SIMCON - Grup de Recerca de Simulació per Ordinador en Matèria Condensada
http://hdl.handle.net/2117/394
2017-03-30T04:52:46ZGapped spectrum in pair-superfluid bosons
http://hdl.handle.net/2117/102969
Gapped spectrum in pair-superfluid bosons
Astrakharchik, Grigori; Zillich, Robert E.; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
We study the ground state of a bilayer system of dipolar bosons with dipoles oriented by an external field perpendicularly to the two parallel planes. By decreasing the interlayer distance, for a fixed value of the strength of the dipolar interaction, the system undergoes a quantum phase transition from an atomic to a pair superfluid. We investigate the excitation spectrum on both sides of this transition by using two microscopic approaches. Quantum Monte Carlo methods are employed to obtain the static structure factors and intermediate scattering functions in imaginary time. The dynamic response is calculated using both the correlated basis functions (CBF) method and the approximate inversion of the Laplace transform of the quantum Monte Carlo imaginary time data. In the atomic phase, both the density and spin excitations are gapless. However, in the pair-superfluid phase a gap opens in the excitation energy of the spin mode. For small separation between layers, the minimal spin excitation energy equals the binding energy of a dimer and is twice the gap value.
2017-03-28T13:25:11ZAstrakharchik, GrigoriZillich, Robert E.Mazzanti Castrillejo, Fernando PabloBoronat Medico, JordiWe study the ground state of a bilayer system of dipolar bosons with dipoles oriented by an external field perpendicularly to the two parallel planes. By decreasing the interlayer distance, for a fixed value of the strength of the dipolar interaction, the system undergoes a quantum phase transition from an atomic to a pair superfluid. We investigate the excitation spectrum on both sides of this transition by using two microscopic approaches. Quantum Monte Carlo methods are employed to obtain the static structure factors and intermediate scattering functions in imaginary time. The dynamic response is calculated using both the correlated basis functions (CBF) method and the approximate inversion of the Laplace transform of the quantum Monte Carlo imaginary time data. In the atomic phase, both the density and spin excitations are gapless. However, in the pair-superfluid phase a gap opens in the excitation energy of the spin mode. For small separation between layers, the minimal spin excitation energy equals the binding energy of a dimer and is twice the gap value.Structure and dynamics of water at carbon-based interfaces
http://hdl.handle.net/2117/102926
Structure and dynamics of water at carbon-based interfaces
Martí Rabassa, Jordi; Calero, Carles; Franzese, Giancarlo
Water structure and dynamics are affected by the presence of a nearby interface. Here,
first we review recent results by molecular dynamics simulations about the effect of different carbon-based materials, including armchair carbon nanotubes and a variety of graphene sheets—flat and with corrugation—on water structure and dynamics. We discuss the calculations of binding energies, hydrogen bond distributions, water’s diffusion coefficients and their relation with surface’s geometries at different thermodynamical conditions. Next, we present new results of the crystallization and dynamics of water in a rigid graphene sieve. In particular, we show that the
diffusion of water confined between parallel walls depends on the plate distance in a non-monotonic way and is related to the water structuring, crystallization, re-melting and evaporation for decreasing inter-plate distance. Our results could be relevant in those applications where water is in contact with nanostructured carbon materials at ambient or cryogenic temperatures, as in man-made superhydrophobic materials or filtration membranes, or in techniques that take advantage of hydrated graphene interfaces, as in aqueous electron cryomicroscopy for the analysis of proteins adsorbed on graphene.
2017-03-27T17:11:41ZMartí Rabassa, JordiCalero, CarlesFranzese, GiancarloWater structure and dynamics are affected by the presence of a nearby interface. Here,
first we review recent results by molecular dynamics simulations about the effect of different carbon-based materials, including armchair carbon nanotubes and a variety of graphene sheets—flat and with corrugation—on water structure and dynamics. We discuss the calculations of binding energies, hydrogen bond distributions, water’s diffusion coefficients and their relation with surface’s geometries at different thermodynamical conditions. Next, we present new results of the crystallization and dynamics of water in a rigid graphene sieve. In particular, we show that the
diffusion of water confined between parallel walls depends on the plate distance in a non-monotonic way and is related to the water structuring, crystallization, re-melting and evaporation for decreasing inter-plate distance. Our results could be relevant in those applications where water is in contact with nanostructured carbon materials at ambient or cryogenic temperatures, as in man-made superhydrophobic materials or filtration membranes, or in techniques that take advantage of hydrated graphene interfaces, as in aqueous electron cryomicroscopy for the analysis of proteins adsorbed on graphene.Topological structure and the H index in complex networks
http://hdl.handle.net/2117/102685
Topological structure and the H index in complex networks
Pastor Satorras, Romualdo; Castellano, Claudio
The generalized H(n) Hirsch index of order n has been recently introduced and shown to interpolate between the degree and the K-core centrality in networks. We provide a detailed analytic characterization of the properties of sets of nodes having the same H(n), within the annealed network approximation. The connection between the Hirsch indices and the degree is highlighted. Numerical tests in synthetic uncorrelated networks and real-world correlated ones validate the findings. We also test the use of the Hirsch index for the identification of influential spreaders in networks, finding that it is in general outperformed by the recently introduced nonbacktracking centrality.
2017-03-20T19:23:16ZPastor Satorras, RomualdoCastellano, ClaudioThe generalized H(n) Hirsch index of order n has been recently introduced and shown to interpolate between the degree and the K-core centrality in networks. We provide a detailed analytic characterization of the properties of sets of nodes having the same H(n), within the annealed network approximation. The connection between the Hirsch indices and the degree is highlighted. Numerical tests in synthetic uncorrelated networks and real-world correlated ones validate the findings. We also test the use of the Hirsch index for the identification of influential spreaders in networks, finding that it is in general outperformed by the recently introduced nonbacktracking centrality.One-dimensional multicomponent Fermi gas in a trap: quantum Monte Carlo study
http://hdl.handle.net/2117/102684
One-dimensional multicomponent Fermi gas in a trap: quantum Monte Carlo study
Matveeva, N.; Astrakharchik, Grigori
A one-dimensional world is very unusual as there is an interplay between quantum statistics and geometry, and a strong short-range repulsion between atoms mimics Fermi exclusion principle, fermionizing the system. Instead, a system with a large number of components with a single atom in each, on the opposite acquires many bosonic properties. We study the ground-state properties of a multicomponent repulsive Fermi gas trapped in a harmonic trap by a fixed-node diffusion Monte Carlo method. The interaction between all components is considered to be the same. We investigate how the energetic properties (energy, contact) and correlation functions (density profile and momentum distribution) evolve as the number of components is changed. It is shown that the system fermionizes in the limit of strong interactions. Analytical expressions are derived in the limit of weak interactions within the local density approximation for an arbitrary number of components and for one plus one particle using an exact solution.
2017-03-20T18:57:22ZMatveeva, N.Astrakharchik, GrigoriA one-dimensional world is very unusual as there is an interplay between quantum statistics and geometry, and a strong short-range repulsion between atoms mimics Fermi exclusion principle, fermionizing the system. Instead, a system with a large number of components with a single atom in each, on the opposite acquires many bosonic properties. We study the ground-state properties of a multicomponent repulsive Fermi gas trapped in a harmonic trap by a fixed-node diffusion Monte Carlo method. The interaction between all components is considered to be the same. We investigate how the energetic properties (energy, contact) and correlation functions (density profile and momentum distribution) evolve as the number of components is changed. It is shown that the system fermionizes in the limit of strong interactions. Analytical expressions are derived in the limit of weak interactions within the local density approximation for an arbitrary number of components and for one plus one particle using an exact solution.Maxwell equations in the Debye potential formalism
http://hdl.handle.net/2117/102159
Maxwell equations in the Debye potential formalism
Fayos Vallés, Francisco; Llanta Salleras, Estanislao; Llosa, J
The authors propose a method for computing the electromagnetic test-field created by a given distribution of charges and currents using the Debye potential formalism in curved space-times. This framework has been applied explicitly to the Schwarzschild case.
2017-03-08T18:24:18ZFayos Vallés, FranciscoLlanta Salleras, EstanislaoLlosa, JThe authors propose a method for computing the electromagnetic test-field created by a given distribution of charges and currents using the Debye potential formalism in curved space-times. This framework has been applied explicitly to the Schwarzschild case.Luttinger parameter of quasi-one-dimensional para- H2
http://hdl.handle.net/2117/101993
Luttinger parameter of quasi-one-dimensional para- H2
Ferré Porta, Guillem; Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
We have studied the ground-state properties of para-hydrogen in one dimension and in quasi-one-dimensional configurations using the path-integral ground-state Monte Carlo method. This method produces zero-temperature exact results for a given interaction and geometry. The quasi-one-dimensional setup has been implemented in two forms: the inner channel inside a carbon nanotube coated with H2 and a harmonic confinement of variable strength. Our main result is the dependence of the Luttinger parameter on the density within the stable regime. Going from one dimension to quasi-one dimension, keeping the linear density constant, produces a systematic increase of the Luttinger parameter. This increase is, however, not enough to reach the superfluid regime and the system always remain in the quasicrystal regime, according to Luttinger liquid theory.
2017-03-06T20:05:14ZFerré Porta, GuillemGordillo Bargueño, Maria CarmenBoronat Medico, JordiWe have studied the ground-state properties of para-hydrogen in one dimension and in quasi-one-dimensional configurations using the path-integral ground-state Monte Carlo method. This method produces zero-temperature exact results for a given interaction and geometry. The quasi-one-dimensional setup has been implemented in two forms: the inner channel inside a carbon nanotube coated with H2 and a harmonic confinement of variable strength. Our main result is the dependence of the Luttinger parameter on the density within the stable regime. Going from one dimension to quasi-one dimension, keeping the linear density constant, produces a systematic increase of the Luttinger parameter. This increase is, however, not enough to reach the superfluid regime and the system always remain in the quasicrystal regime, according to Luttinger liquid theory.Structural and dipolar fluctuations in liquid water: a Car–Parrinello molecular dynamics study
http://hdl.handle.net/2117/101697
Structural and dipolar fluctuations in liquid water: a Car–Parrinello molecular dynamics study
Skarmoutsos, Ioannis; Masia, Marco; Guàrdia Manuel, Elvira
A Car–Parrinello molecular dynamics simulation was performed to investigate the local tetrahedral order, molecular dipole fluctuations and their interrelation with hydrogen bonding in liquid water. Water molecules were classified in three types, exhibiting low, intermediate and high tetrahedral order. Transitions from low to high tetrahedrally ordered structures take place only through transitions to the intermediate state. The molecular dipole moments depend strongly on the tetrahedral order and hydrogen bonding. The average dipole moment of water molecules with a strong tetrahedral order around them comes in excellent agreement with previous estimations of the dipole moment of ice Ih molecules.
2017-02-28T13:26:41ZSkarmoutsos, IoannisMasia, MarcoGuàrdia Manuel, ElviraA Car–Parrinello molecular dynamics simulation was performed to investigate the local tetrahedral order, molecular dipole fluctuations and their interrelation with hydrogen bonding in liquid water. Water molecules were classified in three types, exhibiting low, intermediate and high tetrahedral order. Transitions from low to high tetrahedrally ordered structures take place only through transitions to the intermediate state. The molecular dipole moments depend strongly on the tetrahedral order and hydrogen bonding. The average dipole moment of water molecules with a strong tetrahedral order around them comes in excellent agreement with previous estimations of the dipole moment of ice Ih molecules.Solvation dynamics in water: 2. Energy fluxes on excited- and ground-state surfaces
http://hdl.handle.net/2117/101568
Solvation dynamics in water: 2. Energy fluxes on excited- and ground-state surfaces
Rey Oriol, Rosendo; Hynes, James T.
This series’ first installment introduced an approach to solvation dynamics focused on expressing the emission frequency shift (following electronic excitation of, and resulting charge change or redistribution in, a solute) in terms of energy fluxes, a work and power perspective. This approach, which had been previously exploited for rotational and vibrational excitation-induced energy flow, has the novel advantage of providing a quantitative view and understanding of the molecular-level mechanisms involved in the solvation dynamics via tracing of the energy flow induced by the electronic excitation’s charge change or redistribution in the solute. This new methodology, which was illustrated for the case in which only the excited electronic state surface contributes to the frequency shift (ionization of a monatomic solute in water), is here extended to the general case in which both the excited and ground electronic states may contribute. Simple monatomic solute model variations allow a discussion of the (sometimes surprising) issues involved in assessing each surface’s contribution. The calculation of properly defined energy fluxes/work allows a more complete understanding of the solvation dynamics even when the real work for one of the surfaces does not directly contribute to the frequency shift, an aspect further emphasizing the utility of an energy flux approach.
2017-02-24T16:03:23ZRey Oriol, RosendoHynes, James T.This series’ first installment introduced an approach to solvation dynamics focused on expressing the emission frequency shift (following electronic excitation of, and resulting charge change or redistribution in, a solute) in terms of energy fluxes, a work and power perspective. This approach, which had been previously exploited for rotational and vibrational excitation-induced energy flow, has the novel advantage of providing a quantitative view and understanding of the molecular-level mechanisms involved in the solvation dynamics via tracing of the energy flow induced by the electronic excitation’s charge change or redistribution in the solute. This new methodology, which was illustrated for the case in which only the excited electronic state surface contributes to the frequency shift (ionization of a monatomic solute in water), is here extended to the general case in which both the excited and ground electronic states may contribute. Simple monatomic solute model variations allow a discussion of the (sometimes surprising) issues involved in assessing each surface’s contribution. The calculation of properly defined energy fluxes/work allows a more complete understanding of the solvation dynamics even when the real work for one of the surfaces does not directly contribute to the frequency shift, an aspect further emphasizing the utility of an energy flux approach.Quantum halo states in helium tetramers
http://hdl.handle.net/2117/101558
Quantum halo states in helium tetramers
Stipanovic, P.; Vranješ Markic, Leandra; Boronat Medico, Jordi
The universality of quantum halo states enables a comparison of systems from different fields of physics, as demonstrated in two- and three-body clusters. In the present work, we studied weakly bound helium tetramers in order to test whether some of these four-body realistic systems qualify as halos. Their ground-state binding energies and structural properties were thoroughly estimated using the diffusion Monte Carlo method with pure estimators. Helium tetramer properties proved to be less sensitive on the potential model than previously evaluated trimer properties. We predict the existence of realistic four-body halo 4He23He2, whereas 4He4 and 4He33He are close to the border and thus can be used as prototypes of quasi-halo systems. Our results could be tested by the experimental determination of the tetramers’ structural properties using a setup similar to the one developed for the study of helium trimers.
2017-02-24T15:10:38ZStipanovic, P.Vranješ Markic, LeandraBoronat Medico, JordiThe universality of quantum halo states enables a comparison of systems from different fields of physics, as demonstrated in two- and three-body clusters. In the present work, we studied weakly bound helium tetramers in order to test whether some of these four-body realistic systems qualify as halos. Their ground-state binding energies and structural properties were thoroughly estimated using the diffusion Monte Carlo method with pure estimators. Helium tetramer properties proved to be less sensitive on the potential model than previously evaluated trimer properties. We predict the existence of realistic four-body halo 4He23He2, whereas 4He4 and 4He33He are close to the border and thus can be used as prototypes of quasi-halo systems. Our results could be tested by the experimental determination of the tetramers’ structural properties using a setup similar to the one developed for the study of helium trimers.Ground-state properties of weakly bound helium-alkali trimers
http://hdl.handle.net/2117/100340
Ground-state properties of weakly bound helium-alkali trimers
Stipanovic, P.; Vranješ Markic, Leandra; Zaric, D.; Boronat Medico, Jordi
Weakly bound triatomic molecules consisting of two helium atoms and one alkali metal atom are studied by means of the diffusion Monte Carlo method. We determined the stability of 4He2A, 4He3HeA, and 3He2A, where A is one of the alkali atoms Li, Na, K, Rb, or Cs. Some of the trimers with 3He are predicted to be self-bound for the first time, but this is observed to be dependent on the He–A interaction potential model. In addition to the ground-state energy of the trimers, we determined their density, radial, and angular distributions. Many of them are spatially very extended, which qualifies them as quantum halo states
2017-01-30T19:26:00ZStipanovic, P.Vranješ Markic, LeandraZaric, D.Boronat Medico, JordiWeakly bound triatomic molecules consisting of two helium atoms and one alkali metal atom are studied by means of the diffusion Monte Carlo method. We determined the stability of 4He2A, 4He3HeA, and 3He2A, where A is one of the alkali atoms Li, Na, K, Rb, or Cs. Some of the trimers with 3He are predicted to be self-bound for the first time, but this is observed to be dependent on the He–A interaction potential model. In addition to the ground-state energy of the trimers, we determined their density, radial, and angular distributions. Many of them are spatially very extended, which qualifies them as quantum halo states