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1 to 34 of 34 results
  • A new approach to decimation in high order Boltzmann machines

     Farguell Matasanz, Enric
    Universitat Ramon Llull
    Theses

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  • GRUP DE SIMULACIO PER ORDINADOR EN MATERIA CONDENSADA

     Pastor Satorras, Romualdo; Sese Castel, Gemma; Rey Oriol, Rosendo; Casulleras Ambros, Joaquin; Trullàs Simó, Joaquim; Bitrian Varea, Vicente; Baronchelli, Andrea; Osychenco, Oleg; Canales Gabriel, Manel; Boronat Medico, Jordi; Marti Rabassa, Jordi; Macia Rey, Adrian; Calvo Minguillon, Ausias March; Ortiz de Urbina Viade, Jordi; Mazzanti Castrillejo, Fernando Pablo; Astrakharchik, Grigori; Rota, Riccardo; Sala2 Viñas, Jonàs; Alcaraz Sendra, Olga; Guardia Manuel, Elvira
    Competitive project

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  • SUPERFLUIDEZ Y CONDESACION DE BOSE-EINSTEIN EN MATERIA ULTRAFRIA

     Rota, Riccardo; Casulleras Ambros, Joaquin; Astrakharchik, Grigori; Mazzanti Castrillejo, Fernando Pablo; Osychenco, Oleg; Macia Rey, Adrian; Boronat Medico, Jordi
    Competitive project

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  • Static Properties of a SYstem of Hard Rods in One Dimension

     Mazzanti Castrillejo, Fernando Pablo
    International Conference on Recent Progress in Many-Body Theories (XIV)
    Presentation's date: 2007-07-16
    Presentation of work at congresses

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  • Variational Description of Weakly Interacting Bose Gases in 3 Dimensions

     Mazzanti Castrillejo, Fernando Pablo; Saarela, M; Apaja, V
    Date of publication: 2006-09-30
    Book chapter

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  • Variational Description of Weakly Interacting Bose Gases in 3 Dimensions

     Mazzanti Castrillejo, Fernando Pablo
    13th International Conference Recent Progress in Many-Body Theories
    Presentation's date: 2005-12-06
    Presentation of work at congresses

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  • High momentum response of liquid 3He

     Boronat Medico, Jordi; Mazzanti Castrillejo, Fernando Pablo; Polls, A
    Date of publication: 2003-08-31
    Book chapter

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  • Momentum distributions and condensate fraction in liquid 3he-4he mixtures

     Boronat Medico, Jordi; Pous, A; Mazzanti Castrillejo, Fernando Pablo; Fabrocini, A
    RESEARCH CONFERENCE ON BOSE-EINSTEIN CONDENSATION
    p. 44
    Presentation of work at congresses

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  • Dynamic structure function of 3he-4he mixtures in the deep inelastic regime

     Polls, A; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi; Dalfovo, F; Fabrocini, A
    RECENT PROGRESS IN MAN-BODY THEORIES
    p. 101-107
    Presentation of work at congresses

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  • Ground-state properties of a one-dimensional system of hard rods

     Mazzanti Castrillejo, Fernando Pablo; Astrakharchik, Grigori; Boronat Medico, Jordi; Casulleras Ambros, Joaquin
    Physical review letters
    Vol. 1, num. 2, p. 1-4
    Date of publication: 2008-01
    Journal article

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  • Off-diagonal ground-state properties of a one-dimensional gas of Fermi hard rods

     Mazzanti Castrillejo, Fernando Pablo; Astrakharchik, Grigori; Boronat Medico, Jordi; Casulleras Ambros, Joaquin
    Physical review A
    Vol. 77, num. 4, p. 43632
    DOI: 10.1103/PhysRevA.77.043632
    Date of publication: 2008-04
    Journal article

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  • Dynamic structure funcion in 3he-4he mixtures

     Boronat Medico, Jordi; Dalfovo, F; Mazzanti Castrillejo, Fernando Pablo; Polls, A
    Physica B. Condensed matter
    Vol. 194-96, num. 1, p. 859-860
    Date of publication: 1994-06
    Journal article

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  • Final-state efects on superfluid 4he in the deep inelastic regime

     Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi; Polls, A
    Physical review B: condensed matter and materials physics
    Vol. 53, num. 9, p. 5661-5669
    Date of publication: 1996-03
    Journal article

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  • Coherent and incoherent dynamic structure functions of the free fermi gas

     Mazzanti Castrillejo, Fernando Pablo; Polls, A; Boronat Medico, Jordi
    Physics letters A
    Vol. 220, num. 1, p. 251-257
    Date of publication: 1996-09
    Journal article

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  • High-momentum dynamic structure function of liquid 3He-4He mixtures: a microscopic approach

     Mazzanti Castrillejo, Fernando Pablo; Polls, A; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 63, p. 054521--11
    Date of publication: 2001-02
    Journal article

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  • Mecanica cuantica de los condensados de Bose-Einstein

     Polls, A; Boronat Medico, Jordi; Mazzanti Castrillejo, Fernando Pablo
    Investigación y ciencia
    num. 293, p. 66-74
    Date of publication: 2001-02
    Journal article

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  • Variational description of weakly interacting Bose gases in 3 dimensions

     Mazzanti Castrillejo, Fernando Pablo; Saarela, M; Apaja, V
    International journal of modern physics B
    Vol. 20, num. 30-31, p. 5061-5070
    Date of publication: 2006-12
    Journal article

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  • Dynamic structure function of quantum bose systems; condensate fraction and momentum distribution

     Saarela, M; Mazzanti Castrillejo, Fernando Pablo; Apaja, V
    International journal of modern physics B
    Vol. 22, num. 25-26, p. 4327-4337
    Date of publication: 2008-10
    Journal article

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  • Boltzmann Machines Reduction by High-Order Decimation

     Farguell, E; Mazzanti Castrillejo, Fernando Pablo; Gomez-Ramirez, E
    IEEE transactions on neural networks
    Vol. 19, num. 10, p. 1816-1821
    DOI: 10.1109/TNN.2008.2003249
    Date of publication: 2008-10
    Journal article

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  • Condensate fraction in the dynamic structure function of Bose fluids

     Saarela, M; Mazzanti Castrillejo, Fernando Pablo; Apaja, V
    International journal of modern physics B
    Vol. 21, num. 13-14, p. 2169-2180
    Date of publication: 2007-05
    Journal article

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  • Dynamics of a Two-Dimensional System of Quantum Dipoles

     Mazzanti Castrillejo, Fernando Pablo; Zillich, Robert E.; Astrakharchik, Grigori; Boronat Medico, Jordi
    Physical review letters
    Vol. 102, num. 110405, p. 1
    Date of publication: 2009-03
    Journal article

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  • High momentum response of liquid 3He

     Mazzanti Castrillejo, Fernando Pablo; Polls, A; Boronat Medico, Jordi; Casulleras Ambros, Joaquin
    Physical review letters
    Vol. 92, p. 085301--4
    Date of publication: 2004-02
    Journal article

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  • The effective two-particle interaction of cold atoms as derived from bragg scattering

     Sarjonen, R.; Saarela, M.; Mazzanti Castrillejo, Fernando Pablo
    Journal of low temperature physics
    Vol. 169, num. 5-6, p. 400-422
    DOI: 10.1007/s10909-012-0745-x
    Date of publication: 2012-12
    Journal article

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    We study the dynamic structure function of ultracold alkali-metal gases for large scattering lengths and momenta where corrections to the mean field approximation become important. We compare our result with the Bragg-scattering measurements in 85Rb by Papp et al. (Phys. Rev. Lett. 101:135301, 2008) and show that these experiments set very strict limits to the shape of the effective two-particle interaction ruling out the contact and hard spheres potentials. Using the Feshbach resonance approximation we derive the effective interaction, which turns out to be very similar to the soft spheres potential in momentum space. At large scattering lengths the interaction becomes universal and could be directly measured by Bragg scattering. We also discuss the experimental conditions needed for the appearance of the maxon-roton structure in the excitation spectrum and finally show that when the scattering length becomes larger than 2000 Bohr radii the uniform gas phase undergoes a phase transition into the density wave state.

  • Excitations and stripe phase formation in a two-dimensional dipolar bose gas with tilted polarization

     Macia Rey, Adrian; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
    Physical review letters
    Vol. 109, num. 23
    DOI: 10.1103/PhysRevLett.109.235307
    Date of publication: 2012-12-05
    Journal article

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  • Ground state properties and excitation spectrum of a two dimensional gas of bosonic dipoles

     Macia Rey, Adrian; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
    European physical journal D
    Vol. 66, num. 11
    DOI: 10.1140/epjd/e2012-30455-y
    Date of publication: 2012-11
    Journal article

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    Ferromagnetic transition of a two-component Fermi gas of hard spheres  Open access

     Arias de Saavedra, F.; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi; Polls, A.
    Physical review A
    Vol. 85, num. 3, p. 1-6
    DOI: 10.1103/PhysRevA.85.033615
    Date of publication: 2012-03-12
    Journal article

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    We use microscopic many-body theory to analyze the problem of itinerant ferromagnetism in a repulsive atomic Fermi gas of hard spheres. Using simple arguments we show that the available theoretical predictions for the onset of the ferromagnetic transition predict a transition point at a density (kF a ∼ 1) that is too large to be compatible with the universal low-density expansion of the energy. We present variational calculations for the hard-sphere Fermi gas, in the framework of Fermi hypernetted chain theory, that shift the transition to higher densities (kF a ∼ 1.8). Backflow correlations, which are mainly active in the unpolarized system, are essential for this shift.

  • Single-particle versus pair superfluidity in a bilayer system of dipolar bosons

     Macia Rey, Adrian; Astrakharchik, Grigori; Mazzanti Castrillejo, Fernando Pablo; Giorgini, 0Stefano; Boronat Medico, Jordi
    Physical review A
    Vol. 90, num. 4, p. 1-5
    DOI: 10.1103/PhysRevA.90.043623
    Date of publication: 2014-10-21
    Journal article

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    We consider the ground state of a bilayer system of dipolar bosons, where dipoles are oriented by an external field in the direction perpendicular to the parallel planes. Quantum Monte Carlo methods are used to calculate the ground-state energy, the one-body and two-body density matrix, and the superfluid response as a function of the separation between layers. We find that by decreasing the interlayer distance for fixed value of the strength of the dipolar interaction, the system undergoes a quantum phase transition from a single-particle to a pair superfluid. The single-particle superfluid is characterized by a finite value of both the atomic condensate and the super-counterfluid density. The pair superfluid phase is found to be stable against formation of many-body cluster states and features a gap in the spectrum of elementary excitations.

  • Scaling of the interaction in BECs at large scattering lengths

     Sarjonen, R.; Saarela, M.; Mazzanti Castrillejo, Fernando Pablo
    European physical journal D
    Vol. 65, num. 1-2, p. 25-32
    DOI: 10.1140/epjd/e2011-20102-8
    Date of publication: 2011-11
    Journal article

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    Low-dimensional weakly interacting Bose gases: nonuniversal equations of state  Open access

     Astrakharchik, Grigori; Boronat Medico, Jordi; Kurbakov, I L; Lozovik, Y E; Mazzanti Castrillejo, Fernando Pablo
    Physical review A
    Vol. 81, num. 1, p. 3612-3622
    DOI: 10.1103/PhysRevA.81.013612
    Date of publication: 2010-01
    Journal article

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    The zero-temperature equation of state is analyzed in low-dimensional bosonic systems. We propose to use the concept of energy-dependent s-wave scattering length for obtaining estimations of nonuniversal terms in the energy expansion. We test this approach by making a comparison to exactly solvable one-dimensional problems and find that the generated terms have the correct structure. The applicability to two-dimensional systems is analyzed by comparing with results of Monte Carlo simulations. The prediction for the nonuniversal behavior is qualitatively correct and the densities, at which the deviations from the universal equation of state become visible, are estimated properly. Finally, the possibility of observing the nonuniversal terms in experiments with trapped gases is also discussed.

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    High-order time expansion path integral ground state  Open access

     Rota, Riccardo; Casulleras Ambros, Joaquin; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
    Physical review E: statistical, nonlinear, and soft matter physics
    Vol. 81, num. 1, p. 6707
    DOI: 10.1103/PhysRevE.81.016707
    Date of publication: 2010-01
    Journal article

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    The feasibility of path integral Monte Carlo ground state calculations with very few beads using a high-order short-time Green’s function expansion is discussed. An explicit expression of the evolution operator which provides dramatic enhancements in the quality of ground-state wave functions is examined. The efficiency of the method makes possible to remove the trial wave function and thus obtain completely model-independent results still with a very small number of beads. If a single iteration of the method is used to improve a given model wave function, the result is invariably a shadow-type wave function, whose precise content is provided by the high-order algorithm employed.

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    Microscopic description of anisotropic low-density dipolar Bose gases in two dimensions  Open access

     Macia Rey, Adrian; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi; Zillich, Robert E.
    Physical review A
    Vol. 84, num. 3, p. 1-10
    DOI: 10.1103/PhysRevA.84.033625
    Date of publication: 2011-09-19
    Journal article

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    A microscopic description of the zero-energy two-body ground state and many-body static properties of anisotropic homogeneous gases of bosonic dipoles in two dimensions at low densities is presented and discussed. By changing the polarization angle with respect to the plane, we study the impact of the anisotropy, present in the dipole-dipole interaction, on the energy per particle, comparing the results with mean-field predictions. We restrict the analysis to the regime where the interaction is always repulsive, although the strength of the repulsion depends on the orientation with respect to the polarization field. We present a series expansion of the solution of the zero-energy two-body problem, which allows us to find the scattering length of the interaction and to build a suitable Jastrow factor that we use as a trial wave function for both a variational and diffusion Monte Carlo simulation of the infinite system. We find that the anisotropy has an almost negligible impact on the ground-state properties of the many-body system in the universal regime where the scattering length governs the physics of the system. We also show that scaling in the gas parameter persists in the dipolar case up to values where other isotropic interactions with the same scattering length yield different predictions.

  • Elementary excitations and universal interaction in Bose-Einstein condensates at large scattering lengths

     Sarjonen, R.; Saarela, M.; Mazzanti Castrillejo, Fernando Pablo
    Physical review A
    Vol. 84, num. 4, p. 1-4
    DOI: 10.1103/PhysRevA.84.041602
    Date of publication: 2011-10-10
    Journal article

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    Zero-temperature phase diagram of Yukawa bosons  Open access

     Osychenko, O.N.; Astrakharchik, Grigori; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
    Physical review A
    Vol. 85, num. 6, p. 1-6
    DOI: 10.1103/PhysRevA.85.063604
    Date of publication: 2012
    Journal article

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    We study the zero-temperature phase diagram of bosons interacting via screened Coulomb (Yukawa) potential by means of the diffusion Monte Carlo method. The Yukawa potential is used as a model interaction in the neutron matter, dusty plasmas, and charged colloids. As shown by Petrov et al. [Phys. Rev. Lett. 99, 130407 (2007)], interactions between weakly bound molecules of heavy and light fermionic atoms are described by an effective Yukawa potential with a strength related to the heavy-light mass ratio M/m, which might lead to crystallization in a two-dimensional geometry if the mass ratio of heavy-light fermions exceeds a certain critical value. In the present work we do a thorough study of the quantum three-dimensional Yukawa system. For strong interactions (equivalently, large mass ratios) the system experiences several phase transitions as the density is increased, passing from gas to solid and to gas phase again.Weakly interacting Yukawa particles do not crystallize at any density. We find the minimal interaction strength at which the crystallization happens. In terms of the two-component fermionic system, this strength corresponds to a heavy-light mass ratio of M/m ∼ 180, so that it is impossible to realize the gas-crystal transition in a conventional bulk system. For the Yukawa model of fermionic mixtures we also analyze the possibility of building molecular systems with very large effective mass ratios by confining the heavy component to a sufficiently deep optical lattice. We show how the effective mass of the heavy component can be made arbitrarily large by increasing the lattice depth, thus leading to a tunable effective mass ratio that can be used to realize a molecular superlattice.