Articles de revista
http://hdl.handle.net/2117/395
Wed, 17 Jan 2018 11:02:13 GMT2018-01-17T11:02:13ZDipolar 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 one-body 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 large-distance asymptotic value in the s-wave component of the one-body 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 Andreev-Lifshitz mechanism.
Tue, 09 Jan 2018 18:02:29 GMThttp://hdl.handle.net/2117/1125502018-01-09T18:02:29ZBombín Escudero, RaulBoronat Medico, JordiMazzanti Castrillejo, Fernando PabloWe 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 one-body 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 large-distance asymptotic value in the s-wave component of the one-body 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 Andreev-Lifshitz mechanism.Field theory for a reaction-diffusion model of quasispecies dynamics
http://hdl.handle.net/2117/112295
Field theory for a reaction-diffusion 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 so-called error threshold. This threshold is in fact a critical point beyond which genetic information is lost through a second-order phase transition, which has been dubbed as the “error catastrophe.” Here we explore this phenomenon using a field theory approximation to the spatially extended Swetina-Schuster quasispecies model [J. Swetina and P. Schuster, Biophys. Chem. 16, 329 (1982)], a single-sharp-peak landscape. In analogy with standard absorbing-state phase transitions, we develop a reaction-diffusion model whose discrete rules mimic the Swetina-Schuster model. The field theory representation of the reaction-diffusion system is constructed. The proposed field theory belongs to the same universality class as a conserved reaction-diffusion 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 mean-field predictions previously considered, leading to the definition of characteristic exponents that could be experimentally measurable.
Tue, 19 Dec 2017 15:48:10 GMThttp://hdl.handle.net/2117/1122952017-12-19T15:48:10ZPastor Satorras, RomualdoSolé, Ricard VicençRNA viruses are known to replicate with extremely high mutation rates. These rates are actually close to the so-called error threshold. This threshold is in fact a critical point beyond which genetic information is lost through a second-order phase transition, which has been dubbed as the “error catastrophe.” Here we explore this phenomenon using a field theory approximation to the spatially extended Swetina-Schuster quasispecies model [J. Swetina and P. Schuster, Biophys. Chem. 16, 329 (1982)], a single-sharp-peak landscape. In analogy with standard absorbing-state phase transitions, we develop a reaction-diffusion model whose discrete rules mimic the Swetina-Schuster model. The field theory representation of the reaction-diffusion system is constructed. The proposed field theory belongs to the same universality class as a conserved reaction-diffusion 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 mean-field 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 one-dimensional systems of ultracold atoms. By combining a non-perturbative 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 two-phonon 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 radio-frequency 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 weak-to-intermediate 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 multi-particle bound states.
Thu, 30 Nov 2017 17:25:24 GMThttp://hdl.handle.net/2117/1113952017-11-30T17:25:24ZGrusdt, FabianAstrakharchik, GrigoriDemler, EugeneMobile 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 one-dimensional systems of ultracold atoms. By combining a non-perturbative 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 two-phonon 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 radio-frequency 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 weak-to-intermediate 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 multi-particle 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 real-world topologies. We also present evidence of the consequences of these findings for broad classes of dynamics taking place on the networks. As a by-product, 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.
Thu, 30 Nov 2017 16:17:00 GMThttp://hdl.handle.net/2117/1113942017-11-30T16:17:00ZCastellano, ClaudioPastor Satorras, RomualdoThe 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 real-world topologies. We also present evidence of the consequences of these findings for broad classes of dynamics taking place on the networks. As a by-product, 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 sub-nanometric radiation induced defects in a-iron
http://hdl.handle.net/2117/111001
Edge dislocations as sinks for sub-nanometric radiation induced defects in a-iron
Anento Moreno, Napoleón; Malerba, L.; Serra Tort, Ana María
The role of edge dislocations as sinks for small radiation induced defects in bcc-Fe 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 stress-strain 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.
Tue, 21 Nov 2017 15:22:59 GMThttp://hdl.handle.net/2117/1110012017-11-21T15:22:59ZAnento Moreno, NapoleónMalerba, L.Serra Tort, Ana MaríaThe role of edge dislocations as sinks for small radiation induced defects in bcc-Fe 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 stress-strain 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.Free-energy surfaces of ionic adsorption in cholesterol-free and cholesterol-rich phospholipid membranes
http://hdl.handle.net/2117/110069
Free-energy surfaces of ionic adsorption in cholesterol-free and cholesterol-rich phospholipid membranes
Martí Rabassa, Jordi
Free energy surfaces associated to the adsorption of metal cations ((Formula presented.), (Formula presented.), (Formula presented.), and (Formula presented.)) in biological environments have been computed by metadynamics simulations. In all cases, the systems were modelled using the CHARMM36 force field. The free-energy landscapes unveil specific binding behaviour of metal cations. So, (Formula presented.) and (Formula presented.) are more likely to stay in the aqueous solution, and can easily bind to a few lipid oxygens by overcoming low free-energy barriers. Differently, (Formula presented.) is most stable when bound to four lipid oxygens of the membranes, rather than being hydrated in the aqueous solution. Finally, (Formula presented.) is tightly hydrated, and can hardly lose a hydration water and bind directly to the membranes. When cholesterol is included inside the membrane at concentration up to 50%, the resulting free-energy landscapes reveal the competition between binding of sodium to water and to lipid head groups, although the binding competitiveness of lipid head groups is diminished by cholesterol contents. When cholesterol concentration is greater than 30%, the ionic binding is significantly reduced, which coincides with the phase transition point of DMPC-cholesterol membranes from a liquid-disordered phase to a liquid-ordered phase.
Mon, 06 Nov 2017 19:42:35 GMThttp://hdl.handle.net/2117/1100692017-11-06T19:42:35ZMartí Rabassa, JordiFree energy surfaces associated to the adsorption of metal cations ((Formula presented.), (Formula presented.), (Formula presented.), and (Formula presented.)) in biological environments have been computed by metadynamics simulations. In all cases, the systems were modelled using the CHARMM36 force field. The free-energy landscapes unveil specific binding behaviour of metal cations. So, (Formula presented.) and (Formula presented.) are more likely to stay in the aqueous solution, and can easily bind to a few lipid oxygens by overcoming low free-energy barriers. Differently, (Formula presented.) is most stable when bound to four lipid oxygens of the membranes, rather than being hydrated in the aqueous solution. Finally, (Formula presented.) is tightly hydrated, and can hardly lose a hydration water and bind directly to the membranes. When cholesterol is included inside the membrane at concentration up to 50%, the resulting free-energy landscapes reveal the competition between binding of sodium to water and to lipid head groups, although the binding competitiveness of lipid head groups is diminished by cholesterol contents. When cholesterol concentration is greater than 30%, the ionic binding is significantly reduced, which coincides with the phase transition point of DMPC-cholesterol membranes from a liquid-disordered phase to a liquid-ordered phase.Composite boson description of a low-density gas of excitons
http://hdl.handle.net/2117/110068
Composite boson description of a low-density gas of excitons
Golomedov, A. E.; Lozovik, Yu. E.; Astrakharchik, Grigori; Boronat Medico, Jordi
Ground-state properties of a fermionic Coulomb gas are calculated using the fixed-node diffusion Monte Carlo method. The validity of the composite boson description is tested for different densities. We extract the exciton–exciton s-wave scattering length by solving the four-body problem in a harmonic trap and mapping the energy to that of two trapped bosons. The equation of state is consistent with the Bogoliubov theory for composite bosons interacting with the obtained s-wave scattering length. The perturbative expansion at low density has contributions physically coming from (a) exciton binding energy, (b) mean-field Gross–Pitaevskii interaction between excitons, and (c) quantum depletion of the excitonic condensate (Lee–Huang–Yang terms for composite bosons). In addition, for low densities we find a good agreement with the Bogoliubov bosonic theory for the condensate fraction of excitons. The equation of state in the opposite limit of large density is found to be well described by the perturbative theory including (a) mixture of two ideal Fermi gases and (b) exchange energy. We find that for low densities both energetic and coherent properties are correctly described by the picture of composite bosons (excitons).
Mon, 06 Nov 2017 19:27:49 GMThttp://hdl.handle.net/2117/1100682017-11-06T19:27:49ZGolomedov, A. E.Lozovik, Yu. E.Astrakharchik, GrigoriBoronat Medico, JordiGround-state properties of a fermionic Coulomb gas are calculated using the fixed-node diffusion Monte Carlo method. The validity of the composite boson description is tested for different densities. We extract the exciton–exciton s-wave scattering length by solving the four-body problem in a harmonic trap and mapping the energy to that of two trapped bosons. The equation of state is consistent with the Bogoliubov theory for composite bosons interacting with the obtained s-wave scattering length. The perturbative expansion at low density has contributions physically coming from (a) exciton binding energy, (b) mean-field Gross–Pitaevskii interaction between excitons, and (c) quantum depletion of the excitonic condensate (Lee–Huang–Yang terms for composite bosons). In addition, for low densities we find a good agreement with the Bogoliubov bosonic theory for the condensate fraction of excitons. The equation of state in the opposite limit of large density is found to be well described by the perturbative theory including (a) mixture of two ideal Fermi gases and (b) exchange energy. We find that for low densities both energetic and coherent properties are correctly described by the picture of composite bosons (excitons).Grain boundary mediated plasticity: the role of grain boundary atomic structure and thermal activation
http://hdl.handle.net/2117/109068
Grain boundary mediated plasticity: the role of grain boundary atomic structure and thermal activation
Terentyev, Dimitry; Bakaev, A.; Serra Tort, Ana María; Pavia, F.; Baker, K. L.; Anento Moreno, Napoleón
The interaction of dislocation pile-ups with several tilt grain boundaries (GB) is studied in copper by using a hybrid continuum-atomistic approach. The effects of temperature, pile-up intensity and GB structure on absorption and transmission of slip as a function of local stress state are explored. By considering several high-angle GBs with different misorientation angles, we demonstrate that GB atomic structure primarily defines its ability to accommodate incoming pile-up dislocations, thus limiting the direct transmission of pile-ups through the interface.
Tue, 24 Oct 2017 15:35:10 GMThttp://hdl.handle.net/2117/1090682017-10-24T15:35:10ZTerentyev, DimitryBakaev, A.Serra Tort, Ana MaríaPavia, F.Baker, K. L.Anento Moreno, NapoleónThe interaction of dislocation pile-ups with several tilt grain boundaries (GB) is studied in copper by using a hybrid continuum-atomistic approach. The effects of temperature, pile-up intensity and GB structure on absorption and transmission of slip as a function of local stress state are explored. By considering several high-angle GBs with different misorientation angles, we demonstrate that GB atomic structure primarily defines its ability to accommodate incoming pile-up dislocations, thus limiting the direct transmission of pile-ups through the interface.Lepton flavor changing Higgs decays in the littlest Higgs model with T-parity
http://hdl.handle.net/2117/108150
Lepton flavor changing Higgs decays in the littlest Higgs model with T-parity
del Aguila Jiménez, Francisco; Ametller Congost, Lluís; Illana, José Ignacio; Santiago, Jose; Talavera, Pere; Vega-Morales, Roberto
We calculate loop induced lepton flavor violating Higgs decays in the Littlest
Higgs model with T-parity. We find that a finite amplitude is obtained only when all
contributions from the T-odd lepton sector are included. This is in contrast to lepton fla-
vor violating processes mediated by gauge bosons where the partners of the right-handed
mirror leptons can be decoupled from the spectrum. These partners are necessary to can-
cel the divergence in the Higgs mass introduced by the mirror leptons but are otherwise
unnecessary and assumed to be decoupled in previous phenomenological studies. Further-
more, as we emphasize, including the partner leptons in the spectrum also introduces a
new source of lepton flavor violation via their couplings to the physical pseudo-Goldstone
electroweak triplet scalar. Although this extra source also affects lepton flavor changing
gauge transitions, it decouples from these amplitudes in the limit of heavy mass for the
partner leptons. We find that the corresponding Higgs branching ratio into taus and muons
can be as large as ~ 0.2 × 10 -6 for T-odd masses of the order a few TeV, a demanding
challenge even for the high luminosity LHC.
Fri, 29 Sep 2017 09:56:05 GMThttp://hdl.handle.net/2117/1081502017-09-29T09:56:05Zdel Aguila Jiménez, FranciscoAmetller Congost, LluísIllana, José IgnacioSantiago, JoseTalavera, PereVega-Morales, RobertoWe calculate loop induced lepton flavor violating Higgs decays in the Littlest
Higgs model with T-parity. We find that a finite amplitude is obtained only when all
contributions from the T-odd lepton sector are included. This is in contrast to lepton fla-
vor violating processes mediated by gauge bosons where the partners of the right-handed
mirror leptons can be decoupled from the spectrum. These partners are necessary to can-
cel the divergence in the Higgs mass introduced by the mirror leptons but are otherwise
unnecessary and assumed to be decoupled in previous phenomenological studies. Further-
more, as we emphasize, including the partner leptons in the spectrum also introduces a
new source of lepton flavor violation via their couplings to the physical pseudo-Goldstone
electroweak triplet scalar. Although this extra source also affects lepton flavor changing
gauge transitions, it decouples from these amplitudes in the limit of heavy mass for the
partner leptons. We find that the corresponding Higgs branching ratio into taus and muons
can be as large as ~ 0.2 × 10 -6 for T-odd masses of the order a few TeV, a demanding
challenge even for the high luminosity LHC.Local structural fluctuations, hydrogen bonding and structural transitions in supercritical water
http://hdl.handle.net/2117/108130
Local structural fluctuations, hydrogen bonding and structural transitions in supercritical water
Skarmoutsos, Ioannis; Guàrdia Manuel, Elvira; Samios, Jannis
The contribution of hydrogen bonding interactions to the formation of local density inhomogeneities in supercritical water at near-critical conditions has been extensively studied by means of molecular dynamics simulations. The results obtained have revealed the strong effect of water molecules forming one and two hydrogen bonds on the determination of the local density augmentation in the fluid. The local structural order has also been studied in terms of the trigonal and tetrahedral order parameters, revealing the correlation between local orientational order and hydrogen bonding. The dynamics of the structural order parameters exhibit similarities with local density ones. The local structural analysis performed in terms of nearest neighbors around the individual molecules provides additional significant evidence about the existence of a liquid-like to gas-like structural transition in supercritical water at the density range close to 0.2 ¿c, further supporting previous suggestions based on the interpretation of experimental thermodynamic data.
Thu, 28 Sep 2017 18:17:02 GMThttp://hdl.handle.net/2117/1081302017-09-28T18:17:02ZSkarmoutsos, IoannisGuàrdia Manuel, ElviraSamios, JannisThe contribution of hydrogen bonding interactions to the formation of local density inhomogeneities in supercritical water at near-critical conditions has been extensively studied by means of molecular dynamics simulations. The results obtained have revealed the strong effect of water molecules forming one and two hydrogen bonds on the determination of the local density augmentation in the fluid. The local structural order has also been studied in terms of the trigonal and tetrahedral order parameters, revealing the correlation between local orientational order and hydrogen bonding. The dynamics of the structural order parameters exhibit similarities with local density ones. The local structural analysis performed in terms of nearest neighbors around the individual molecules provides additional significant evidence about the existence of a liquid-like to gas-like structural transition in supercritical water at the density range close to 0.2 ¿c, further supporting previous suggestions based on the interpretation of experimental thermodynamic data.