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http://hdl.handle.net/2117/394
2015-04-01T13:15:26ZSharp crossover from composite fermionization to phase separation in microscopic mixtures of ultracold bosons
http://hdl.handle.net/2117/27031
Title: Sharp crossover from composite fermionization to phase separation in microscopic mixtures of ultracold bosons
Authors: Garcia March, Miguel Angel; Julia Diaz, Bruno; Astrakharchik, Grigori; Busch, T; Boronat Medico, Jordi; Rios Polls, Artur
Abstract: We show that a two-component mixture of a few repulsively interacting ultracold atoms in a one-dimensional trap possesses very diverse quantum regimes and that the crossover between them can be induced by tuning the interactions in one of the species. Starting from the composite fermionization regime, in which the interactions between both components are large and neither gas is phase coherent, our results show that a phase-separated state can be reached by increasing the interaction in one of the species. In this regime, the weakly interacting component stays at the center of the trap and becomes almost fully phase coherent, while the strongly interacting one is expelled to the edges of the trap. The crossover is sharp, as can be witnessed in the system's energy and in the occupation of the lowest natural orbital of the weakly interacting species. We show that such a transition is a few-atom effect which disappears for a large population imbalance.2015-03-25T16:00:33ZTemporal percolation in activity-driven networks
http://hdl.handle.net/2117/26924
Title: Temporal percolation in activity-driven networks
Authors: Starnini, Michele; Pastor Satorras, Romualdo
Abstract: We study the temporal percolation properties of temporal networks by taking as a representative example the recently proposed activity-driven-network model [N. Perra et al., Sci. Rep. 2, 469 ( 2012)]. Building upon an analytical framework based on a mapping to hidden variables networks, we provide expressions for the percolation time T-p marking the onset of a giant connected component in the integrated network. In particular, we consider both the generating function formalism, valid for degree-uncorrelated networks, and the general case of networks with degree correlations. We discuss the different limits of the two approaches, indicating the parameter regions where the correlated threshold collapses onto the uncorrelated case. Our analytical predictions are confirmed by numerical simulations of the model. The temporal percolation concept can be fruitfully applied to study epidemic spreading on temporal networks. We show in particular how the susceptible-infected-removed model on an activity-driven network can be mapped to the percolation problem up to a time given by the spreading rate of the epidemic process. This mapping allows us to obtain additional information on this process, not available for previous approaches.2015-03-23T09:53:58ZStopping criteria in contrastive divergence: Alternatives to the reconstruction error
http://hdl.handle.net/2117/26625
Title: Stopping criteria in contrastive divergence: Alternatives to the reconstruction error
Authors: Buchaca, David; Romero Merino, Enrique; Mazzanti Castrillejo, Fernando Pablo; Delgado Pin, Jordi
Abstract: Restricted Boltzmann Machines (RBMs) are general unsupervised learning devices to ascertain generative models of data distributions.
RBMs are often trained using the Contrastive Divergence learning algorithm (CD), an approximation to the gradient of the data log-likelihood.
A simple reconstruction error is often used to decide whether the approximation provided by the CD algorithm is good enough, though several authors (Schulz et al., 2010; Fischer & Igel, 2010) have raised doubts concerning the feasibility of this procedure. However, not many alternatives to the reconstruction error have been used in the literature. In this manuscript we investigate simple alternatives to the reconstruction error in order to detect as soon as possible the decrease in the log-likelihood during learning.2015-03-09T12:28:53ZShort range order of CCl4: RMC and MD methods
http://hdl.handle.net/2117/26574
Title: Short range order of CCl4: RMC and MD methods
Authors: Silva-Santisteban López, Alvaro; Henao Aristizábal, Andrés; Pothoczki, Szilvia; Bermejo, F. Javier; Tamarit Mur, José Luis; Guàrdia Manuel, Elvira; Cuello, Gabriel Julio; Pardo Soto, Luis Carlos
Abstract: The main objective of this paper is to present a method to fully determine the six
degrees of freedom regarding position and orientation of a neighboring molecule around a central
one, i.e. the g(rCM,¿) distribution function. This is accomplished by completely determining the
short range structure of liquid carbon tetrachloride, while employing results from two different
methods, Molecular Dynamics (MD) [1] and Reverse Monte Carlo (RMC) [2]. Exclusively, the
structural ordering of the first four molecules will be detailed.2015-03-04T12:11:45ZDynamic structure function of a cold Fermi gas at unitarity
http://hdl.handle.net/2117/26519
Title: Dynamic structure function of a cold Fermi gas at unitarity
Authors: Astrakharchik, Grigori; Boronat Medico, Jordi; Krotscheck, Eckhard; Lichtenegger, Thomas
Abstract: We present a theoretical study of the dynamic structure function of a resonantly interacting two-component Fermi gas at zero temperature. Our approach is based on dynamic many-body theory able to describe excitations in strongly correlated Fermi systems. The fixed-node diffusion Monte Carlo method is used to produce the ground-state correlation functions which are used as an input for the excitation theory. Our approach reproduces recent Bragg scattering data in both the density and the spin channel. In the BCS regime, the response is close to that of the ideal Fermi gas. On the BEC side, the Bose peak associated with the formation of dimers dominates the density channel of the dynamic response. When the fraction of dimers is large our theory departs from the experimental data, mainly in the spin channel.2015-02-26T08:58:44ZUniversality in molecular halo clusters
http://hdl.handle.net/2117/26455
Title: Universality in molecular halo clusters
Authors: Stipanovic, P.; Markic, L. Vranjes; Beslic, Ivana; Boronat Medico, Jordi
Abstract: The ground state of weakly bound dimers and trimers with a radius extending well into the classically forbidden region is explored, with the goal to test the predicted universality of quantum halo states. The focus of the study is molecules consisting of T down arrow, D down arrow, He-3, He-4, and alkali atoms, where the interaction between particles is much better known than in the case of nuclei, which are traditional examples of quantum halos. The study of realistic systems is supplemented by model calculations in order to analyze how low-energy properties depend on the interaction potential. The use of variational and diffusion Monte Carlo methods enabled a very precise calculation of both the size and binding energy of the trimers. In the quantum halo regime, and for large values of scaled binding energies, all clusters follow almost the same universal line. As the scaled binding energy decreases, Borromean states separate from tango trimers.2015-02-20T16:17:53ZFirst-principles modeling of quantum nuclear effects and atomic interactions in solid He-4 at high pressure
http://hdl.handle.net/2117/26453
Title: First-principles modeling of quantum nuclear effects and atomic interactions in solid He-4 at high pressure
Authors: Cazorla Silva, Claudio; Boronat Medico, Jordi
Abstract: We present a first-principles computational study of solid He-4 at T = 0 K and pressures up to similar to 160 GPa. Our computational strategy consists in using van der Waals density functional theory (DFT-vdW) to describe the electronic degrees of freedom in this material, and the diffusion Monte Carlo (DMC) method to solve the Schrodinger equation describing the behavior of the quantum nuclei. For this, we construct an analytical interaction function based on the pairwise Aziz potential that closely matches the volume variation of the cohesive energy calculated with DFT-vdW in dense helium. Interestingly, we find that the kinetic energy of solid He-4 does not increase appreciably with compression for P >= 85 GPa. Also, we show that the Lindemann ratio in dense solid He-4 amounts to 0.10 almost independently of pressure. The reliability of customary quasiharmonic DFT (QH DFT) approaches in describing quantum nuclear effects in solids is also studied. We find that QH DFT simulations, although provide a reasonable equation of state in agreement with experiments, are not able to reproduce correctly these critical effects in compressed He-4. In particular, we disclose huge discrepancies of at least similar to 50% in the calculated He-4 kinetic energies using both the QH DFT and present DFT-DMC methods.2015-02-20T15:30:21ZBeam focalization in reflection from flat dielectric subwavelength gratings
http://hdl.handle.net/2117/26401
Title: Beam focalization in reflection from flat dielectric subwavelength gratings
Authors: Cheng, Yu Chieh; Zeng, H; Trullàs Simó, Joaquim; Cojocaru, Crina; Malinauskas, Mangirdas; Jukna, T; Wiersma, D.S.; Staliunas, Kestutis
Abstract: We experimentally demonstrate the recently predicted effect of near-field focusing for light beams from flat dielectric subwavelength gratings (SWGs). This SWGs were designed for visible light 532 nm and fabricated by direct laser writing in a negative photoresist, with the refractive index n = 1.5 and the period d = 314 nm. The laterally invariant gratings can focus light beams without any optical axis to achieve the transversal invariance. We show that focal distances can be obtained up to 13 mu m at normal reflection for TE polarization.2015-02-18T07:43:19ZLuttinger-liquid behavior of one-dimensional He-3
http://hdl.handle.net/2117/26200
Title: Luttinger-liquid behavior of one-dimensional He-3
Authors: Astrakharchik, Grigori; Boronat Medico, Jordi
Abstract: The ground-state properties of one-dimensional He-3 are studied using quantum Monte Carlo methods. The equation of state is calculated in a wide range of physically relevant densities and is well interpolated by a power-series fit. The Luttinger liquid theory is found to describe the long-range properties of the correlation functions. The density dependence of the Luttinger parameter is explicitly found, and interestingly it shows a nonmonotonic behavior. Depending on the density, the static structure factor can be a smooth function of the momentum or might contain a peak of a finite or infinite height. Although no phase transitions are present in the system, we identify a number of physically different regimes, including an ideal Fermi gas, a2015-02-03T18:41:49ZQuantum phase transition with a simple variational ansatz
http://hdl.handle.net/2117/25919
Title: Quantum phase transition with a simple variational ansatz
Authors: Lutsyshyn, Yaroslav; Astrakharchik, Grigori; Cazorla, C; Boronat Medico, Jordi
Abstract: We study the zero-temperature quantum phase transition between liquid and hcp solid He-4. We use the variational method with a simple yet exchange-symmetric and fully explicit wave function. It is found that the optimized wave function undergoes spontaneous symmetry breaking and describes the quantum solidification of helium at 22 atm. The explicit form of the wave function allows us to consider various contributions to the phase transition. We find that the employed wave function is an excellent candidate for describing both a first-order quantum phase transition and the ground state of a Bose solid.2015-01-19T15:23:44ZDistinguishability, degeneracy, and correlations in three harmonically trapped bosons in one dimension
http://hdl.handle.net/2117/25917
Title: Distinguishability, degeneracy, and correlations in three harmonically trapped bosons in one dimension
Authors: Garcia March, Miguel Angel; Julia Diaz, Bruno; Astrakharchik, Grigori; Boronat Medico, Jordi; Polls, A.
Abstract: We study a system of two bosons of one species and a third atom of a second species in a one-dimensional parabolic trap at zero temperature. We assume contact repulsive inter- and intraspecies interactions. By means of an exact diagonalization method we calculate the ground and excited states for the whole range of interactions. We use discrete group theory to classify the eigenstates according to the symmetry of the interaction potential. We also propose and validate analytical Ansatze gaining physical insight over the numerically obtained wave functions. We show that, for both approaches, it is crucial to take into account that the distinguishability of the third atom implies the absence of any restriction over the wave function when interchanging this boson with any of the other two. We find that there are degeneracies in the spectra in some limiting regimes, that is, when the interspecies and/or the intraspecies interactions tend to infinity. This is in contrast with the three-identical boson system, where no degeneracy occurs in these limits. We show that, when tuning both types of interactions through a protocol that keeps them equal while they are increased towards infinity, the systems's ground state resembles that of three indistinguishable bosons. Contrarily, the systems's ground state is different from that of three-identical bosons when both types of interactions are increased towards infinity through protocols that do not restrict them to be equal. We study the coherence and correlations of the system as the interactions are tuned through different protocols, which permit us to build up different correlations in the system and lead to different spatial distributions of the three atoms.2015-01-19T14:47:48ZPhase diagram of dipolar bosons in two dimensions with tilted polarization
http://hdl.handle.net/2117/25911
Title: Phase diagram of dipolar bosons in two dimensions with tilted polarization
Authors: Macia Rey, Adrián; Boronat Medico, Jordi; Mazzanti Castrillejo, Fernando Pablo
Abstract: We analyze the ground state of a system of dipolar bosons moving in the XY plane and such that their dipolar moments are all aligned in a fixed direction in space. We focus on the general case where the polarization field forms a generic angle a with respect to the Z axis. We use the path-integral ground-state method to analyze the static properties of the system as both a and the density n vary over a wide range where the system is stable. We use the maximum of the static structure function as an order parameter to characterize the different phases and the transition lines among them. We find that, in addition to a superfluid gas and a solid phase, the system reaches a stripe phase at large tilting angles that is entirely induced by the anisotropic character of the interaction. We also show that the quantum phase transition from the gas to the stripe phase is of second order and report approximate values for the critical exponents.2015-01-19T14:16:59ZMolecular dynamics simulation of single ions in aqueous solutions: effects of the flexibility of the water molecules
http://hdl.handle.net/2117/25529
Title: Molecular dynamics simulation of single ions in aqueous solutions: effects of the flexibility of the water molecules
Authors: Guàrdia Manuel, Elvira
Abstract: We performed molecular dynamics simulations of single Na’ and F ions in aqueous solutions. Two single point charge water models with and without internal degrees of freedom were considered. Structural (radial distribution functions, orientation angles), dynamical (translational, vibrational, and reorientational motions), and other microscopic properties (hydration numbers, residence times) of ions and water molecules of their hydration shell were calculated. Our results are compared both with experimental data and with other simulation results using different interaction models. The influence of the flexibility of water molecules on the different properties is carefully discussed.2015-01-14T19:01:42ZIonic association of Na+-Cl-, Na+-Na+ and Cl--Cl- in methanol: mean force potentials and friction kernels
http://hdl.handle.net/2117/25524
Title: Ionic association of Na+-Cl-, Na+-Na+ and Cl--Cl- in methanol: mean force potentials and friction kernels
Authors: Sesé Castel, Gemma; Guàrdia Manuel, Elvira; Padro Cardenas, Joan Angel
Abstract: Kinetic investigation of 1-octene bromination in AOT-isooctane-water microemulsions (13 = w = [H2O]/[AOT] = 24 and 6 = z = [IO]/[AOT] = 57) shows that the reaction is first-order in alkene and first-order in bromine, as usually found in protic media. Although both reagents are mainly located in the isooctane phase (Ktr, transfer coefficients from isooctane to water, are 1.5 × 10-5 and 8.8 × 10-3 for alkene and bromine, respectively), bromination occurs in an aqueous microenvironment, as illustrated by the high sensitivity of the bromination rate to the water content of the microemulsion. A kinetic pseudophase model describes the rate constant dependence on microemulsion composition satisfactorily by assuming competition between reactions at the interface and in the aqueous phase. Reasonable values for the coefficients of reagent partition between the interface and the two microphases and for the local bromination rate constants are obtained from the kinetic equations derived from the model. In particular, spectroscopically observed AOT-bromine complexation is in agreement with the high bromine concentration at the interface (K2, bromine partition coefficient from isooctane to interface, = 6.8). The water-phase bromination rate constant, kw = 1 × 108 M-1 s-1, is in the same range as that measured in bulk water. The lower limit for the interfacial rate constant, ki, is 103 M-1 s-1, a value close to that observed in poorly aqueous methanol (MeOH/H2O = 95/5 v/v). These data are compared with those recently obtained in the same microemulsions for solvolysis, a reaction which, like bromination, is water-promoted but supposed to take place at the interface only. The results are discussed in terms of the chemical properties of the water molecules encased in the microemulsion droplets.2015-01-14T17:29:40ZStructure and dynamics of liquid ethanol
http://hdl.handle.net/2117/25516
Title: Structure and dynamics of liquid ethanol
Authors: Saiz, L; Padro Cardenas, Joan Angel; Guàrdia Manuel, Elvira
Abstract: Molecular dynamics simulations of liquid ethanol at four thermodynamic states ranging from T = 173 K to T = 348 K were carried out using the transferable OPLS potential model of Jorgensen (J. Phys. Chem. 1986, 90, 1276). Both static and dynamic properties are analyzed. The resulting properties show an overall agreement with available experimental data. Special attention is paid to the hydrogen bonds and to their influence on the molecular behavior. Results for liquid ethanol are compared with those for methanol in earlier computer simulation studies.2015-01-14T15:38:22Z