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1 to 34 of 34 results
  • 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
    Date of publication: 2012-12-05
    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
    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.

  • 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
    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.

  • 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
    Date of publication: 2012-11
    Journal article

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  • Monte Carlo Study of Quantum Phase Transitions at Zero Temperature  Open access

     Osychenko, Oleg
    Defense's date: 2012-12-20
    Department of Applied Physics, Universitat Politècnica de Catalunya
    Theses

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    The Thesis is devoted to simulations of quantum phase transitions by means of Quantum Monte Carlo techniques. Quantum phase transition is a transition between phases at zero or low enough temperature, where quantum effects play an important role. The recent advances in the field of ultracold atom manipulation and optical lattices allowed to produce the systems with unique properties. This opened a perspective to observe quantum phase transitions in many-body systems with non-trivial interparticle interactions in a wide range of the system's characteristic physical parameters and geometries. First, we develop the explicit expressions for the Ewald sums in systems with an interaction potential of a generic 1/r^k type, and in 3D, 2D and 1D geometry. These generalizations can be useful in simulating systems with important interaction potentials as the dipole-dipole, van der Waals interaction, etc. In this Thesis we give the functional forms for the terms of the Ewald sums, ready for implementation in a code. The derivation and the functional form of the results differ in the cases of short-ranged, long-ranged and "marginal" forces, and for a jellium model. It is argued that in the case of some short-range potentials the Ewald method can be advantageous with respect to a direct summation due to a faster convergence rate. We also give a discussion of the convergence properties of a quasi-neutral Coulomb system. We have obtained the zero-temperature phase diagram of bosons interacting through Yukawa forces. We have used a diffusion Monte Carlo simulation starting from a good approximation to the optimal variational ground-state wave function obtained by solving the corresponding Euler-Lagrange hypernetted chain equations. The phase diagram shows that any fermionic mixture of pure elements will always be seen in gaseous form, as the mass ratios required for crystallization of weakly bound fermionic molecules are far beyond the ones that can be achieved in nature. We investigate an alternative mechanism based on the confinement of one of the species to a deep optical lattice which increases its effective mass. The resulting mass ratio of the mixture created in this way can then be tuned at will and could be used to check experimentally the predicted phase diagram both in the gas and crystal (superlattice) phases. We performed a QMC study of the system, comrised of Rydberg atoms. The applied QMC techniques allowed to parametrize a model with isotropic van der Waals interactions into a universal phase diagram. We have characterized the phase diagram of Rydberg atoms by considering a model of bosons with repulsive van der Waals 1/r^6 interaction, and determined solidification and Bose-Einstein condensation conditions. Relaxation mechanisms other than thermal motion should be considered if one considers Rydberg systems on timescales of several tenths of microseconds. We have also studied the excitation spectrum within the approximation of a classical harmonic crystal. We also discuss that interactions between Rydberg excitations open a possibility of new supersolid scenarios. In the last Chapter of the Thesis I present a study of the system of para-hydrogen atoms at low temperatures below the point of crystallization by means of QMC methods. The zero-temperature simulation was performed in order to investigate the properties of a metastable liquid phase and to find the fraction of the Bose-Einstein condensate in the relevant range of densities. The methods of choice for the zero-temperature simulations of the para-H2 system were VMC and DMC techniques. The results of the zero-temperature simulations suggest that the metastable liquid para-hydrogen is a strongly correlated liquid, which again serves as an evidence of high instability of this hypothetical system. The calculation of the Bose-Einstein condensate shows that the condensate fraction is substantially lower than in the liquid helium He4.

    Los avances recientes en manipulación de átomos ultrafrios y retículos ópticos abrieron la posibilidad de observar las transiciones de fase en sistemas de muchos cuerpos con las interacciones interparticulares no triviales para un amplio rango de los parámetros físicos característicos y geometrías del sistema. En principio desarrollamos las expresiones explicitas para las sumas de Ewald en el caso del potencial de interacción genérico 1/r^k, y en las geometrías arbitrarias: 3D, 2D y 1D. Dichas generalizaciones pueden ser útiles para simular sistemas con los potenciales importantes como dipolo-dipolo, interacción de van der Waals, etc. En la Tesis presentamos las formas funcionales para los términos de las sumas de Ewald, listas para la implementación actual. La derivación y las formas funcionales cambian en función de la potencial de corto, largo o alcance "marginal", y en particular para el modelo de jellium. Argüimos que en el caso del potencial de corto alcance el método de Ewald puede ser ventajoso respecto a la sumatorio directo gracias a la convergencia más rápida. También presentamos la discusión sobre las propiedades de convergencia del sistema de Coulomb сuasi-neutro. Hemos obtenido el diagrama de fase a temperatura cero de los bosones interactuando mediante a las fuerzas de Yukawa. Hemos usado la simulación de Monte Carlo difusivo empezando de una buena aproximación a la función de onda óptima del estado de base obtenida a través de la solución de las ecuaciones de Euler-Lagrange del método HNC. El diagrama de fase demuestra que la mezcla fermiónica de los elementos puros siempre aparece en la forma gaseosa, como los parametros requeridos para la cristalización de estas moléculas fermiónicas están fuera de lo que puede ser visto en la naturaleza. Investigamos el mecanismo alternativo basado en el confinamiento de una de las especies en el retículo óptico, que aumenta su masa efectiva. El cociente de masas de la mezcla creada de esta manera puede ser ajustada arbitrariamente y usada para comprobar el diagrama de fase predicha en el estudio tanto en fase liquida como en la cristalina. Hemos hecho el estudio QMC del sistema de los átomos de Rydberg. Las técnicas de Monte Carlo cuánticas aplicadas nos permitieron parametrizar el modelo mediante la interacción de van der Waals isotrópica y así obtener el diagrama de fase universal. Caracterizamos el diagrama de fase de los átomos de Rydberg considerando el modelo de bosones con la interacción repulsiva 1/r^6, y determinamos las condiciones de solidificación y condensación de Bose-Einstein. Los mecanismos de relajación aparte del movimiento térmico deben de ser tenidos en cuenta a escala de tiempo de decenas de microsegundos. Estudiamos también el espectro de excitaciones dentro de la aproximación de cristal clásico harmónico. Finalmente, discutimos que las interacciones entre las excitaciones de Rydberg abren la posibilidad de los escenarios nuevos del supersólido.

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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
    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.

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

     Sarjonen, R.; Saarela, M.; Mazzanti Castrillejo, Fernando Pablo
    Physical review A
    Date of publication: 2011-10-10
    Journal article

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  • Path Integral Monte Carlo and Bose-Einstein condensation in quantum fluids and solids  Open access

     Rota, Riccardo
    Defense's date: 2011-12-20
    Department of Applied Physics, Universitat Politècnica de Catalunya
    Theses

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    Several microscopic theories point out that Bose-Einstein condensation (BEC), i.e., a macroscopic occupation of the lowest energy single particle state in many-boson systems, may appear also in quantum fluids and solids and that it is at the origin of the phenomenon of superfluidity. Nevertheless, the connection between BEC and superfluidity is still matter of debate, since the experimental evidences indicating a non zero condensate fraction in superfluid helium are only indirect. In the theoretical study of BEC in quantum fluids and solids, perturbative approaches are useless because of the strong correlations between the atoms, arising both from the interatomic potential and from the quantum nature of the system. Microscopic Quantum Monte Carlo simulations provide a reliable description of these systems. In particular, the Path Integral Monte Carlo (PIMC) method is very suitable for this purpose. This method is able to provide exact results for the properties of the quantum system, both at zero and finite temperature, only with the definition of the Hamiltonian and of the symmetry properties of the system, giving an easy picture for superfluidity and BEC in many-boson systems. In this thesis, we apply PIMC methods to the study of several quantum fluids and solids. We describe in detail all the features of PIMC, from the sampling methods to the estimators of the physical properties. We present also the most recent techniques, such as the high-order approximations for the thermal density matrix and the worm algorithm, used in PIMC to provide reliable simulations. We study the liquid phase of condensed 4He, providing unbiased estimations of the one-body density matrix g1(r). We analyze the model for g1(r) used to fit the experimental data, highlighting its merits and its faults. In particular we see that, even if it presents some difficulties in the description of the overall behavior of g1(r), it can provide an accurate estimation of the kinetic energy K and of the condensate fraction n0 of the system. Furthermore, we show that our results for n0 as a function of the pressure are in a good agreement with the most recent experimental results. The study of the solid phase of 4He is the most significant part of this thesis. The recent observation of non classical rotational inertia (NCRI) effects in solid helium has generated big interest in the study of an eventual supersolid phase, characterized at the same time by crystalline order and superfluidity. Nevertheless, until now it has been impossible to give a theoretical model able to describe all the experimental evidences. In this work, we perform PIMC simulations of 4He at high densities, according to different microscopic configurations of the atoms. In commensurate crystals we see that BEC does not appear, our model being able to reproduce the momentum distribution obtained form neutron scattering experiments. In a crystal with vacancies, we have been able to see a transition to a superfluid phase at temperatures in agreement with experimental results if the vacancy concentration is low enough. In amorphous solids, superfluid effects are enhanced but appear at temperatures higher than the experimental estimation for the transition temperature. Finally, we study also metastable disordered configurations in molecular para-hydrogen at low temperature. The aim of this study is to investigate if a Bose liquid other than helium can display superfluidity. Choosing accurately a ¿quantum liquid¿ initial configuration and the dimensions of the simulation box, we have been able to frustrate the formation of the crystal and to calculate the temperature dependence of the superfluid density, showing a transition to a superfluid phase at temperatures close to 1 K.

  • A new approach to decimation in high order Boltzmann machines

     Farguell Matasanz, Enric
    Defense's date: 2011-01-20
    Universitat Ramon Llull
    Theses

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  • Scaling of the interaction in BECs at large scattering lengths

     Sarjonen, R.; Saarela, M.; Mazzanti Castrillejo, Fernando Pablo
    European physical journal D
    Date of publication: 2011-11
    Journal article

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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
    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.

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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
    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
    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.

  • 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; Guardia Manuel, Elvira
    Participation in a competitive project

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  • SUPERFLUIDEZ Y CONDESACION DE BOSE-EINSTEIN EN MATERIA ULTRAFRIA

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

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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
    Date of publication: 2009-03
    Journal article

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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
    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
    Date of publication: 2008-04
    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
    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
    Date of publication: 2008-10
    Journal article

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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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  • Condensate fraction in the dynamic structure function of Bose fluids

     Saarela, M; Mazzanti Castrillejo, Fernando Pablo; Apaja, V
    International journal of modern physics B
    Date of publication: 2007-05
    Journal article

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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; Saarela, M; Apaja, V
    International journal of modern physics B
    Date of publication: 2006-12
    Journal article

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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

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

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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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  • 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
    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
    Date of publication: 2001-02
    Journal article

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  • ANALYSIS OF THE HIGH MOMENTUM RESPONSE OF HELIUM LIQUIDS

     Mazzanti Castrillejo, Fernando Pablo
    Defense's date: 1997-06-20
    UB
    Theses

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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
    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
    Date of publication: 1996-09
    Journal article

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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
    Presentation of work at congresses

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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
    Date of publication: 1994-06
    Journal article

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