secondary structure formation of single-stranded DNA at monovalent and divalent salt conditions
Bosco, Alessandro; Camuñas Soler, Joan; Ritort Farran, Fèlix
Single-stranded DNA (ssDNA) plays a major role in several biological processes. It is therefore of fundamental interest to understand how the elastic response and the formation of secondary structures are modulated by the interplay between base pairing and electrostatic interactions. Here we measure force-extension curves (FECs) of ssDNA molecules in optical tweezers set up over two orders of magnitude of monovalent and divalent salt conditions, and obtain its elastic parameters by fitting the FECs to semiflexible models of polymers. For both monovalent and divalent salts, we find that the electrostatic contribution to the persistence length is proportional to the Debye screening length, varying as the inverse of the square root of cation concentration. The intrinsic persistence length is equal to 0.7 nm for both types of salts, and the effectivity of divalent cations in screening electrostatic interactions appears to be 100-fold as compared with monovalent salt, in line with what has been recently reported for single-stranded RNA. Finally, we propose an analysis of the FECs using a model that accounts for the effective thickness of the filament at low salt condition and a simple phenomenological description that quantifies the formation of non-specific secondary structure at low forces.
http://www.recercat.cat:80/handle/2072/239422