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  • Microscopic description of two dimensional dipolar quantum gases  Open access

     Macia Rey, Adrian
    Department of Physics and Nuclear Engineering, Universitat Politècnica de Catalunya
    Theses

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    En este trabajo presentamos una descripción de las propiedades de los gases homogéneos de dipolos bosónicos en dos dimensiones. Cambiando el ángulo de polarización respecto a la perpendicular al plano donde las partículas están confinadas estudiamos el impacto de la anisotropía de la interacción dipolar en diferentes magnitudes físicas. El análisis se restringe al rango de ángulos de polarización en que la interacción es repulsiva aunque la intensidad pueda depender fuertemente de la orientación respecto a la dirección de polarización. El análisis del problema a dos cuerpos a energía cero nos permite evaluar la longitud de difusión de la interacción y construir una función de onda de tipo Jastrow para el sistema de muchos cuerpos. Esta función de onda será usada como función de prueba para las simulaciones Monte Carlo del sistema homogéneo de dipolos bosónicos en dos dimensiones. En la primera parte de la tesis hemos estudiado el gas de Bose dipolar en el régimen de bajas densidades, observando que el impacto de la anisotropía es negligible en las propiedades macroscópicas en el regimen donde la longitud de difusión gobierna la física del sistema. Hemos comprobado también que el escalado en el parámetro de gas persiste en el caso dipolar hasta valores donde otras interacciones isótropas con la misma longitud de difusión llevan a distintas predicciones. Hemos evaluado el espectro de excitaciones elementales del gas de Bose dipolar en el contexto de la aproximación de Feynman, comparando los resultados con los obtenidos mediante la aproximación de Bogoliubov. Como cabría esperar, las dos aproximaciones coinciden a bajas densidades y se alejan progresivamente al aumentar la densidad.Al aumentar la densidad del sistema vemos que el comportamiento del gas depende del valor del ángulo de polarización de los momentos dipolares. A altas densidades y valores moderados del ángulo de polarización el sistema experimenta una transición de fase de primer orden pasando de una fase gaseosa a una cristalina. Hemos observado también que la anisotropía de la interacción dipolar causa una elongación de la red cristalina en la dirección de interacción más intensa. Para valores elevados del ángulo de polarización y densidades moderadas el sistema muestra transición de fase, esta vez de segundo orden, en la que el sistema pasa de la fase gaseosa a una fase de bandas. Los exponentes críticos de esta transición de fase son independientes del ángulo de polarización y, dentro de los errores estadísticos de las simulaciones, son compatibles con las clases de universalidad del modelo de Ising y XY en tres dimensiones. Finalmente, a altas densidades y valores grandes del ángulo de polarización el sistema muestra otra transición de fase de primer orden entre la fase cristalina y la fase de bandas. La pendiente de esta curva de transición es extremadamente grande indicando que, debido a la anisotropía de la interacción, la fase cristalina deja de ser estable si el potencial de interacción dipolo-dipolo es muy anisótropo.En la última parte de la tesis estudiamos el estado fundamental de un sistema bicapa de dipolos bosónicos, que es una configuración en la que se confinan las partículas en dos planos paralelos mediante un potencial externo. Consideramos la situación más simple en la que los momentos dipolares están orientados por un campo externo en la dirección perpendicular a los planos. Hemos evaluado la energía del estado fundamental y las matrices densidad a uno y dos cuerpos en función de la distancia entre capas usando métodos Monte Carlo. Hemos encontrado que disminuyendo la distancia entre planos para un valor fijo de la intensidad de la interacción, el comportamiento de todos los observables estudiados es compatible con la existencia de una transición de fase de segundo orden modulada por la distancia entre capas. En este sentido, los resultados obtenidos en este trabajo muestran buen acuerdo con estudios previos de este sistema.

    A microscopic description of the many-body properties of anisotropic homogeneous gases of bosonic dipoles in two dimensions 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 different physical quantities. We restrict the analysis to the range of polarization angles where the interaction is always repulsive, although the strength of the repulsion can be strongly dependent on the orientation with respect to the polarization field. We present a study of the zero energy two-body problem which allows us to find the scattering length of the interaction and to build a suitable Jastrow many-body wave function that will be used as a trial wave function for Monte Carlo simulations of the bulk two-dimensional system of bosonic dipoles. In the first part of this work we have studied the low-density dipolar Bose gas and 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. We also evaluate the excitation spectrum of the dipolar Bose gas in the context of the Feynman approximation and compare the results obtained with the Bogoliubov ones. As expected, we find that these two approximations agree at very low densities, while they start to deviate from each other as the density increases. When the density of the system is increased we find that the behavior of the system depends on the value of the polarization angle of the dipolar moments of the system. At large densities and moderate values of the polarization angle the system undergoes a first-order quantum phase transition from a gas and a crystal phase. We also find that the anisotropy of the dipole-dipole potential causes an elongation of the crystalline lattice of the system in the direction where the interaction is stronger. At large polarization angles and moderate densities the system undergoes a second-order quantum phase transition from a gas to a stripe phase. Interestingly, the critical exponents of this second order transition are nearly independent of the tilting angle and are compatible with the 3D Ising and 3D XY model universality classes within the statistical uncertainty of our simulations. Finally, at high densities and large tilting angles the system shows a first order phase transition between the crystal and stripe phases. The slope of this transition curve is extremely large indicating that, due to the anisotropy of the interaction, the crystal phase of the system is no longer stable if the dipole - dipole potential is highly anisotropic. We consider the ground state of a bilayer system of dipolar bosons, which is a configuration consisting in the continement of the particles in two paralel planes by means of a trapping potential. We consider the simplest situation where dipole moments 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 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 behavior of all the physical observables studied are compatible with the existence of a second order phase transition modulated by the inter-layer distance. In this sense, the results presented in this work are in good agreement with some previous studies of dipolar gases in a bilayer setup

    En este trabajo presentamos una descripción de las propiedades de los gases homogéneos de dipolos bosónicos en dos dimensiones. Cambiando el ángulo de polarización respecto a la perpendicular al plano donde las partículas están confinadas estudiamos el impacto de la anisotropía de la interacción dipolar en diferentes magnitudes físicas. El análisis se restringe al rango de ángulos de polarización en que la interacción es repulsiva aunque la intensidad pueda depender fuertemente de la orientación respecto a la dirección de polarización. El análisis del problema a dos cuerpos a energía cero nos permite evaluar la longitud de difusión de la interacción y construir una función de onda de tipo Jastrow para el sistema de muchos cuerpos. Esta función de onda será usada como función de prueba para las simulaciones Monte Carlo del sistema homogéneo de dipolos bosónicos en dos dimensiones. En la primera parte de la tesis hemos estudiado el gas de Bose dipolar en el régimen de bajas densidades, observando que el impacto de la anisotropía es negligible en las propiedades macroscópicas en el regimen donde la longitud de difusión gobierna la física del sistema. Hemos comprobado también que el escalado en el parámetro de gas persiste en el caso dipolar hasta valores donde otras interacciones isótropas con la misma longitud de difusión llevan a distintas predicciones. Hemos evaluado el espectro de excitaciones elementales del gas de Bose dipolar en el contexto de la aproximación de Feynman, comparando los resultados con los obtenidos mediante la aproximación de Bogoliubov. Como cabría esperar, las dos aproximaciones coinciden a bajas densidades y se alejan progresivamente al aumentar la densidad. Al aumentar la densidad del sistema vemos que el comportamiento del gas depende del valor del ángulo de polarización de los momentos dipolares. A altas densidades y valores moderados del ángulo de polarización el sistema experimenta una transición de fase de primer orden pasando de una fase gaseosa a una cristalina. Hemos observado también que la anisotropía de la interacción dipolar causa una elongación de la red cristalina en la dirección de interacción más intensa. Para valores elevados del ángulo de polarización y densidades moderadas el sistema muestra transición de fase, esta vez de segundo orden, en la que el sistema pasa de la fase gaseosa a una fase de bandas. Los exponentes críticos de esta transición de fase son independientes del ángulo de polarización y, dentro de los errores estadísticos de las simulaciones, son compatibles con las clases de universalidad del modelo de Ising y XY en tres dimensiones. Finalmente, a altas densidades y valores grandes del ángulo de polarización el sistema muestra otra transición de fase de primer orden entre la fase cristalina y la fase de bandas. La pendiente de esta curva de transición es extremadamente grande indicando que, debido a la anisotropía de la interacción, la fase cristalina deja de ser estable si el potencial de interacción dipolo-dipolo es muy anisótropo. En la última parte de la tesis estudiamos el estado fundamental de un sistema bicapa de dipolos bosónicos, que es una configuración en la que se confinan las partículas en dos planos paralelos mediante un potencial externo. Consideramos la situación más simple en la que los momentos dipolares están orientados por un campo externo en la dirección perpendicular a los planos. Hemos evaluado la energía del estado fundamental y las matrices densidad a uno y dos cuerpos en función de la distancia entre capas usando métodos Monte Carlo. Hemos encontrado que disminuyendo la distancia entre planos para un valor fijo de la intensidad de la interacción, el comportamiento de todos los observables estudiados es compatible con la existencia de una transición de fase de segundo orden modulada por la distancia entre capas. En este sentido, los resultados obtenidos en este trabajo muestran buen acuerdo con estudios previos de este sistema.

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    First-principles modeling of quantum nuclear effects and atomic interactions in solid He-4 at high pressure  Open access

     Cazorla Silva, Claudio; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 91, num. 2, p. 1-9
    DOI: 10.1103/PhysRevB.91.024103
    Date of publication: 2015-01-07
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    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.

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    Luttinger-liquid behavior of one-dimensional He-3  Open access

     Astrakharchik, Grigori; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 90, num. 23, p. 1-5
    DOI: 10.1103/PhysRevB.90.235439
    Date of publication: 2014-12-30
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    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, a "Bose gas." a "super-Tonks-Girardeau" regime, and a "quasicrystal." The obtained results are applicable to unpolarized, partially, or fully polarized He-3.

    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, a

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    Universality in molecular halo clusters  Open access

     Stipanovic, P.; Markic, L. Vranjes; Beslic, Ivana; Boronat Medico, Jordi
    Physical review letters
    Vol. 113, num. 25, p. 1-5
    DOI: 10.1103/PhysRevLett.113.253401
    Date of publication: 2014-12-16
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    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.

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    Quantum phase transition with a simple variational ansatz  Open access

     Lutsyshyn, Yaroslav; Astrakharchik, Grigori; Cazorla, C; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 90, num. 21, p. 1-5
    DOI: 10.1103/PhysRevB.90.214512
    Date of publication: 2014-12-11
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    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.

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    Phase diagram of dipolar bosons in two dimensions with tilted polarization  Open access

     Macia Rey, Adrian; Boronat Medico, Jordi; Mazzanti Castrillejo, Fernando Pablo
    Physical review A
    Vol. 90, num. 6, p. 1-5
    DOI: 10.1103/PhysRevA.90.061601
    Date of publication: 2014-12-01
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    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.

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    Distinguishability, degeneracy, and correlations in three harmonically trapped bosons in one dimension  Open access

     Garcia March, Miguel Angel; Julia Diaz, Bruno; Astrakharchik, Grigori; Boronat Medico, Jordi; Polls, A.
    Physical review A
    Vol. 90, num. 6, p. 1-13
    DOI: 10.1103/PhysRevA.90.063605
    Date of publication: 2014-12-01
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    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.

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     Boronat Medico, Jordi
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  • 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
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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.

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    Quantum correlations and spatial localization in one-dimensional ultracold bosonic mixtures  Open access

     Garcia March, Miguel Angel; Julia Diaz, Bruno; Astrakharchik, Grigori; Busch, Th; Boronat Medico, Jordi; Polls, A.
    New journal of physics
    Vol. 16, p. 1-18
    DOI: 10.1088/1367-2630/16/10/103004
    Date of publication: 2014-10-07
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    We present the complete phase diagram for one-dimensional binary mixtures of bosonic ultracold atomic gases in a harmonic trap. We obtain exact results with direct numerical diagonalization for a small number of atoms, which permits us to quantify quantum many-body correlations. The quantum Monte Carlo method is used to calculate energies and density profiles for larger system sizes. We study the system properties for a wide range of interaction parameters. For the extreme values of these parameters, different correlation limits can be identified, where the correlations are either weak or strong. We investigate in detail how the correlations evolve between the limits. For balanced mixtures in the number of atoms in each species, the transition between the different limits involves sophisticated changes in the one-and two-body correlations. Particularly, we quantify the entanglement between the two components by means of the von Neumann entropy. We show that the limits equally exist when the number of atoms is increased for balanced mixtures. Also, the changes in the correlations along the transitions among these limits are qualitatively similar. We also show that, for imbalanced mixtures, the same limits with similar transitions exist. Finally, for strongly imbalanced systems, only two limits survive, i.e., a miscible limit and a phase-separated one, resembling those expected with a mean-field approach.

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    Dynamic structure function of a cold Fermi gas at unitarity  Open access

     Astrakharchik, Grigori; Boronat Medico, Jordi; Krotscheck, Eckhard; Lichtenegger, Thomas
    Journal of physics: conference series
    Vol. 529, num. 1
    DOI: 10.1088/1742-6596/529/1/012009
    Date of publication: 2014-08-14
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    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.

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    Atomic monolayer deposition on the surface of nanotube mechanical resonators  Open access

     Tavernarakis, Alexandros; Chaste, Julien; Eichler, Alexander; Ceballo, Gustavo; Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi; Bachtold, Adrian
    Physical review letters
    Vol. 112, num. 19, p. 196103-1-196103-5
    DOI: 10.1103/PhysRevLett.112.196103
    Date of publication: 2014-05-14
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    We study monolayers of noble gas atoms (Xe, Kr, Ar, and Ne) deposited on individual ultraclean suspended nanotubes. For this, we record the resonance frequency of the mechanical motion of the nanotube, since it provides a direct measure of the coverage. The latter is the number of adsorbed atoms divided by the number of the carbon atoms of the suspended nanotube. Monolayers form when the temperature is lowered in a constant pressure of noble gas atoms. The coverage of Xe monolayers remains constant at 1/6 over a large temperature range. This finding reveals that Xe monolayers are solid phases with a triangular atomic arrangement, and are commensurate with the underlying carbon nanotube. By comparing our measurements to theoretical calculations, we identify the phases of Ar and Ne monolayers as fluids, and we tentatively describe Kr monolayers as solid phases. These results underscore that mechanical resonators made from single nanotubes are excellent probes for surface science.

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    Spin-polarized hydrogen adsorbed on the surface of superfluid He-4  Open access

     Marín, J. M.; Vranješ Markic, Leandra; Boronat Medico, Jordi
    Journal of chemical physics
    Vol. 139, num. 22, p. 224708-1-224708-7
    DOI: 10.1063/1.4843375
    Date of publication: 2013-12-14
    Journal article

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    The experimental realization of a thin layer of spin-polarized hydrogen H double down arrow adsorbed on top of the surface of superfluid He-4 provides one of the best examples of a stable, nearly two-dimensional(2D) quantum Bose gas. We report a theoretical study of this system using quantum Monte Carlo methods in the limit of zero temperature. Using the full Hamiltonian of the system, composed of a superfluid He-4 slab and the adsorbed H double down arrow layer, we calculate the main properties of its ground state using accurate models for the pair interatomic potentials. Comparing the results for the layer with the ones obtained for a strictly 2D setup, we analyze the departure from the 2D character when the density increases. Only when the coverage is rather small the use of a purely 2D model is justified. The condensate fraction of the layer is significantly larger than in 2D at the same surface density, being as large as 60% at the largest coverage studied. (c) 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4843375]

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    Possible superfluidity of molecular hydrogen in a two-dimensional crystal phase of sodium  Open access

     Cazorla Silva, Claudio; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 88, num. 22, p. 224501-1-224501-6
    DOI: 10.1103/PhysRevB.88.224501
    Date of publication: 2013-12-03
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    We theoretically investigate the ground-state properties of a molecular para-hydrogen (p-H2) film in which crystallization is energetically frustrated by embedding sodium (Na) atoms periodically distributed in a triangular lattice. In order to fully deal with the quantum nature of p-H 2 molecules, we employ the diffusion Monte Carlo method and realistic semiempirical pairwise potentials describing the interactions between H 2-H2 and Na-H2 species. In particular, we calculate the energetic, structural, and superfluid properties of two-dimensional Na-H2 systems within a narrow density interval around equilibrium at zero temperature. In contrast to previous computational studies considering other alkali metal species such as rubidium and potassium, we find that the p-H2 ground state is a liquid with a significantly large superfluid fraction of ¿s/¿=0.29(2). The appearance of p-H2 superfluid response is due to the fact that the interactions between Na atoms and H2 molecules are less attractive than between H2 molecules. This induces a considerable reduction of the hydrogen density which favors the stabilization of the liquid phase.

    We theoretically investigate the ground-state properties of a molecular para-hydrogen (p-H 2 ) film in which crystallization is energetically frustrated by embedding sodium (Na) atoms periodically distributed in a triangular lattice. In order to fully deal with the quantum nature of p-H 2 molecules, we employ the diffusion Monte Carlo method and realistic semiempirical pairwise potentials describing the interactions between H 2 -H 2 and Na-H 2 species. In particular, we calculate the energetic, structural, and superfluid properties of two-dimensional Na-H 2 systems within a narrow density interval around equilibrium at zero temperature. In contrast to previous computational studies considering other alkali metal species such as rubidium and potassium, we find that the p-H 2 ground state is a liquid with a significantly large superfluid fraction of ρ s /ρ=0.29(2) . The appearance of p-H 2 superfluid response is due to the fact that the interactions between Na atoms and H 2 molecules are less attractive than between H 2 molecules. This induces a considerable reduction of the hydrogen density which favors the stabilization of the liquid phase.

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    Sharp crossover from composite fermionization to phase separation in microscopic mixtures of ultracold bosons  Open access

     Garcia March, Miguel Angel; Julia Diaz, Bruno; Astrakharchik, Grigori; Busch, T; Boronat Medico, Jordi; Rios Polls, Artur
    Physical review A
    Vol. 88, num. 6, p. 063604-1-063604-5
    DOI: 10.1103/PhysRevA.88.063604
    Date of publication: 2013-12-03
    Journal article

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    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.

  • Zero-temperature phase diagram of D2 physisorbed on graphane

     Carbonell Coronado, Carmen; De Soto Borrera, Feliciano Carlos; Cazorla Silva, Claudio; Boronat Medico, Jordi; Gordillo Bargueño, Maria Carmen
    Journal of physics: condensed matter
    Vol. 25, num. 44, p. 1-6
    DOI: 10.1088/0953-8984/25/44/445011
    Date of publication: 2013-10-06
    Journal article

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    We determined the zero-temperature phase diagram of D2 physisorbed on graphane using the diffusion Monte Carlo method. The substrate used was C-graphane, an allotropic form of the compound that has been experimentally obtained through hydrogenation of graphene. We found that the ground state is the d phase, a commensurate structure observed experimentally when D2 is adsorbed on graphite, and not the registered structure characteristic of H2 on the same substrate.

    We determined the zero-temperature phase diagram of D2 physisorbed on graphane using the diffusion Monte Carlo method. The substrate used was C-graphane, an allotropic form of the compound that has been experimentally obtained through hydrogenation of graphene. We found that the ground state is the δ phase, a commensurate structure observed experimentally when D2 is adsorbed on graphite, and not the registered structure characteristic of H2 on the same substrate.

  • Spin-polarized hydrogen and its isotopes: A rich class of quantum phases (Review Article)

     Beslic, Ivana; Vranješ Markic, Leandra; Boronat Medico, Jordi
    Low temperature physics
    Vol. 39, num. 10, p. 1104-1122
    DOI: 10.1063/1.4823490
    Date of publication: 2013-10
    Journal article

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    We review the recent activity in the theoretical description of spin-polarized atomic hydrogen and its isotopes at very low temperatures. Spin-polarized hydrogen is the only system in nature that remains stable in the gas phase even in the zero temperature limit due to its small mass and weak interatomic interaction. Hydrogen and its heavier isotope tritium are bosons, the heavier mass of tritium producing a self-bound (liquid) system at zero temperature. The other isotope, deuterium, is a fermion with nuclear spin one making possible the study of three different quantum systems depending on the population of the three degenerate spin states. From the theoretical point of view, spin-polarized hydrogen is specially appealing because its interatomic potential is very accurately known making possible its precise quantum many-body study. The experimental study of atomic hydrogen has been very difficult due to its high recombination rate, but it finally led to its Bose-Einstein condensate state in 1998. Degeneracy has also been observed in thin films of hydrogen adsorbed on the 4He surface allowing for thepossibility of observing the Berezinskii-Kosterlitz-Thouless superfluid transition. © 2013 I. Bešlic, L. Vranješ Markic, and J. Boronat.

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    Elastic constants of incommensurate solid 4He from diffusion Monte Carlo simulations  Open access

     Cazorla Silva, Claudio; Lutsyshyn, Yaroslav; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 87, num. 21, p. 1-5
    DOI: 10.1103/PhysRevB.87.214522
    Date of publication: 2013-06-28
    Journal article

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    We study the elastic properties of incommensurate solid 4He in the limit of zero temperature. Specifically, we calculate the pressure dependence of the five elastic constants (C11, C12, C13, C33, and C44), longitudinal and transversal speeds of sound, and the T=0 Debye temperature of incommensurate and commensurate hcp 4He using the diffusion Monte Carlo method. Our results show that under compression, the commensurate crystal is globally stiffer than the incommensurate, however at pressures close to melting (i.e., P~25 bar) some of the elastic constants accounting for strain deformations of the hcp basal plane (C12 and C13) are slightly larger in the incommensurate solid. Also, we find that upon the introduction of tiny concentrations of point defects, the shear modulus of 4He (C44) undergoes a small reduction.

    We study the elastic properties of incommensurate solid 4He in the limit of zero temperature. Specifically, we calculate the pressure dependence of the five elastic constants (C11, C12, C13, C33, and C44), longitudinal and transversal speeds of sound, and the T=0 Debye temperature of incommensurate and commensurate hcp 4He using the diffusion Monte Carlo method. Our results show that under compression, the commensurate crystal is globally stiffer than the incommensurate, however at pressures close to melting (i.e., P∼25 bar) some of the elastic constants accounting for strain deformations of the hcp basal plane (C12 and C13) are slightly larger in the incommensurate solid. Also, we find that upon the introduction of tiny concentrations of point defects, the shear modulus of 4He (C44) undergoes a small reduction.

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    Phase diagrams of He-4 on flat and curved environments  Open access

     Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
    Journal of low temperature physics
    Vol. 171, num. 5-6, p. 606-612
    DOI: 10.1007/s10909-012-0790-5
    Date of publication: 2013-06
    Journal article

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    By means of diffusion Monte Carlo calculations, we obtained the phase diagrams of a first and second layer of He-4 on graphene and on the outside of different isolated armchair carbon nanotubes with radii in the range 3.42 to 10.85 . That corresponds to tubes between the (5, 5) and (16, 16) in standard nomenclature. In both cases, the ground state is either a liquid (second layer on graphene and on nanotubes whose radii is greater than similar to 7 ) or an incommensurate solid (for thinner tubes). In the former case, upon a density increase, the system undergoes a first-order phase transition to another incommensurate solid. A study of the influence of the C-He potential (isotropic or anisotropic) on the phase diagrams is also presented.

    By means of diffusion Monte Carlo calculations, we obtained the phase diagrams of a first and second layer of 4He on graphene and on the outside of different isolated armchair carbon nanotubes with radii in the range 3.42 to 10.85 Å. That corresponds to tubes between the (5, 5) and (16, 16) in standard nomenclature. In both cases, the ground state is either a liquid (second layer on graphene and on nanotubes whose radii is greater than ∼7 Å) or an incommensurate solid (for thinner tubes). In the former case, upon a density increase, the system undergoes a first-order phase transition to another incommensurate solid. A study of the influence of the C–He potential (isotropic or anisotropic) on the phase diagrams is also presented.

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    H-2 physisorbed on graphane  Open access

     Carbonell Coronado, Carmen; De Soto Borrera, Feliciano Carlos; Cazorla Silva, Claudio; Boronat Medico, Jordi; Gordillo Bargueño, Maria Carmen
    Journal of low temperature physics
    Vol. 171, num. 5-6, p. 619-625
    DOI: 10.1007/s10909-012-0828-8
    Date of publication: 2013-06
    Journal article

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    We study the zero-temperature phase diagrams of H-2 adsorbed on the three structures predicted for graphane (chair, boat and washboard graphane), using a diffusion Monte Carlo technique. Graphane is the hydrogenated version of graphene, in which each carbon atom changes its hybridization to sp (3) and forms a covalent bond with a hydrogen atom. Our results show that the ground state of H-2 adsorbed on all three types of graphane is a solid, similar to the structures found both for H-2 and D-2 on graphene. When the H-2 density increases, the system undergoes a first order phase transition to a triangular incommensurate solid. This change is direct in the case of washboard graphane, but indirect via different commensurate structures in the other cases. The total hydrogen weight percentage on the three graphane types in their ground states is in the range 10 % to 12 %, depending on if one or both graphane surfaces are covered with H-2.

    We study the zero-temperature phase diagrams of H2 adsorbed on the three structures predicted for graphane (chair, boat and washboard graphane), using a diffusion Monte Carlo technique. Graphane is the hydrogenated version of graphene, in which each carbon atom changes its hybridization to sp3 and forms a covalent bond with a hydrogen atom. Our results show that the ground state of H2 adsorbed on all three types of graphane is a 3 √ ×3 √ solid, similar to the structures found both for H2 and D2 on graphene. When the H2 density increases, the system undergoes a first order phase transition to a triangular incommensurate solid. This change is direct in the case of washboard graphane, but indirect via different commensurate structures in the other cases. The total hydrogen weight percentage on the three graphane types in their ground states is in the range 10 % to 12 %, depending on if one or both graphane surfaces are covered with H2.

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    Two-dimensional Spin-polarized hydrogen at zero temperature  Open access

     Vranješ Markic, Leandra; Boronat Medico, Jordi
    Journal of low temperature physics
    Vol. 171, num. 5-6, p. 685-692
    DOI: 10.1007/s10909-012-0756-7
    Date of publication: 2013-06
    Journal article

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    The ground-state properties of spin polarized hydrogen H¿ in two dimensions (2D) are obtained by means of diffusion Monte Carlo calculations. Using the most accurate to date ab initio H¿–H¿ interatomic potential we have studied hydrogen gas phase, from the very dilute regime until densities above its freezing point. For very low densities, the equation of state of the gas can be described in terms of the gas parameter na2, where a is the s-wave scattering length in 2D. The solid phase in 2D has also been studied up to high pressures and the gas-solid phase transition determined using the double-tangent Maxwell construction.

    The ground-state properties of spin polarized hydrogen H↓ in two dimensions (2D) are obtained by means of diffusion Monte Carlo calculations. Using the most accurate to date ab initio H↓–H↓ interatomic potential we have studied hydrogen gas phase, from the very dilute regime until densities above its freezing point. For very low densities, the equation of state of the gas can be described in terms of the gas parameter na2, where a is the s-wave scattering length in 2D. The solid phase in 2D has also been studied up to high pressures and the gas-solid phase transition determined using the double-tangent Maxwell construction.

  • Influence of the Interaction Potential on the D¿ 1 Equation of State

     Beslic, Ivana; Vranješ Markic, Leandra; Casulleras Ambros, Joaquin; Boronat Medico, Jordi
    Journal of low temperature physics
    Vol. 171, num. 3-4, p. 436-442
    DOI: 10.1007/s10909-012-0773-6
    Date of publication: 2013-05-01
    Journal article

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    Second layer of H2 and D2 adsorbed on graphene  Open access

     Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 87, num. 16, p. 1-6
    DOI: 10.1103/PhysRevB.87.165403
    Date of publication: 2013-04-03
    Journal article

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    We report diffusion Monte Carlo calculations on the phase diagrams of para-H2 and ortho-D2 adsorbed on top of a first layer of the same substances on graphene. We found that the ground state of the second layer is a triangular incommensurate solid for both isotopes. The densities for promotion to a second layer and for the onset of a two-dimensional solid on that second layer compare favorably with available experimental data in both cases.

  • Quantum Monte Carlo study of spin-polarized deuterium

     Beslic, Ivana; Vranješ Markic, Leandra; Casulleras Ambros, Joaquin; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 88, p. 1-10
    DOI: 10.1103/PhysRevB.88.024507
    Date of publication: 2013
    Journal article

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    The ground-state properties of spin-polarized deuterium (D¿) at zero temperature are obtained by means of diffusion Monte Carlo calculations within the fixed-node approximation. Three D¿ species have been investigated (D¿1, D¿2, D¿3), corresponding respectively to one, two, and three equally occupied nuclear-spin states. The influence of the backflow correlations on the ground-state energy of the systems is explored. The equations of state of liquid D¿2 and D¿3 are obtained and compared with the ones obtained in previous approximate predictions. The density and pressure at which D¿1 experiences a gas-liquid transition at T=0 are obtained.

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

     Osychenko, Oleg
    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.

  • 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
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  • Stability of resonantly interacting heavy-light Fermi mixtures

     Astrakharchik, Grigori; Giorgini, Stefano; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 86, num. 17, p. 1-5
    DOI: 10.1103/PhysRevB.86.174518
    Date of publication: 2012-11
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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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    4He adsorbed outside a single carbon nanotube  Open access

     Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 86, num. 16, p. 1-6
    DOI: 10.1103/PhysRevB.86.165409
    Date of publication: 2012-10-08
    Journal article

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    The phase diagrams of 4He adsorbed on the external surfaces of single armchair carbon nanotubes with radii in the range 3.42–10.85 Å are calculated using the diffusion Monte Carlo method. For nanotubes narrower than a (10,10) one, the ground state is an incommensurate solid similar to the one found for H2 on the same substrates. For wider nanotubes, the phase with the minimum energy per particle is a liquid layer. Curved v3×v3 registered solids similar to the ones found on graphene and graphite were unstable for all the tubes considered.

    The phase diagrams of 4He adsorbed on the external surfaces of single armchair carbon nanotubes with radii in the range 3.42–10.85 Å are calculated using the diffusion Monte Carlo method. For nanotubes narrower than a (10,10) one, the ground state is an incommensurate solid similar to the one found for H2 on the same substrates. For wider nanotubes, the phase with the minimum energy per particle is a liquid layer. Curved √3×√3 registered solids similar to the ones found on graphene and graphite were unstable for all the tubes considered.

  • A microscopic description of vacancies in solid He-4

     Rota, Riccardo; Lutsyshyn, Yaroslav; Cazorla, C; Boronat Medico, Jordi
    Journal of low temperature physics
    Vol. 168, num. 3-4, p. 150-161
    DOI: 10.1007/s10909-012-0616-5
    Date of publication: 2012-08
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    Superfluidity of metastable glassy bulk para-hydrogen at low temperature  Open access

     Osychenko, Oleg; Rota, Riccardo; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 85, num. 22, p. 1-10
    DOI: 10.1103/PhysRevB.85.224513
    Date of publication: 2012-06-13
    Journal article

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    Molecular para-hydrogen has been proposed theoretically as a possible candidate for superfluidity, but the eventual superfluid transition is hindered by its crystallization. In this work, we study a metastable non crystalline phase of bulk p-H2 by means of the Path Integral Monte Carlo method in order to investigate at which temperature this system can support superfluidity. By choosing accurately the initial configuration and using a non commensurate simulation box, we have been able to frustrate the formation of the crystal in the simulated system and to calculate the temperature dependence of the one-body density matrix and of the superfluid fraction. We observe a transition to a superfluid phase at temperatures around 1 K. The limit of zero temperature is also studied using the diffusion Monte Carlo method. Results for the energy, condensate fraction, and structure of the metastable liquid phase at T=0 are reported and compared with the ones obtained for the stable solid phase.

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    Zero-temperature phase diagram of the second layer of 4He adsorbed on graphene  Open access

     Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 85, num. 19, p. 1-6
    DOI: 10.1103/PhysRevB.85.195457
    Date of publication: 2012-05-25
    Journal article

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    The phase diagram at zero temperature of 4He adsorbed on a helium incommensurate triangular solid on top of a single graphene sheet has been obtained using the diffusion Monte Carlo method. We have found that, in accordance with previous experimental and simulation results for graphite, the ground state of 4He on this setup is a liquid that, upon compression, transforms into a triangular solid. To define the stability limits of both liquid and solid phases, we considered not only the adsorption energies of the atoms located on the second layer but the average energy of the atoms in both layers. Our results show that the lower density limit for a stable liquid in the second layer is 0.163±0.005 Å−2 and that the lower limit for the existence of an incommensurate solid on the second layer is 0.186±0.003 Å−2. Both values are in overall agreement with the results of torsional oscillator experiments and heat capacity measurements on graphite. The 4/7 and 7/12 registered solids are found to be metastable with respect to triangular incommensurate arrangements of the same density.

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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.

  • Condensate Fraction in Liquid (4)He at Zero Temperature

     Rota, Riccardo; Boronat Medico, Jordi
    Journal of low temperature physics
    Vol. 166, num. 1-2, p. 21-32
    DOI: 10.1007/s10909-011-0410-9
    Date of publication: 2012-01
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    Zero-temperature phase diagram of Yukawa bosons  Open access

     Osychenko, Oleg; 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.

  • Onset Temperature of Bose-Einstein Condensation in Incommensurate Solid 4He

     Rota, Riccardo; Boronat Medico, Jordi
    Physical review letters
    Vol. 108, num. 4, p. 1-5
    DOI: 10.1103/PhysRevLett.108.045308
    Date of publication: 2012
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  • Ewald method for polytropic potentials in arbitrary dimensionality

     Osychenko, Oleg; Astrakharchik, Grigori; Boronat Medico, Jordi
    Molecular physics
    Vol. 110, num. 4, p. 227-247
    DOI: 10.1080/00268976.2011.640291
    Date of publication: 2012
    Journal article

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

     Rota, Riccardo
    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.

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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.

  • Monte Carlo study of quantum phase diagram of Rydberg atoms with repulsive 1/r6 interaction

     Astrakharchik, Grigori; Boronat Medico, Jordi; Lutsyshyn, Yaroslav; Osychenko, Oleg; Lozovik, Yu. E.
    Bose-Einstein Condensation
    Presentation's date: 2011-09-15
    Presentation of work at congresses

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  • Solidification of small p-H(2) clusters at zero temperature

     Sola, E.; Boronat Medico, Jordi
    Russian journal of physical chemistry A
    Vol. 115, num. 25, p. 7071-7076
    DOI: 10.1021/jp112378h
    Date of publication: 2011-06-30
    Journal article

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    We have determined the ground-state energies of para-H2 clusters at zero temperature using the diffusion Monte Carlo method. The liquid or solid character of each cluster is investigated by restricting the phase through the use of proper importance sampling. Our results show inhomogeneous crystallization of clusters, with alternating behavior between liquid and solid phases up to N = 55. From there on, all clusters are solid. The ground-state energies in the range N = 13−75 are established, and the stable phase of each cluster is determined. In spite of the small differences observed between the energy of liquid and solid clusters, the corresponding density profiles are significantly different, a feature that can help to solve ambiguities in the determination of the specific phase of H2 clusters.

  • Two-component Fermi gas of unequal masses at unitarity.

     Astrakharchik, Grigori; Boronat Medico, Jordi; Giorgini, Stefano
    Workshop on Frontiers in Ultracold Fermi Gases
    Presentation's date: 2011-06-06
    Presentation of work at congresses

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    Supersolidity in quantum films adsorbed on graphene and graphite  Open access

     Gordillo Bargueño, Maria Carmen; Cazorla, C; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 83, num. 12, p. 1-4
    DOI: 10.1103/PhysRevB.83.121406
    Date of publication: 2011-03-17
    Journal article

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    Using quantum Monte Carlo we have studied the superfluid density of the first layer of 4He and H2 adsorbed on graphene and graphite. Our main focus has been on the equilibrium ground state of the system, which corresponds to a registered √3 × √3 phase. The perfect solid phase of H2 shows no superfluid signal, whereas 4He has a finite but small superfluid fraction (0.67%). The introduction of vacancies in the crystal makes the superfluidity increase, showing values as large as 14% in 4He without destroying the spatial solid order.

  • On the stability of small vacancy clusters in solid 4He

     Lutsyshyn, Yaroslav; Rota, Riccardo; Boronat Medico, Jordi
    Journal of low temperature physics
    Vol. 162, num. 5-6, p. 455-463
    DOI: 10.1007/s10909-010-0301-5
    Date of publication: 2011-03
    Journal article

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    We study numerically properties of multiple vacancies in solid 4He at zero temperature. Up to four vacancies were introduced into the solid through incommensuration between the number of available lattice sites and the actual number of atoms. Vacancy-vacancy correlation function increases at very short distances indicating effective vacancy attraction between vacancies located on nearby lattice sites. The decay of the pair correlation function at large distances puts an upper bound on the absolute value of the binding energy varying from 4 mK at melting density to 150 mK at the highest considered density and no lower bound; either the four-vacancy clusters are unbound, or are bound too weakly for the temperatures of the supersolid experiments.

  • Ground state of small mixed helium and spin-polarized tritium clusters: A quantum Monte Carlo study

     Stipanovic, P.; Markic, L. Vranjes; Boronat Medico, Jordi; Kezic, B.
    Journal of chemical physics
    Vol. 134, num. 5, p. 054509-1-054509-8
    DOI: 10.1063/1.3530837
    Date of publication: 2011-02-07
    Journal article

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    We report results for the ground-state energy and structural properties of small 4He–T↓ clusters consisting of up to four T↓ and eight 4He atoms. These results have been obtained using very well-known 4He–4He and T↓– T↓ interaction potentials and several models for the 4He– T↓ interatomic potential. All the calculations have been performed with variational and diffusion Monte Carlo methods. It takes at least three atoms to form a mixed bound state. In particular, for small clusters the binding energies are significantly affected by the precise form of the 4He– T↓ interatomic potential but the stability limits remain unchanged. The only exception is the 4He2T↓ trimer whose stability in the case of the weakest 4He– T↓ interaction potential is uncertain while it seems stable for other potentials. The mixed trimer 4He(T↓)2, a candidate for the Borromean state, is not bound. All other studied clusters are stable. Some of the weakest bound clusters can be classified as quantum halo as a consequence of having high probability of being in a classically forbidden region.

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    Path integral Monte Carlo calculation of momentum distribution in solid He-4  Open access

     Rota, Riccardo; Boronat Medico, Jordi
    Journal of low temperature physics
    Vol. 162, num. 3-4, p. 146-153
    DOI: 10.1007/s10909-010-0249-5
    Date of publication: 2011-02
    Journal article

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    We perform calculations of the momentum distribution n(k) in solid 4He by means of path integral Monte Carlo methods.We see that, in perfect crystals, n(k)does not depend on temperature T and that is different from the classical Gaussian shape of the Maxwell-Boltzmann distribution, even though these discrepancies decrease when the density of the system increases. In crystals presenting vacancies, we see that for T ≥ 0.75 K, n(k) presents the same behavior as in the perfect crystal, but, at lower T , it presents a peak when k→0.

    Postprint (author’s final draft)

  • Elastic constants of solid 4He under pressure: Diffusion Monte Carlo study

     Cazorla, C; Lutsyshyn, Yaroslav; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 85, num. 2, p. 1-8
    DOI: 10.1103/PhysRevB.85.024101
    Date of publication: 2011-01-04
    Journal article

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  • Phase transitions of H(2) adsorbed on the surface of single carbon nanotubes

     Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Vol. 84, num. 3, p. 1-4
    DOI: 10.1103/PhysRevB.84.033406
    Date of publication: 2011
    Journal article

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  • Phase diagram of Rydberg atoms with repulsive van der Waals interaction

     Osychenko, Oleg; Astrakharchik, Grigori; Lutsyshyn, Yaroslav; Lozovik, Yu. E.; Boronat Medico, Jordi
    Physical review A
    Vol. 84, num. 6, p. 1-5
    DOI: 10.1103/PhysRevA.84.063621
    Date of publication: 2011
    Journal article

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  • Microscopic approach to the bcc phase of solid 4He

     Rota, Riccardo; Boronat Medico, Jordi
    Molecular physics
    Vol. 109, num. 23-24, p. 2963-2968
    DOI: 10.1080/00268976.2011.620025
    Date of publication: 2011
    Journal article

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