Articles de revista
http://hdl.handle.net/2117/395
2016-04-29T16:31:33ZPhase diagram of a quantum Coulomb wire
http://hdl.handle.net/2117/85102
Phase diagram of a quantum Coulomb wire
Ferré Porta, Guillem; Astrakharchik, Grigori; Boronat Medico, Jordi
2016-04-04T10:20:09ZFerré Porta, GuillemAstrakharchik, GrigoriBoronat Medico, JordiOne-dimensional Bose gas in optical lattices of arbitrary strength
http://hdl.handle.net/2117/84244
One-dimensional Bose gas in optical lattices of arbitrary strength
Astrakharchik, Grigori; Krutitsky, K.V.; Lewenstein, Maciej; Mazzanti Castrillejo, Fernando Pablo
One-dimensional Bose gas with contact interaction in optical lattices at zero temperature is investigated by means of the exact diffusion Monte Carlo algorithm. The results obtained from the fundamental continuous model are compared with those obtained from the lattice (discrete) Bose-Hubbard model, using exact diagonalization, and from the quantum sine-Gordon model. We map out the complete phase diagram of the continuous model and determine the regions of applicability of the Bose-Hubbard model. Various physical quantities characterizing the systems are calculated, and it is demonstrated that the sine-Gordon model used for shallow lattices is inaccurate.
2016-03-11T17:29:02ZAstrakharchik, GrigoriKrutitsky, K.V.Lewenstein, MaciejMazzanti Castrillejo, Fernando PabloOne-dimensional Bose gas with contact interaction in optical lattices at zero temperature is investigated by means of the exact diffusion Monte Carlo algorithm. The results obtained from the fundamental continuous model are compared with those obtained from the lattice (discrete) Bose-Hubbard model, using exact diagonalization, and from the quantum sine-Gordon model. We map out the complete phase diagram of the continuous model and determine the regions of applicability of the Bose-Hubbard model. Various physical quantities characterizing the systems are calculated, and it is demonstrated that the sine-Gordon model used for shallow lattices is inaccurate.Distinct types of eigenvector localization in networks
http://hdl.handle.net/2117/83693
Distinct types of eigenvector localization in networks
Pastor Satorras, Romualdo; Castellano, Claudio
The spectral properties of the adjacency matrix provide a trove of information about the structure and function of complex networks. In particular, the largest eigenvalue and its associated principal eigenvector are crucial in the understanding of nodes' centrality and the unfolding of dynamical processes. Here we show that two distinct types of localization of the principal eigenvector may occur in heterogeneous networks. For synthetic networks with degree distribution P(q) ~ q -'¿, localization occurs on the largest hub if ¿ > 5/2; for ¿ < 5/2 a new type of localization arises on a mesoscopic subgraph associated with the shell with the largest index in the K-core decomposition. Similar evidence for the existence of distinct localization modes is found in the analysis of real-world networks. Our results open a new perspective on dynamical processes on networks and on a recently proposed alternative measure of node centrality based on the non-backtracking matrix.
2016-03-02T10:56:42ZPastor Satorras, RomualdoCastellano, ClaudioThe spectral properties of the adjacency matrix provide a trove of information about the structure and function of complex networks. In particular, the largest eigenvalue and its associated principal eigenvector are crucial in the understanding of nodes' centrality and the unfolding of dynamical processes. Here we show that two distinct types of localization of the principal eigenvector may occur in heterogeneous networks. For synthetic networks with degree distribution P(q) ~ q -'¿, localization occurs on the largest hub if ¿ > 5/2; for ¿ < 5/2 a new type of localization arises on a mesoscopic subgraph associated with the shell with the largest index in the K-core decomposition. Similar evidence for the existence of distinct localization modes is found in the analysis of real-world networks. Our results open a new perspective on dynamical processes on networks and on a recently proposed alternative measure of node centrality based on the non-backtracking matrix.First-principles modeling of three-body interactions in highly compressed solid helium
http://hdl.handle.net/2117/83684
First-principles modeling of three-body interactions in highly compressed solid helium
Cazorla Silva, Claudio; Boronat Medico, Jordi
We present a set of three-body interaction models based on the Slater-Kirkwood (SK) potential that are suitable for the study of the energy, structural, and elastic properties of solid He4 at high pressure. Our effective three-body potentials are obtained from the fit to total energies and atomic forces computed with the van der Waals density functional theory method due to Grimme, and represent an improvement with respect to previously reported three-body interaction models. In particular, we show that some of the introduced SK three-body potentials reproduce closely the experimental equation of state and bulk modulus of solid helium up to a pressure of ~60 GPa, when used in combination with standard pairwise interaction models in diffusion Monte Carlo simulations. Importantly, we find that recent predictions reporting a surprisingly small variation of the kinetic energy and Lindeman ratio on quantum crystals under increasing pressure are likely to be artifacts deriving from the use of incomplete interaction models. Also, we show that the experimental variation of the shear modulus, C44, at pressures 0=P=25 GPa can be quantitatively described by our set of SK three-body potentials. At higher compression, however, the agreement between our C44 calculations and experiments deteriorates and thus we argue that higher order many-body terms in the expansion of the atomic interactions probably are necessary in order to better describe elasticity in very dense solid He4.
2016-03-02T09:15:05ZCazorla Silva, ClaudioBoronat Medico, JordiWe present a set of three-body interaction models based on the Slater-Kirkwood (SK) potential that are suitable for the study of the energy, structural, and elastic properties of solid He4 at high pressure. Our effective three-body potentials are obtained from the fit to total energies and atomic forces computed with the van der Waals density functional theory method due to Grimme, and represent an improvement with respect to previously reported three-body interaction models. In particular, we show that some of the introduced SK three-body potentials reproduce closely the experimental equation of state and bulk modulus of solid helium up to a pressure of ~60 GPa, when used in combination with standard pairwise interaction models in diffusion Monte Carlo simulations. Importantly, we find that recent predictions reporting a surprisingly small variation of the kinetic energy and Lindeman ratio on quantum crystals under increasing pressure are likely to be artifacts deriving from the use of incomplete interaction models. Also, we show that the experimental variation of the shear modulus, C44, at pressures 0=P=25 GPa can be quantitatively described by our set of SK three-body potentials. At higher compression, however, the agreement between our C44 calculations and experiments deteriorates and thus we argue that higher order many-body terms in the expansion of the atomic interactions probably are necessary in order to better describe elasticity in very dense solid He4.Quantum versus mean-field collapse in a many-body system
http://hdl.handle.net/2117/83079
Quantum versus mean-field collapse in a many-body system
Astrakharchik, Grigori; Malomed, Boris A.
The recent analysis, based on the mean-field approximation (MFA), has predicted that the critical quantum collapse of the bosonic wave function, pulled to the center by the inverse-square potential in the three-dimensional space, is suppressed by the repulsive cubic nonlinearity in the bosonic gas, the collapsing ground state being replaced by a regular one. We demonstrate that a similar stabilization acts in a quantum many-body system, beyond the MFA. While the collapse remains possible, repulsive two-particle interactions give rise to a metastable gaseous state, which is separated by a potential barrier from the collapsing regime. The stability of this state improves with the increase of the number of particles. The results are produced by calculations of the variational energy, with the help of the Monte Carlo method.
2016-02-17T15:33:53ZAstrakharchik, GrigoriMalomed, Boris A.The recent analysis, based on the mean-field approximation (MFA), has predicted that the critical quantum collapse of the bosonic wave function, pulled to the center by the inverse-square potential in the three-dimensional space, is suppressed by the repulsive cubic nonlinearity in the bosonic gas, the collapsing ground state being replaced by a regular one. We demonstrate that a similar stabilization acts in a quantum many-body system, beyond the MFA. While the collapse remains possible, repulsive two-particle interactions give rise to a metastable gaseous state, which is separated by a potential barrier from the collapsing regime. The stability of this state improves with the increase of the number of particles. The results are produced by calculations of the variational energy, with the help of the Monte Carlo method.Interaction of He and He-V clusters with self-interstitials and dislocations defects in bcc Fe
http://hdl.handle.net/2117/81803
Interaction of He and He-V clusters with self-interstitials and dislocations defects in bcc Fe
Terentyev, Dimitry; Anento Moreno, Napoleón; Serra Tort, Ana María; Ortiz, C. J.; zhurkin, E. E.
The understanding of helium effects in synergy with radiation damage is crucial for the development of structural steels for fusion applications. Recent investigations in ultra-pure iron, taken as a basic model, have shown a drastic impact of dual beam (helium and iron) exposure on the accumulation of radiation-induced dislocation loops in terms of strong bias towards a(0)/2 < 111 > loops, while a(0)< 100 > loops are mostly observed upon iron ion beam. In this work we perform a series of atomistic studies to rationalize possible mechanisms through which He could affect the evolution of microstructure and bias the population of a(0)/2 < 111 > loops. It is shown that He atoms are dragged by gliding a(0)/2 < 111 > loops. This strongly suppresses loop diffusivity and in turn it prohibits the mutual interaction of a(0)/2 < 111 > loops, being prerequisite of the formation of a(0)< 100 > loops, as well as it reduces the disappearance of a(0)/2 < 111 > loops at sinks. A scenario for the microstructural evolution in the single-and dual-beam conditions is discussed. (C) 2014 Elsevier B. V. All rights reserved.
2016-01-21T12:29:01ZTerentyev, DimitryAnento Moreno, NapoleónSerra Tort, Ana MaríaOrtiz, C. J.zhurkin, E. E.The understanding of helium effects in synergy with radiation damage is crucial for the development of structural steels for fusion applications. Recent investigations in ultra-pure iron, taken as a basic model, have shown a drastic impact of dual beam (helium and iron) exposure on the accumulation of radiation-induced dislocation loops in terms of strong bias towards a(0)/2 < 111 > loops, while a(0)< 100 > loops are mostly observed upon iron ion beam. In this work we perform a series of atomistic studies to rationalize possible mechanisms through which He could affect the evolution of microstructure and bias the population of a(0)/2 < 111 > loops. It is shown that He atoms are dragged by gliding a(0)/2 < 111 > loops. This strongly suppresses loop diffusivity and in turn it prohibits the mutual interaction of a(0)/2 < 111 > loops, being prerequisite of the formation of a(0)< 100 > loops, as well as it reduces the disappearance of a(0)/2 < 111 > loops at sinks. A scenario for the microstructural evolution in the single-and dual-beam conditions is discussed. (C) 2014 Elsevier B. V. All rights reserved.Evolution of matrix-twin interfaces of (101¯2) twin in magnesium
http://hdl.handle.net/2117/80919
Evolution of matrix-twin interfaces of (101¯2) twin in magnesium
Ostapovets, Andriy; Serra Tort, Ana María
Recently, the presence of basal-prismatic interfaces in hexagonal close packed metals became subject of intensive investigation. We model the {101¯2} twin in magnesium bounded by two types of boundaries, i.e. {101¯2} interface and basal-prismatic facets. The migration of all boundary types is mediated by the motion of interfacial disconnections. It was shown that basal-prismatic interfaces play an important role in twin growth. The lengths of basal-prismatic facets remain constant during migration independently of the applied strain. In contrast, the {101¯2} interfaces increase their lengths during growth.
2015-12-18T18:44:03ZOstapovets, AndriySerra Tort, Ana MaríaRecently, the presence of basal-prismatic interfaces in hexagonal close packed metals became subject of intensive investigation. We model the {101¯2} twin in magnesium bounded by two types of boundaries, i.e. {101¯2} interface and basal-prismatic facets. The migration of all boundary types is mediated by the motion of interfacial disconnections. It was shown that basal-prismatic interfaces play an important role in twin growth. The lengths of basal-prismatic facets remain constant during migration independently of the applied strain. In contrast, the {101¯2} interfaces increase their lengths during growth.Slow relaxation dynamics and aging in random walks on activity driven temporal networks
http://hdl.handle.net/2117/77865
Slow relaxation dynamics and aging in random walks on activity driven temporal networks
Sousa da Mata, Angelica; Pastor Satorras, Romualdo
We investigate the dynamic relaxation of random walks on temporal networks by focusing in the recently proposed activity driven model [N. Perra, B. Gon, calves, R. Pastor-Satorras, A. Vespignani, Sci. Rep. 2, 469 (2012)]. For realistic activity distributions with a power-law form, we observe the presence of a very slow relaxation dynamics compatible with aging effects. A theoretical description of this processes in achieved by means of a mapping to Bouchaud's trap model. The mapping highlights the profound difference in the dynamics of the random walks according to the value of the exponent gamma in the activity distribution.
2015-10-19T09:20:07ZSousa da Mata, AngelicaPastor Satorras, RomualdoWe investigate the dynamic relaxation of random walks on temporal networks by focusing in the recently proposed activity driven model [N. Perra, B. Gon, calves, R. Pastor-Satorras, A. Vespignani, Sci. Rep. 2, 469 (2012)]. For realistic activity distributions with a power-law form, we observe the presence of a very slow relaxation dynamics compatible with aging effects. A theoretical description of this processes in achieved by means of a mapping to Bouchaud's trap model. The mapping highlights the profound difference in the dynamics of the random walks according to the value of the exponent gamma in the activity distribution.Insights into Q2Qbar2 states from an effective perspective
http://hdl.handle.net/2117/77626
Insights into Q2Qbar2 states from an effective perspective
Ametller Congost, Lluís; Talavera Sánchez, Pedro
We discuss the two-photon coupling of the lightest scalar meson on the basis of an extension of χPT. Using low-energy data on the pion form factor and the γγ→π+π−(π0π0) cross sections as inputs, we find Γ(σ→γγ)≅0.126 keV. The smallness of the result and the relative weight between its components, Γγγ→S1Γγγ→ππ→S1≤1, suggests that the scalar 0++ meson is mainly a Q2Q¯2 state.
2015-10-13T12:55:24ZAmetller Congost, LluísTalavera Sánchez, PedroWe discuss the two-photon coupling of the lightest scalar meson on the basis of an extension of χPT. Using low-energy data on the pion form factor and the γγ→π+π−(π0π0) cross sections as inputs, we find Γ(σ→γγ)≅0.126 keV. The smallness of the result and the relative weight between its components, Γγγ→S1Γγγ→ππ→S1≤1, suggests that the scalar 0++ meson is mainly a Q2Q¯2 state.Epidemic processes in complex networks
http://hdl.handle.net/2117/77005
Epidemic processes in complex networks
Pastor Satorras, Romualdo; Castellano, Claudio; Van Mieghem, Piet; Vespignani, Alessandro
In recent years the research community has accumulated overwhelming evidence for the emergence of complex and heterogeneous connectivity patterns in a wide range of biological and sociotechnical systems. The complex properties of real-world networks have a profound impact on the behavior of equilibrium and nonequilibrium phenomena occurring in various systems, and the study of epidemic spreading is central to our understanding of the unfolding of dynamical processes in complex networks. The theoretical analysis of epidemic spreading in heterogeneous networks requires the development of novel analytical frameworks, and it has produced results of conceptual and practical relevance. A coherent and comprehensive review of the vast research activity concerning epidemic processes is presented, detailing the successful theoretical approaches as well as making their limits and assumptions clear. Physicists, mathematicians, epidemiologists, computer, and social scientists share a common interest in studying epidemic spreading and rely on similar models for the description of the diffusion of pathogens, knowledge, and innovation. For this reason, while focusing on the main results and the paradigmatic models in infectious disease modeling, the major results concerning generalized social contagion processes are also presented. Finally, the research activity at the forefront in the study of epidemic spreading in coevolving, coupled, and time-varying networks is reported.
2015-09-22T08:32:01ZPastor Satorras, RomualdoCastellano, ClaudioVan Mieghem, PietVespignani, AlessandroIn recent years the research community has accumulated overwhelming evidence for the emergence of complex and heterogeneous connectivity patterns in a wide range of biological and sociotechnical systems. The complex properties of real-world networks have a profound impact on the behavior of equilibrium and nonequilibrium phenomena occurring in various systems, and the study of epidemic spreading is central to our understanding of the unfolding of dynamical processes in complex networks. The theoretical analysis of epidemic spreading in heterogeneous networks requires the development of novel analytical frameworks, and it has produced results of conceptual and practical relevance. A coherent and comprehensive review of the vast research activity concerning epidemic processes is presented, detailing the successful theoretical approaches as well as making their limits and assumptions clear. Physicists, mathematicians, epidemiologists, computer, and social scientists share a common interest in studying epidemic spreading and rely on similar models for the description of the diffusion of pathogens, knowledge, and innovation. For this reason, while focusing on the main results and the paradigmatic models in infectious disease modeling, the major results concerning generalized social contagion processes are also presented. Finally, the research activity at the forefront in the study of epidemic spreading in coevolving, coupled, and time-varying networks is reported.