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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
    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.

  • Phase diagrams of He-4 on flat and curved environments

     Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
    Journal of low temperature physics
    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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    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
    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.

  • H-2 physisorbed on graphane

     Carbonell Coronado, Carmen; De Soto Borrera, Feliciano Carlos; Cazorla Silva, Claudio; Boronat Medico, Jordi; Gordillo Bargueño, Maria Carmen
    Journal of low temperature physics
    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.

  • Two-dimensional Spin-polarized hydrogen at zero temperature

     Vranje¿ Markic, Leandra; Boronat Medico, Jordi
    Journal of low temperature physics
    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.

  • 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
    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.

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

     Cazorla Silva, Claudio; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Date of publication: 2013-12-03
    Journal article

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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.

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

     Marín, J. M.; Vranje¿ Markic, Leandra; Boronat Medico, Jordi
    Journal of chemical physics
    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]

  • 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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    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
    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.

  • Stability of resonantly interacting heavy-light Fermi mixtures

     Astrakharchik, Grigori; Giorgini, Stefano; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Date of publication: 2012-11
    Journal article

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  • A microscopic description of vacancies in solid He-4

     Rota, Riccardo; Lutsyshyn, Yaroslav; Cazorla, C; Boronat Medico, Jordi
    Journal of low temperature physics
    Date of publication: 2012-08
    Journal article

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

     Rota, Riccardo; Boronat Medico, Jordi
    Physical review letters
    Date of publication: 2012
    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.

  • Ewald method for polytropic potentials in arbitrary dimensionality

     Osychenko, O.N.; Astrakharchik, Grigori; Boronat Medico, Jordi
    Molecular physics
    Date of publication: 2012
    Journal article

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

     Macia Rey, Adrian; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
    European physical journal D
    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.

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

     Rota, Riccardo; Boronat Medico, Jordi
    Journal of low temperature physics
    Date of publication: 2012-01
    Journal article

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    Superfluidity of metastable glassy bulk para-hydrogen at low temperature  Open access

     Osychenko, O.N.; Rota, Riccardo; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    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 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.

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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
    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.

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

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

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

     Rota, Riccardo; Boronat Medico, Jordi
    Journal of low temperature physics
    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)

  • 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
    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.

  • Supersolidity in quantum films adsorbed on graphene and graphite

     Gordillo Bargueño, Maria Carmen; Cazorla, C; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Date of publication: 2011-03-17
    Journal article

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

     Lutsyshyn, Yaroslav; Rota, Riccardo; Boronat Medico, Jordi
    Journal of low temperature physics
    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.

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

     Sola, E.; Boronat Medico, Jordi
    Russian journal of physical chemistry A
    Date of publication: 2011-06-30
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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
    Date of publication: 2011
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  • Microscopic approach to the bcc phase of solid 4He

     Rota, Riccardo; Boronat Medico, Jordi
    Molecular physics
    Date of publication: 2011
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  • Phase diagram of Rydberg atoms with repulsive van der Waals interaction

     Osychenko, O.N.; Astrakharchik, Grigori; Lutsyshyn, Yaroslav; Lozovik, Yu. E.; Boronat Medico, Jordi
    Physical review A
    Date of publication: 2011
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  • Generation of spin squeezing in an ensemble of cold rubidium 87

     Koschorreck, Marco
    Defense's date: 2011-01-13
    Department of Applied Physics, Universitat Politècnica de Catalunya
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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.

  • 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
    Date of publication: 2011-01-04
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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.

  • Ground-state properties and superfluidity of two- and quasi-two-dimensional solid He-4

     Cazorla, C; Astrakharchik, Grigori; Casulleras Ambros, Joaquin; Boronat Medico, Jordi
    Journal of physics: condensed matter
    Date of publication: 2010-04-28
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  • Instability of Vacancy Clusters in Solid 4He

     Lutsyshyn, Yaroslav; Cazorla Silva, Claudio; Boronat Medico, Jordi
    Journal of low temperature physics
    Date of publication: 2010-02
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  • Properties of vacancy formation in hcp 4He crystals at zero temperature and fixed pressure

     Lutsyshyn, Yaroslav; Cazorla, C; Astrakharchik, Grigori; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Date of publication: 2010-08-09
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    Phase diagram of H-2 adsorbed on graphene  Open access

     Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Date of publication: 2010-04-15
    Journal article

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    The phase diagram of the first layer of H2 adsorbed on top of a single graphene sheet has been calculated by means of a series of diffusion Monte Carlo simulations. We have found that, as in the case of 4He, the ground state of molecular hydrogen is a √3×√3 commensurate structure, followed, upon a pressure increase, by an incommensurate triangular solid. A striped phase of intermediate density was also considered and found lying on top of the equilibrium curve separating both commensurate and incommensurate solids.

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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.

  • Ultracold Dipolar Gases in Optical Lattices  Open access

     Trefzger, Christian
    Defense's date: 2010-04-19
    Department of Physics and Nuclear Engineering, Universitat Politècnica de Catalunya
    Theses

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    Esta tesis es un trabajo teórico, en el que estudiamos la física de los átomos dipolares bosónicos ultrafríos en retículos ópticos. Éstos gases consisten de átomos o moléculas bosónicas, enfriados bajo la temperatura de degeneración cuántica, típicamente del orden de nK. En éstas condiciones, en una trampa armónica tridimensional (3D), los bosones que interaccionan débilmente condensan y forman un Condensado de Bose Einstein (BEC). Cuando se carga un BEC en un retículo óptico producido por ondas estacionarias de luz láser, se producen nuevos fenómenos físicos. Estos sistemas entonces realizan modelos de tipo Hubbard y pueden ser llevados a regimenes fuertemente correlacionados.En 1989, M. Fisher et. al. predecían que el modelo de Bose-Hubbard homogéneo (BH) presenta la transición de fase cuántica Superfluid-Mott insulator (SF-MI). En 2002, la transición entre éstas dos fases fue observada experimentalmente por primera vez en el grupo de T. Esslinger e I. Bloch. La realización experimental de un BEC dipolar de cromo en el grupo de T. Pfau, y los progresos recientes en las técnicas de enfriamiento y atrapamiento de moléculas dipolares en los grupos de D. Jin e J. Ye, han abierto el camino hacia los gases cuánticos ultra-fríos dominados por la interacción dipolar. La evolución natural, y el reto de hoy en día por parte experimental, es de cargar BEC dipolares en retículos ópticos y estudiar los gases dipolares fuertemente correlacionados.Antes de éste trabajo de doctorado, estudios sobre modelos de BH con interacciones extendidas a los primeros vecinos mostraron la evidencia de nuevas fases cuánticas, como el supersólido (SS) y la fase checkerboard (CB). Debido al carácter de largo alcance de la interacción dipolo-dipolo, que decae con la potencia cúbica inversa de la distancia, es necesario incluir más de un primer vecino para obtener una descripción fiel y cuantitativa de los sistemas dipolares. De hecho, al incluir más vecinos se permiten y se estabilizan aún más nuevas fases.En esta tesis estudiamos modelos de BH con interacciones dipolares, investigando más allá del estado fundamental. Estudiamos un retículo bidimensional (2D) donde los dipolos están polarizados en dirección perpendicular al plano 2D, dando lugar a una interacción dipolar repulsiva e isotrópica. Utilizamos aproximaciones de campo-medio y un ansatz Gutzwiller, que son suficientemente correctos y adecuados para describir este sistema. Encontramos que los gases dipolares en 2D presentan una multitud de estados metaestables de tipo MI, que compiten con el estado fundamental, de modo parecido a sistemas desordenados. Estudiamos en detalle el destino de estos estados metaestables: como pueden ser preparados de manera controlada, como pueden ser detectados, cual es su tiempo de vida debido al tunnelling, y cual es su rol en los procesos de enfriamiento. Además, encontramos que el estado fundamental está caracterizado por estados MI de tipo checkerboard con coeficiente de ocupación n fraccionario (numero medio de partículas por sitio) que depende del cut-off utilizado en el radio de alcance de la interacción. Confirmamos esta predicción estudiando el mismo sistema con métodos Quantum Monte Carlo (worm algorithm). En este caso no utilizamos ningún cut-off en el radio de alcance de la interacción, y encontramos pruebas de una "Devil's staircase" en el estado fundamental, i.e. donde las fases MI aparecen en todos los n racionales del retículo subyacente. Encontramos además, regiones de los parámetros donde el estado fundamental es supersólido, obtenido drogando los sólidos con partículas o con agujeros.En este trabajo, investigamos también como cambia la estructura precedente en 3D. Nos focalizamos en el retículo 3D más sencillo compuesto de dos planos 2D, en el cual los dipolos están polarizados perpendicularmente a los planos; la interacción dipolar es entonces repulsiva por partículas del mismo plano, mientras es atractiva por partículas en el mismo sitio de dos planos diferentes. En cambio suprimimos el tunnelling entre los planos, lo cual hace el sistema equivalente a una mezcla bosónica en un retículo 2D. Nuestros cálculos muestran que las partículas se juntan en parejas, y demostramos la existencia de la nueva fase cuántica Pair Super Solid (PSS).Actualmente estamos estudiando un retículo 2D donde los dipolos están libres de apuntar en ambas direcciones perpendicularmente al plano, lo cual resulta en una interacción a primeros vecinos repulsiva (atractiva) por dipolos alineados (anti-alineados). Encontramos regiones de parámetros donde el estado fundamental es ferromagnético u anti-ferromagnético, y encontramos pruebas de la existencia de la fase cuántica Counterflow Super Solid (CSS).Las nuestras predicciones tienen directas consecuencias experimentales, y esperamos que vengan pronto controladas en experimentos con gases dipolares atómicos y moleculares ultra-fríos.

    This thesis is a theoretical work, in which we study the physics of ultra-cold dipolar bosonic gases in optical lattices. Such gases consist of bosonic atoms or molecules, cooled below the quantum degeneracy temperature, typically in the nK range. In such conditions, in a three-dimensional (3D) harmonic trap, weakly interacting bosons condense and form a Bose-Einstein Condensate (BEC). When a BEC is loaded into an optical lattice produced by standing waves of laser light, new kinds of physical phenomena occur.These systems realize then Hubbard-type models and can be brought to a strongly correlated regime. In 1989, M. Fisher et. al. predicted that the homogeneous Bose-Hubbard model (BH) exhibits the Superfluid-Mott insulator (SF-MI) quantum phase transition. In 2002 the transition between these two phases were observed experimentally for the first time in the group of T. Esslinger and I. Bloch. The experimental realisation of a dipolar BEC of Chromium by the group of T. Pfau, and the recent progresses in trapping and cooling of dipolar molecules by the groups of D. Jin and J. Ye, have opened the path towards ultra-cold quantum gases with dominant dipole interactions. A natural evolution and present challenge, on the experimental side is then to load dipolar BECs into optical lattices and study strongly correlated ultracold dipolar lattice gases.Before this PhD work, studies of BH models with interactions extended to nearest neighbours had pointed out that novel quantum phases, like supersolid (SS) and checkerboard phases (CB) are expected. Due to the long-range character of the dipole-dipole interaction, which decays as the inverse cubic power of the distance, it is necessary to include more than one nearest neighbour to have a faithful quantitative description of dipolar systems. In fact, longer-range interactions tend to allow for and stabilize more novel phases.In this thesis we study BH models with dipolar interactions, going beyond the ground state search. We consider a two-dimensional (2D) lattice where the dipoles are polarized perpendicularly to the 2D plane, resulting in an isotropic repulsive interaction. We use the mean-field approximations and a Gutzwiller ansatz which are quite accurate and suitable to describe this system. We find that dipolar bosonic gas in 2D exhibits a multitude of insulating metastable states, often competing with the ground state, similarly as in a disordered system. We study in detail the fate of these metastable states: how can they be prepared on demand, how they can be detected, what is their lifetime due to tunnelling, and what is their role in various cooling schemes. Moreover, we find that the ground state is characterized by insulating checkerboard-like states with fractional filling factors v(average number of particles per site) that depend on the cut-off used for the interaction range. We confirm this prediction by studying the same system with Quantum Monte Carlo methods (the worm algorithm). In this case no cut-off is used, and we find evidence for a Devil's staircase in the ground state, i.e. where insulating phases appear at all rational of the underlying lattice. We also find regions of parameters where the ground state is a supersolid, obtained by doping the solids either with particles or vacancies.In this work, we also investigate how the previous scenario changes in 3D. We focus on the simplest 3D lattice composed of two 2D layers in which the dipoles are polarized perpendicularly to the planes; the dipolar interaction is then repulsive for particles laying on the same plane, while it is attractive for particles at the same lattice site on different layers. Instead we consider inter-layer tunnelling to be suppressed, which makes the system analogous to a bosonic mixture in a 2D lattice. Our calculations show that particles pair into composites, and demonstrate the existence of the novel Pair Super Solid (PSS) quantum phase.Currently we are studying a 2D lattice where the dipoles are free to point in both directions perpendicularly to the plane, which results in a nearest neighbour repulsive (attractive) interaction for aligned (antialigned) dipoles. We find regions of parameters where the ground state is ferromagnetic or antiferromagnetic, and find evidences for the existence of a Counterflow Super Solid (CSS) quantum phase.Our predictions have direct experimental consequences, and we hope that they will be soon checked in experiments with ultracold dipolar atomic and molecular gases.

  • Interfacing single ions and single photons for quantum networks

     Schuck, Carsten
    Defense's date: 2010-01-13
    Department of Physics and Nuclear Engineering, Universitat Politècnica de Catalunya
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  • Organización Conferencia/'Correlations in Quantum Gases/'

     Boronat Medico, Jordi
    Participation in a competitive project

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  • Quasiequilibrium supersolid phase of a two-dimensional dipolar crystal

     Kurbakov, I. L.; Lozovik, Yu. E.; Astrakharchik, Grigori; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Date of publication: 2010-07-09
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  • Probing the near-field optical response of plasmon nanostructures with two-photon luminescence microscopy.

     Ghenuche, Petru Virgil
    Defense's date: 2009-04-02
    Department of Applied Physics, Universitat Politècnica de Catalunya
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  • Rubidium resonant squeezed light from a diode-pumped optical-parametric oscillator.

     Predojevic, Ana
    Defense's date: 2009-06-30
    Department of Physics and Nuclear Engineering, Universitat Politècnica de Catalunya
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  • Quantum Monte Carlo study of large spin-polarized tritium clusters

     Be¿li¿, I.; Vranje¿ Marki¿,, L.; Boronat Medico, Jordi
    Journal of chemical physics
    Date of publication: 2009
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    Quantum Monte Carlo simulation of spin-polarized tritium  Open access

     Vranjes, L; Beslic, Ivana; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Date of publication: 2009
    Journal article

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    The ground-state properties of spin-polarized tritium T↓ at zero temperature are obtained by means of diffusion Monte Carlo calculations. Using an accurate ab initio T↓-T↓ interatomic potential we have studied its liquid phase, from the spinodal point until densities above its freezing point. The equilibrium density of the liquid is significantly higher and the equilibrium energy of −3.664(6) K significantly lower than in previous approximate descriptions. The solid phase has also been studied for three lattices up to high pressures and we find that hcp lattice is slightly preferred. The liquid-solid phase transition has been determined using the double-tangent Maxwell construction; at zero temperature, bulk tritium freezes at a pressure of P=9(1) bar.

  • Quantum Fluids in Nanotubes: A Quantum Monte Carlo Approach

     Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
    Journal of low temperature physics
    Date of publication: 2009-11
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    We review quantum Monte Carlo results on energetic and structure properties of quantum fluids adsorbed in a bundle of carbon nanotubes. Using realistic interatomic interactions the different adsorption sites that a bundle offer are accurately studied and compared in some cases with strictly one-dimensional geometries. The study is performed quite extensively for 4He and restricted to the inner part of a single nanotube for H2 and D2. From a theoretical point of view, nanotubes open the real possibility of a quasi-one-dimensional confinement where to study quantum fluids in extremely reduced dimensionality. The results obtained show that in the narrowest configurations the system is nearly one-dimensional reinforcing the interest on the physics of one-dimensional quantum fluids. Experimental confirmation of the theoretical results obtained is still not in a satisfactory situation due to the difficulties on extracting from the data the dominant adsorption sites.