SIMCON  Grup de Recerca de Simulació per Ordinador en Matèria Condensada
http://hdl.handle.net/2117/394
20180319T20:34:52Z

Few trapped quantum dipoles: quantum versus classical structures
http://hdl.handle.net/2117/115017
Few trapped quantum dipoles: quantum versus classical structures
Sánchez Baena, Juan; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
We analyze the ground state of a twodimensional quantum system of a few strongly confined dipolar bosons. Dipoles arrange in different stable structures that depend on the tilting polarization angle and the anisotropy of the confining trap. To this end, we use the exact diffusion Monte Carlo method and the quantum results are compared with classical ones obtained by stochastic optimization using simulated annealing. We establish the stability domains for the different patterns and estimate the transition boundaries delimiting them. Our results show significant differences between the classical and quantum regimes which are mainly due to the quantum kinetic energy.
20180309T14:22:56Z
Sánchez Baena, Juan
Mazzanti Castrillejo, Fernando Pablo
Boronat Medico, Jordi
We analyze the ground state of a twodimensional quantum system of a few strongly confined dipolar bosons. Dipoles arrange in different stable structures that depend on the tilting polarization angle and the anisotropy of the confining trap. To this end, we use the exact diffusion Monte Carlo method and the quantum results are compared with classical ones obtained by stochastic optimization using simulated annealing. We establish the stability domains for the different patterns and estimate the transition boundaries delimiting them. Our results show significant differences between the classical and quantum regimes which are mainly due to the quantum kinetic energy.

AndreevBashkin effect in superfluid cold gases mixtures
http://hdl.handle.net/2117/113354
AndreevBashkin effect in superfluid cold gases mixtures
Nespolo, Jacopo; Astrakharchik, Grigori; Recati, Alessio
© 2017 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft. We study a mixture of two superfluids with densitydensity and currentcurrent (AndreevBashkin) interspecies interactions. The AndreevBashkin coupling gives rise to a dissipationless drag (or entrainment) between the two superfluids. Within the quantum hydrodynamics approximation, we study the relations between speeds of sound, susceptibilities and static structure factors, in a generic model in which the density and spin dynamics decouple. Due to translational invariance, the density channel does not feel the drag. The spin channel, instead, does not satisfy the usual BijlFeynman relation, since the fsum rule is not exhausted by the spin phonons. The very same effect on one dimensional Bose mixtures and their Luttinger liquid description is analysed within perturbation theory. Using diffusion quantum Monte Carlo simulations of a system of dipolar gases in a double layer configuration, we confirm the general results. Given the recent advances in measuring the counterflow instability, we also study the effect of the entrainment on the dynamical stability of a superfluid mixture with nonzero relative velocity.
20180129T18:16:04Z
Nespolo, Jacopo
Astrakharchik, Grigori
Recati, Alessio
© 2017 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft. We study a mixture of two superfluids with densitydensity and currentcurrent (AndreevBashkin) interspecies interactions. The AndreevBashkin coupling gives rise to a dissipationless drag (or entrainment) between the two superfluids. Within the quantum hydrodynamics approximation, we study the relations between speeds of sound, susceptibilities and static structure factors, in a generic model in which the density and spin dynamics decouple. Due to translational invariance, the density channel does not feel the drag. The spin channel, instead, does not satisfy the usual BijlFeynman relation, since the fsum rule is not exhausted by the spin phonons. The very same effect on one dimensional Bose mixtures and their Luttinger liquid description is analysed within perturbation theory. Using diffusion quantum Monte Carlo simulations of a system of dipolar gases in a double layer configuration, we confirm the general results. Given the recent advances in measuring the counterflow instability, we also study the effect of the entrainment on the dynamical stability of a superfluid mixture with nonzero relative velocity.

Observation of dynamic atomatom correlation in liquid helium in real space
http://hdl.handle.net/2117/113282
Observation of dynamic atomatom correlation in liquid helium in real space
Dmowski, Wojtek; Diallo, Souleymane Omar; Lokshin, Konstantin A.; Ehlers, Georg; Ferré Porta, Guillem; Boronat Medico, Jordi; Egami, Takeshi
© The Author(s) 2017. Liquid 4He becomes superfluid and flows without resistance below temperature 2.17 K. Superfluidity has been a subject of intense studies and notable advances were made in elucidating the phenomenon by experiment and theory. Nevertheless, details of the microscopic state, including dynamic atomatom correlations in the superfluid state, are not fully understood. Here using a technique of neutron dynamic pairdensity function (DPDF) analysis we show that 4He atoms in the BoseEinstein condensate have environment significantly different from uncondensed atoms, with the interatomic distance larger than the average by about 10%, whereas the average structure changes little through the superfluid transition. DPDF peak not seen in the snapshot pairdensity function is found at 2.3 A, and is interpreted in terms of atomic tunnelling. The real space picture of dynamic atomatom correlations presented here reveal characteristics of atomic dynamics not recognized so far, compelling yet another look at the phenomenon.
20180126T19:09:42Z
Dmowski, Wojtek
Diallo, Souleymane Omar
Lokshin, Konstantin A.
Ehlers, Georg
Ferré Porta, Guillem
Boronat Medico, Jordi
Egami, Takeshi
© The Author(s) 2017. Liquid 4He becomes superfluid and flows without resistance below temperature 2.17 K. Superfluidity has been a subject of intense studies and notable advances were made in elucidating the phenomenon by experiment and theory. Nevertheless, details of the microscopic state, including dynamic atomatom correlations in the superfluid state, are not fully understood. Here using a technique of neutron dynamic pairdensity function (DPDF) analysis we show that 4He atoms in the BoseEinstein condensate have environment significantly different from uncondensed atoms, with the interatomic distance larger than the average by about 10%, whereas the average structure changes little through the superfluid transition. DPDF peak not seen in the snapshot pairdensity function is found at 2.3 A, and is interpreted in terms of atomic tunnelling. The real space picture of dynamic atomatom correlations presented here reveal characteristics of atomic dynamics not recognized so far, compelling yet another look at the phenomenon.

Evolutionary dynamics of the cryptocurrency market
http://hdl.handle.net/2117/113281
Evolutionary dynamics of the cryptocurrency market
ElBahrawy, Abeer; Alessandretti, Laura; Kandler, Anne; Pastor Satorras, Romualdo; Baronchelli, Andrea
The cryptocurrency market surpassed the barrier of $100 billion market capitalization in June 2017, after months of steady growth. Despite its increasing relevance in the financial world, a comprehensive analysis of the whole system is still lacking, as most studies have focused exclusively on the behaviour of one (Bitcoin) or few cryptocurrencies. Here, we consider the history of the entire market and analyse the behaviour of 1469 cryptocurrencies introduced between April 2013 and May 2017. We reveal that, while new cryptocurrencies appear and disappear continuously and their market capitalization is increasing (super)exponentially, several statistical properties of the market have been stable for years. These include the number of active cryptocurrencies, market share distribution and the turnover of cryptocurrencies. Adopting an ecological perspective, we show that the socalled neutral model of evolution is able to reproduce a number of key empirical observations, despite its simplicity and the assumption of no selective advantage of one cryptocurrency over another. Our results shed light on the properties of the cryptocurrency market and establish a first formal link between ecological modelling and the study of this growing system. We anticipate they will spark further research in this direction.
20180126T17:51:03Z
ElBahrawy, Abeer
Alessandretti, Laura
Kandler, Anne
Pastor Satorras, Romualdo
Baronchelli, Andrea
The cryptocurrency market surpassed the barrier of $100 billion market capitalization in June 2017, after months of steady growth. Despite its increasing relevance in the financial world, a comprehensive analysis of the whole system is still lacking, as most studies have focused exclusively on the behaviour of one (Bitcoin) or few cryptocurrencies. Here, we consider the history of the entire market and analyse the behaviour of 1469 cryptocurrencies introduced between April 2013 and May 2017. We reveal that, while new cryptocurrencies appear and disappear continuously and their market capitalization is increasing (super)exponentially, several statistical properties of the market have been stable for years. These include the number of active cryptocurrencies, market share distribution and the turnover of cryptocurrencies. Adopting an ecological perspective, we show that the socalled neutral model of evolution is able to reproduce a number of key empirical observations, despite its simplicity and the assumption of no selective advantage of one cryptocurrency over another. Our results shed light on the properties of the cryptocurrency market and establish a first formal link between ecological modelling and the study of this growing system. We anticipate they will spark further research in this direction.

Effects of temporal correlations in social multiplex networks
http://hdl.handle.net/2117/113278
Effects of temporal correlations in social multiplex networks
Starnini, Michele; Baronchelli, Andrea; Pastor Satorras, Romualdo
Multilayered networks represent a major advance in the description of natural complex systems, and their study has shed light on new physical phenomena. Despite its importance, however, the role of the temporal dimension in their structure and function has not been investigated in much detail so far. Here we study the temporal correlations between layers exhibited by real social multiplex networks. At a basic level, the presence of such correlations implies a certain degree of predictability in the contact pattern, as we quantify by an extension of the entropy and mutual information analyses proposed for the singlelayer case. At a different level, we demonstrate that temporal correlations are a signature of a ‘multitasking’ behavior of network agents, characterized by a higher level of switching between different social activities than expected in a uncorrelated pattern. Moreover, temporal correlations significantly affect the dynamics of coupled epidemic processes unfolding on the network. Our work opens the way for the systematic study of temporal multiplex networks and we anticipate it will be of interest to researchers in a broad array of fields.
20180126T15:22:22Z
Starnini, Michele
Baronchelli, Andrea
Pastor Satorras, Romualdo
Multilayered networks represent a major advance in the description of natural complex systems, and their study has shed light on new physical phenomena. Despite its importance, however, the role of the temporal dimension in their structure and function has not been investigated in much detail so far. Here we study the temporal correlations between layers exhibited by real social multiplex networks. At a basic level, the presence of such correlations implies a certain degree of predictability in the contact pattern, as we quantify by an extension of the entropy and mutual information analyses proposed for the singlelayer case. At a different level, we demonstrate that temporal correlations are a signature of a ‘multitasking’ behavior of network agents, characterized by a higher level of switching between different social activities than expected in a uncorrelated pattern. Moreover, temporal correlations significantly affect the dynamics of coupled epidemic processes unfolding on the network. Our work opens the way for the systematic study of temporal multiplex networks and we anticipate it will be of interest to researchers in a broad array of fields.

Dipolar Bose Supersolid Stripes
http://hdl.handle.net/2117/112550
Dipolar Bose Supersolid Stripes
Bombín Escudero, Raul; Boronat Medico, Jordi; Mazzanti Castrillejo, Fernando Pablo
We study the superfluid properties of a system of fully polarized dipolar bosons moving in the
XY plane. We focus on the general case where the polarization field forms an arbitrary angle a with respect to the Z axis, while the system is still stable. We use the diffusion Monte Carlo and the path integral ground state methods to evaluate the onebody density matrix and the superfluid fractions in the region of the phase diagram where the system forms stripes. Despite its oscillatory behavior, the presence of a finite largedistance asymptotic value in the swave component of the onebody density matrix indicates the existence of a Bose condensate. The superfluid fraction along the stripes direction is always close to 1, while in the Y direction decreases to a small value that is nevertheless different from zero. These two facts confirm that the stripe phase of the dipolar Bose system is a clear candidate for an intrinsic supersolid without the presence of defects as described by the AndreevLifshitz mechanism.
20180109T18:02:29Z
Bombín Escudero, Raul
Boronat Medico, Jordi
Mazzanti Castrillejo, Fernando Pablo
We study the superfluid properties of a system of fully polarized dipolar bosons moving in the
XY plane. We focus on the general case where the polarization field forms an arbitrary angle a with respect to the Z axis, while the system is still stable. We use the diffusion Monte Carlo and the path integral ground state methods to evaluate the onebody density matrix and the superfluid fractions in the region of the phase diagram where the system forms stripes. Despite its oscillatory behavior, the presence of a finite largedistance asymptotic value in the swave component of the onebody density matrix indicates the existence of a Bose condensate. The superfluid fraction along the stripes direction is always close to 1, while in the Y direction decreases to a small value that is nevertheless different from zero. These two facts confirm that the stripe phase of the dipolar Bose system is a clear candidate for an intrinsic supersolid without the presence of defects as described by the AndreevLifshitz mechanism.

Field theory for a reactiondiffusion model of quasispecies dynamics
http://hdl.handle.net/2117/112295
Field theory for a reactiondiffusion model of quasispecies dynamics
Pastor Satorras, Romualdo; Solé, Ricard Vicenç
RNA viruses are known to replicate with extremely high mutation rates. These rates are actually close to the socalled error threshold. This threshold is in fact a critical point beyond which genetic information is lost through a secondorder phase transition, which has been dubbed as the “error catastrophe.” Here we explore this phenomenon using a field theory approximation to the spatially extended SwetinaSchuster quasispecies model [J. Swetina and P. Schuster, Biophys. Chem. 16, 329 (1982)], a singlesharppeak landscape. In analogy with standard absorbingstate phase transitions, we develop a reactiondiffusion model whose discrete rules mimic the SwetinaSchuster model. The field theory representation of the reactiondiffusion system is constructed. The proposed field theory belongs to the same universality class as a conserved reactiondiffusion model previously proposed [F. van Wijland et al., Physica A 251, 179 (1998)]. From the field theory, we obtain the full set of exponents that characterize the critical behavior at the error threshold. Our results present the error catastrophe from a different point of view and suggest that spatial degrees of freedom can modify several meanfield predictions previously considered, leading to the definition of characteristic exponents that could be experimentally measurable.
20171219T15:48:10Z
Pastor Satorras, Romualdo
Solé, Ricard Vicenç
RNA viruses are known to replicate with extremely high mutation rates. These rates are actually close to the socalled error threshold. This threshold is in fact a critical point beyond which genetic information is lost through a secondorder phase transition, which has been dubbed as the “error catastrophe.” Here we explore this phenomenon using a field theory approximation to the spatially extended SwetinaSchuster quasispecies model [J. Swetina and P. Schuster, Biophys. Chem. 16, 329 (1982)], a singlesharppeak landscape. In analogy with standard absorbingstate phase transitions, we develop a reactiondiffusion model whose discrete rules mimic the SwetinaSchuster model. The field theory representation of the reactiondiffusion system is constructed. The proposed field theory belongs to the same universality class as a conserved reactiondiffusion model previously proposed [F. van Wijland et al., Physica A 251, 179 (1998)]. From the field theory, we obtain the full set of exponents that characterize the critical behavior at the error threshold. Our results present the error catastrophe from a different point of view and suggest that spatial degrees of freedom can modify several meanfield predictions previously considered, leading to the definition of characteristic exponents that could be experimentally measurable.

Bose polarons in ultracold atoms in one dimension: beyond the Fröhlich paradigm
http://hdl.handle.net/2117/111395
Bose polarons in ultracold atoms in one dimension: beyond the Fröhlich paradigm
Grusdt, Fabian; Astrakharchik, Grigori; Demler, Eugene
Mobile impurity atoms immersed in Bose–Einstein condensates provide a new platform for exploring Bose polarons. Recent experimental advances in the field of ultracold atoms make it possible to realize such systems with highly tunable microscopic parameters and to explore equilibrium and dynamical properties of polarons using a rich toolbox of atomic physics. In this paper we present a detailed theoretical analysis of Bose polarons in onedimensional systems of ultracold atoms. By combining a nonperturbative renormalization group approach with numerically exact diffusion Monte Carlo calculations we obtain not only detailed numerical results over a broad range of parameters but also qualitative understanding of different regimes of the system. We find that an accurate description of Bose polarons requires the inclusion of twophonon scattering terms which go beyond the commonly used Fröhlich model. Furthermore we show that when the Bose gas is in the strongly interacting regime, one needs to include interactions between the phonon modes. We use several theoretical approaches to calculate the polaron energy and its effective mass. The former can be measured using radiofrequency spectroscopy and the latter can be studied experimentally using impurity oscillations in a harmonic trapping potential. We compare our theoretical results for the effective mass to the experiments by Catani et al (2012 Phys. Rev. A 85 023623). In the weaktointermediate coupling regimes we obtain excellent quantitative agreement between theory and experiment, without any free fitting parameter. We supplement our analysis by full dynamical simulations of polaron oscillations in a shallow trapping potential. We also use our renormalization group approach to analyze the full phase diagram and identify regions that support repulsive and attractive polarons, as well as multiparticle bound states.
20171130T17:25:24Z
Grusdt, Fabian
Astrakharchik, Grigori
Demler, Eugene
Mobile impurity atoms immersed in Bose–Einstein condensates provide a new platform for exploring Bose polarons. Recent experimental advances in the field of ultracold atoms make it possible to realize such systems with highly tunable microscopic parameters and to explore equilibrium and dynamical properties of polarons using a rich toolbox of atomic physics. In this paper we present a detailed theoretical analysis of Bose polarons in onedimensional systems of ultracold atoms. By combining a nonperturbative renormalization group approach with numerically exact diffusion Monte Carlo calculations we obtain not only detailed numerical results over a broad range of parameters but also qualitative understanding of different regimes of the system. We find that an accurate description of Bose polarons requires the inclusion of twophonon scattering terms which go beyond the commonly used Fröhlich model. Furthermore we show that when the Bose gas is in the strongly interacting regime, one needs to include interactions between the phonon modes. We use several theoretical approaches to calculate the polaron energy and its effective mass. The former can be measured using radiofrequency spectroscopy and the latter can be studied experimentally using impurity oscillations in a harmonic trapping potential. We compare our theoretical results for the effective mass to the experiments by Catani et al (2012 Phys. Rev. A 85 023623). In the weaktointermediate coupling regimes we obtain excellent quantitative agreement between theory and experiment, without any free fitting parameter. We supplement our analysis by full dynamical simulations of polaron oscillations in a shallow trapping potential. We also use our renormalization group approach to analyze the full phase diagram and identify regions that support repulsive and attractive polarons, as well as multiparticle bound states.

Relating topological determinants of complex networks to their spectral properties: structural and dynamical effects
http://hdl.handle.net/2117/111394
Relating topological determinants of complex networks to their spectral properties: structural and dynamical effects
Castellano, Claudio; Pastor Satorras, Romualdo
The largest eigenvalue of a network’s adjacency matrix and its associated principal eigenvector are key elements for determining the topological structure and the properties of dynamical processes mediated by it. We present a physically grounded expression relating the value of the largest eigenvalue of a given network to the largest eigenvalue of two network subgraphs, considered as isolated: the hub with its immediate neighbors and the densely connected set of nodes with maximum
K
core index. We validate this formula by showing that it predicts, with good accuracy, the largest eigenvalue of a large set of synthetic and realworld topologies. We also present evidence of the consequences of these findings for broad classes of dynamics taking place on the networks. As a byproduct, we reveal that the spectral properties of heterogeneous networks built according to the linear preferential attachment model are qualitatively different from those of their static counterparts.
20171130T16:17:00Z
Castellano, Claudio
Pastor Satorras, Romualdo
The largest eigenvalue of a network’s adjacency matrix and its associated principal eigenvector are key elements for determining the topological structure and the properties of dynamical processes mediated by it. We present a physically grounded expression relating the value of the largest eigenvalue of a given network to the largest eigenvalue of two network subgraphs, considered as isolated: the hub with its immediate neighbors and the densely connected set of nodes with maximum
K
core index. We validate this formula by showing that it predicts, with good accuracy, the largest eigenvalue of a large set of synthetic and realworld topologies. We also present evidence of the consequences of these findings for broad classes of dynamics taking place on the networks. As a byproduct, we reveal that the spectral properties of heterogeneous networks built according to the linear preferential attachment model are qualitatively different from those of their static counterparts.

Edge dislocations as sinks for subnanometric radiation induced defects in airon
http://hdl.handle.net/2117/111001
Edge dislocations as sinks for subnanometric radiation induced defects in airon
Anento Moreno, Napoleón; Malerba, L.; Serra Tort, Ana María
The role of edge dislocations as sinks for small radiation induced defects in bccFe is investigated by means of atomistic computer simulation. In this work we investigate by Molecular Statics (T = 0K) the interaction between an immobile dislocation line and defect clusters of small sizes invisible experimentally. The study highlights in particular the anisotropy of the interaction and distinguishes between absorbed and trapped defects. When the considered defect intersects the dislocation glide plane and the distance from the dislocation line to the defect is on the range between 2 nm and 4 nm, either total or partial absorption of the cluster takes place leading to the formation of jogs. Residual defects produced during partial absorption pin the dislocation. By the calculation of stressstrain curves we have assessed the strength of those residues as obstacles for the motion of the dislocation, which is reflected on the unpinning stresses and the binding energies obtained. When the defect is outside this range, but on planes close to the dislocation glide plane, instead of absorption we have observed a capture process. Finally, with a view to introducing explicitly in kinetic Monte Carlo models a sink with the shape of a dislocation line, we have summarized our findings on a table presenting the most relevant parameters, which define the interaction of the dislocation with the defects considered.
20171121T15:22:59Z
Anento Moreno, Napoleón
Malerba, L.
Serra Tort, Ana María
The role of edge dislocations as sinks for small radiation induced defects in bccFe is investigated by means of atomistic computer simulation. In this work we investigate by Molecular Statics (T = 0K) the interaction between an immobile dislocation line and defect clusters of small sizes invisible experimentally. The study highlights in particular the anisotropy of the interaction and distinguishes between absorbed and trapped defects. When the considered defect intersects the dislocation glide plane and the distance from the dislocation line to the defect is on the range between 2 nm and 4 nm, either total or partial absorption of the cluster takes place leading to the formation of jogs. Residual defects produced during partial absorption pin the dislocation. By the calculation of stressstrain curves we have assessed the strength of those residues as obstacles for the motion of the dislocation, which is reflected on the unpinning stresses and the binding energies obtained. When the defect is outside this range, but on planes close to the dislocation glide plane, instead of absorption we have observed a capture process. Finally, with a view to introducing explicitly in kinetic Monte Carlo models a sink with the shape of a dislocation line, we have summarized our findings on a table presenting the most relevant parameters, which define the interaction of the dislocation with the defects considered.