Articles de revista
http://hdl.handle.net/2117/395
2015-08-04T16:33:07ZSolvation dynamics in liquid water: 1: ultrafast energy fluxes
http://hdl.handle.net/2117/28563
Solvation dynamics in liquid water: 1: ultrafast energy fluxes
Rey Oriol, Rosendo; Hynes, James T.
Solvation dynamics in liquid water is addressed via nonequilibrium energy-transfer pathways activated after a neutral atomic solute acquires a unit charge, either positive or negative. It is shown that the well-known nonequilibrium frequency shift relaxation function can be expressed in a novel fashion in terms of energy fluxes, providing a clear-cut and quantitative account of the processes involved. Roughly half of the initial excess energy is transferred into hindered rotations of first hydration shell water molecules, i.e., librational motions, specifically those rotations around the lowest moment of inertia principal axis. After integration over all water solvent molecules, rotations account for roughly 80% of the energy transferred, while translations have a secondary role; transfer to intramolecular water stretch and bend vibrations is negligible. This picture is similar to that for relaxation of a single vibrationally or rotationally excited water molecule in neat liquid water, although solvation relaxation is more nonlocal. In addition, we find a remarkable independence of the main relaxation channels on the newly created charge’s sign. Although the methodology is applied here to the simplest solute case, the approach is rather general, and it should be at least equally useful in more realistic and complex scenarios.
2015-01-30T00:00:00ZThe limit of mechanical stability in quantum crystals: a diffusion Monte Carlo study of solid He-4
http://hdl.handle.net/2117/28560
The limit of mechanical stability in quantum crystals: a diffusion Monte Carlo study of solid He-4
Cazorla Silva, Claudio; Boronat Medico, Jordi
We present a first-principles study of the energy and elastic properties of solid helium at pressures below the range in which it is energetically stable. We find that the limit of mechanical stability in hcp He is bar, which lies significantly below the spinodal pressure found in the liquid phase (i.e., bar). Furthermore, we show that the pressure variation of the transverse and longitudinal sound velocities close to does not follow a power law of the form , in contrast to what is observed in the fluid.
2015-07-01T00:00:00ZGeneralized equipartition theorem and confining walls
http://hdl.handle.net/2117/28219
Generalized equipartition theorem and confining walls
Rey Oriol, Rosendo
It is shown that the generalized equipartition theorem as applied to the momentum may fail, even though the usual conditions of validity are met. This failure is linked to the nature of the confining walls, with ideal walls constituting a singular case. The corresponding analysis illustrates the use of proper sets of canonical coordinates, and the utility of canonical transformations other than point transformations.
2015-06-01T00:00:00ZLifespan method as a tool to study criticality in absorbing-state phase transitions
http://hdl.handle.net/2117/28182
Lifespan method as a tool to study criticality in absorbing-state phase transitions
de Souza Mata, Angelica; Boguñá, Marian; Castellano, Claudio; Pastor Satorras, Romualdo
In a recent work, a new numerical method (the lifespan method) has been introduced to study the critical properties of epidemic processes on complex networks [M. Boguna, C. Castellano, and R. Pastor-Satorras, Phys. Rev. Lett. 111, 068701 (2013)]. Here, we present a detailed analysis of the viability of this method for the study of the critical properties of generic absorbing-state phase transitions in lattices. Focusing on the well-understood case of the contact process, we develop a finite-size scaling theory to measure the critical point and its associated critical exponents. We show the validity of the method by studying numerically the contact process on a one-dimensional lattice and comparing the findings of the lifespan method with the standard quasistationary method. We find that the lifespan method gives results that are perfectly compatible with those of quasistationary simulations and with analytical results. Our observations confirm that the lifespan method is a fully legitimate tool for the study of the critical properties of absorbing phase transitions in regular lattices.
2015-05-12T00:00:00ZCharacterizing ordering in liquids: An information theoretic approach
http://hdl.handle.net/2117/27961
Characterizing ordering in liquids: An information theoretic approach
Pardo Soto, Luis Carlos; Henao Aristizábal, Andrés; Vispa, Alessandro
The determination of special molecular arrangements in disordered phases such as liquids is inherently difficult due to its lack of periodicity, in contrast to the crystalline solids. We have already settled a general method to study molecular liquids capable to unveil the details of the molecular ordering from small molecules to systems as big as a protein. However it would be desirable to extract some general features of a liquid phase without going into such details. In this work we propose a method to achieve this challenge by analyzing the probability distributions describing position and orientational molecular ordering within the framework of information theory. (C) 2014 Elsevier B.V. All rights reserved.
2015-01-01T00:00:00ZBurstiness and aging in social temporal networks
http://hdl.handle.net/2117/27534
Burstiness and aging in social temporal networks
Moinet, Antoine; Starnini, Michele; Pastor Satorras, Romualdo
The presence of burstiness in temporal social networks, revealed by a power-law form of the waiting time distribution of consecutive interactions, is expected to produce aging effects in the corresponding time-integrated network. Here, we propose an analytically tractable model, in which interactions among the agents are ruled by a renewal process, that is able to reproduce this aging behavior. We develop an analytic solution for the topological properties of the integrated network produced by the model, finding that the time translation invariance of the degree distribution is broken. We validate our predictions against numerical simulations, and we check for the presence of aging effects in a empirical temporal network, ruled by bursty social interactions.
2015-03-13T00:00:00ZQuantum Monte Carlo estimation of complex-time correlations for the study of the ground-state dynamic structure function
http://hdl.handle.net/2117/27527
Quantum Monte Carlo estimation of complex-time correlations for the study of the ground-state dynamic structure function
Rota, R; Casulleras Ambrós, Joaquín; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
We present a method based on the path integral Monte Carlo formalism for the calculation of ground-state time correlation functions in quantum systems. The key point of the method is the consideration of time as a complex variable whose phase d acts as an adjustable parameter. By using high-order approximations for the quantum propagator, it is possible to obtain Monte Carlo data all the way from purely imaginary time to d values near the limit of real time. As a consequence, it is possible to infer accurately the spectral functions using simple inversion algorithms. We test this approach in the calculation of the dynamic structure function S(q, omega) of two one-dimensional model systems, harmonic and quartic oscillators, for which S(q, omega) can be exactly calculated. We notice a clear improvement in the calculation of the dynamic response with respect to the common approach based on the inverse Laplace transform of the imaginary-time correlation function. (C) 2015 AIP Publishing LLC.
2015-03-21T00:00:00Z1H nuclear spin relaxation of liquid water from molecular dynamics simulations
http://hdl.handle.net/2117/27329
1H nuclear spin relaxation of liquid water from molecular dynamics simulations
Calero Borrallo, Carles; Martí Rabassa, Jordi; Guàrdia Manuel, Elvira
We have investigated the nuclear spin relaxation properties of 1H in liquid water with the help of molecular dynamics simulations. We have computed the 1H nuclear spin relaxation times T1 and T2 and determined the contribution of the different interactions to the relaxation at different temperatures and for different classical water models (SPC/E, TIP3P, TIP4P, and TIP4P/2005). Among the water models considered, the TIP4P/2005 model exhibits the best agreement with the experiment. The same analysis was performed with Car–Parrinello ab initio molecular dynamics simulations of bulk water at T = 330 K, which provided results close to the experimental values at room temperature. To complete the study, we have successfully accounted for the temperature-dependence of T1 and T2 in terms of a simplified model, which considers the reorientation in finite angle jumps and the diffusive translation of water molecules.
2015-01-13T00:00:00ZSharp crossover from composite fermionization to phase separation in microscopic mixtures of ultracold bosons
http://hdl.handle.net/2117/27031
Sharp crossover from composite fermionization to phase separation in microscopic mixtures of ultracold bosons
Garcia March, Miguel Angel; Julia Diaz, Bruno; Astrakharchik, Grigori; Busch, T; Boronat Medico, Jordi; Rios Polls, Artur
We show that a two-component mixture of a few repulsively interacting ultracold atoms in a one-dimensional trap possesses very diverse quantum regimes and that the crossover between them can be induced by tuning the interactions in one of the species. Starting from the composite fermionization regime, in which the interactions between both components are large and neither gas is phase coherent, our results show that a phase-separated state can be reached by increasing the interaction in one of the species. In this regime, the weakly interacting component stays at the center of the trap and becomes almost fully phase coherent, while the strongly interacting one is expelled to the edges of the trap. The crossover is sharp, as can be witnessed in the system's energy and in the occupation of the lowest natural orbital of the weakly interacting species. We show that such a transition is a few-atom effect which disappears for a large population imbalance.
2013-12-03T00:00:00ZTemporal percolation in activity-driven networks
http://hdl.handle.net/2117/26924
Temporal percolation in activity-driven networks
Starnini, Michele; Pastor Satorras, Romualdo
We study the temporal percolation properties of temporal networks by taking as a representative example the recently proposed activity-driven-network model [N. Perra et al., Sci. Rep. 2, 469 ( 2012)]. Building upon an analytical framework based on a mapping to hidden variables networks, we provide expressions for the percolation time T-p marking the onset of a giant connected component in the integrated network. In particular, we consider both the generating function formalism, valid for degree-uncorrelated networks, and the general case of networks with degree correlations. We discuss the different limits of the two approaches, indicating the parameter regions where the correlated threshold collapses onto the uncorrelated case. Our analytical predictions are confirmed by numerical simulations of the model. The temporal percolation concept can be fruitfully applied to study epidemic spreading on temporal networks. We show in particular how the susceptible-infected-removed model on an activity-driven network can be mapped to the percolation problem up to a time given by the spreading rate of the epidemic process. This mapping allows us to obtain additional information on this process, not available for previous approaches.
2014-03-14T00:00:00Z