Graphic summary
  • Show / hide key
  • Information


Scientific and technological production
  •  

1 to 50 of 236 results
  • Atomic monolayer deposition on the surface of nanotube mechanical resonators

     Tavernarakis, Alexandros; Chaste, Julien; Eichler, Alexander; Ceballo, Gustavo; Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi; Bachtold, Adrian
    Physical review letters
    Date of publication: 2014-05-14
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • 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

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    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; Vranje¿ Markic, Leandra; Boronat Medico, Jordi
    Low temperature physics
    Date of publication: 2013-10
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    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. Be¿lic, L. Vranje¿ Markic, and J. Boronat.

  • Access to the full text
    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
    Date of publication: 2013-06
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Access to the full text
    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
    Date of publication: 2013-12-03
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Access to the full text
    Spin-polarized hydrogen adsorbed on the surface of superfluid He-4  Open access

     Marín, J. M.; Vranje¿ Markic, Leandra; Boronat Medico, Jordi
    Journal of chemical physics
    Date of publication: 2013-12-14
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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]

    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]

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Quantum Monte Carlo study of spin-polarized deuterium

     Beslic, Ivana; Vranje¿ Markic, Leandra; Casulleras Ambros, Joaquin; Boronat Medico, Jordi
    Physical review B: condensed matter and materials physics
    Date of publication: 2013
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Access to the full text
    Phase diagrams of He-4 on flat and curved environments  Open access

     Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
    Journal of low temperature physics
    Date of publication: 2013-06
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

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

     Beslic, Ivana; Vranje¿ Markic, Leandra; Casulleras Ambros, Joaquin; Boronat Medico, Jordi
    Journal of low temperature physics
    Date of publication: 2013-05-01
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Access to the full text
    Two-dimensional Spin-polarized hydrogen at zero temperature  Open access

     Vranje¿ Markic, Leandra; Boronat Medico, Jordi
    Journal of low temperature physics
    Date of publication: 2013-06
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • 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

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Ewald method for polytropic potentials in arbitrary dimensionality

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

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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
    Date of publication: 2012-08
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

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

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

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • 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

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • 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

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • 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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • 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

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • 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

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    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
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • 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
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Microscopic approach to the bcc phase of solid 4He

     Rota, Riccardo; Boronat Medico, Jordi
    Molecular physics
    Date of publication: 2011
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • 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
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • 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
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • 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

     Share Reference managers Reference managers Open in new window

  • 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

     Share Reference managers Reference managers Open in new window

  • 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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • 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
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • 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
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Access to the full text
    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

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    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.

  • 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
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Organización Conferencia/'Correlations in Quantum Gases/'

     Boronat Medico, Jordi
    Participation in a competitive project

     Share

  • 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
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • He4 on a Single Graphene Sheet

     Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
    Physical review letters
    Date of publication: 2009-02
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The phase diagram of the first layer of He4 adsorbed on a single graphene sheet has been calculated by a series of diffusion Monte Carlo calculations including corrugation effects. Since the number of C-He interactions is smaller than in graphite, the binding energy of He4 atoms to graphene is reduced approximately 13.4 K per helium atom. Our results indicate that the phase diagram is qualitatively similar to that of helium on top of graphite. A two-dimensional liquid film on graphene is predicted to be metastable with respect to the commensurate solid but the difference in energy between both phases is very small, opening the possibility of such a liquid film to be experimentally observed.

  • Thermal and quantum fluctuations in chains of ultracold polar molecules

     Boronat Medico, Jordi; Astrakharchik, Grigori; Chiara, De G; Morigi, G
    Journal of physics B. Atomic molecular and optical physics
    Date of publication: 2009-07
    Journal article

     Share Reference managers Reference managers Open in new window

  • Dynamics of a Two-Dimensional System of Quantum Dipoles

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

     Share Reference managers Reference managers Open in new window