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 SIMCON  Computer Simulation in Condensed Matter Research Group
 Department
 Department of Physics and Nuclear Engineering
 School
 Barcelona School of Informatics (FIB)
 jordi.boronatupc.edu
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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: 20140514
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Twodimensional Spinpolarized hydrogen at zero temperature
Vranje¿ Markic, Leandra; Boronat Medico, Jordi
Journal of low temperature physics
Date of publication: 201306
Journal article
Read the abstract Access to the full text Share Reference managersThe groundstate 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 swave scattering length in 2D. The solid phase in 2D has also been studied up to high pressures and the gassolid phase transition determined using the doubletangent Maxwell construction.
The groundstate 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 swave scattering length in 2D. The solid phase in 2D has also been studied up to high pressures and the gassolid phase transition determined using the doubletangent Maxwell construction. 
Zerotemperature 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: 20131006
Journal article
Read the abstract View Share Reference managersWe determined the zerotemperature phase diagram of D2 physisorbed on graphane using the diffusion Monte Carlo method. The substrate used was Cgraphane, 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 zerotemperature phase diagram of D2 physisorbed on graphane using the diffusion Monte Carlo method. The substrate used was Cgraphane, 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. 
Possible superfluidity of molecular hydrogen in a twodimensional crystal phase of sodium
Cazorla Silva, Claudio; Boronat Medico, Jordi
Physical review B: condensed matter and materials physics
Date of publication: 20131203
Journal article
Read the abstract Access to the full text Share Reference managersWe theoretically investigate the groundstate properties of a molecular parahydrogen (pH2) 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 pH 2 molecules, we employ the diffusion Monte Carlo method and realistic semiempirical pairwise potentials describing the interactions between H 2H2 and NaH2 species. In particular, we calculate the energetic, structural, and superfluid properties of twodimensional NaH2 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 pH2 ground state is a liquid with a significantly large superfluid fraction of ¿s/¿=0.29(2). The appearance of pH2 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 groundstate properties of a molecular parahydrogen (pH 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 pH 2 molecules, we employ the diffusion Monte Carlo method and realistic semiempirical pairwise potentials describing the interactions between H 2 H 2 and NaH 2 species. In particular, we calculate the energetic, structural, and superfluid properties of twodimensional NaH 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 pH 2 ground state is a liquid with a significantly large superfluid fraction of ρ s /ρ=0.29(2) . The appearance of pH 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. 
Elastic constants of incommensurate solid 4He from diffusion Monte Carlo simulations
Cazorla Silva, Claudio; Lutsyshyn, Yaroslav; Boronat Medico, Jordi
Physical review B: condensed matter and materials physics
Date of publication: 20130628
Journal article
Read the abstract Access to the full text Share Reference managersWe 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 spinpolarized 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 View Share Reference managersThe groundstate properties of spinpolarized deuterium (D¿) at zero temperature are obtained by means of diffusion Monte Carlo calculations within the fixednode approximation. Three D¿ species have been investigated (D¿1, D¿2, D¿3), corresponding respectively to one, two, and three equally occupied nuclearspin states. The influence of the backflow correlations on the groundstate 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 gasliquid transition at T=0 are obtained. 
Phase diagrams of He4 on flat and curved environments
Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
Journal of low temperature physics
Date of publication: 201306
Journal article
Read the abstract Access to the full text Share Reference managersBy means of diffusion Monte Carlo calculations, we obtained the phase diagrams of a first and second layer of He4 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 firstorder phase transition to another incommensurate solid. A study of the influence of the CHe 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 firstorder 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. 
Second layer of H2 and D2 adsorbed on graphene
Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
Physical review B: condensed matter and materials physics
Date of publication: 20130403
Journal article
Read the abstract Access to the full text Share Reference managersWe report diffusion Monte Carlo calculations on the phase diagrams of paraH2 and orthoD2 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 twodimensional solid on that second layer compare favorably with available experimental data in both cases. 
Spinpolarized 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: 201310
Journal article
Read the abstract View Share Reference managersWe review the recent activity in the theoretical description of spinpolarized atomic hydrogen and its isotopes at very low temperatures. Spinpolarized 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 selfbound (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, spinpolarized hydrogen is specially appealing because its interatomic potential is very accurately known making possible its precise quantum manybody study. The experimental study of atomic hydrogen has been very difficult due to its high recombination rate, but it finally led to its BoseEinstein 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 BerezinskiiKosterlitzThouless superfluid transition. © 2013 I. Be¿lic, L. Vranje¿ Markic, and J. Boronat. 
H2 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: 201306
Journal article
Read the abstract Access to the full text Share Reference managersWe study the zerotemperature 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 sp (3) 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 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.
We study the zerotemperature 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. 
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: 20130501
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Spinpolarized hydrogen adsorbed on the surface of superfluid He4
Marín, J. M.; Vranje¿ Markic, Leandra; Boronat Medico, Jordi
Journal of chemical physics
Date of publication: 20131214
Journal article
Read the abstract Access to the full text Share Reference managersThe experimental realization of a thin layer of spinpolarized hydrogen H double down arrow adsorbed on top of the surface of superfluid He4 provides one of the best examples of a stable, nearly twodimensional(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 He4 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 spinpolarized hydrogen H double down arrow adsorbed on top of the surface of superfluid He4 provides one of the best examples of a stable, nearly twodimensional(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 He4 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] 
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: 201211
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Excitations and stripe phase formation in a twodimensional dipolar bose gas with tilted polarization
Macia Rey, Adrian; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
Physical review letters
Date of publication: 20121205
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Stability of resonantly interacting heavylight Fermi mixtures
Astrakharchik, Grigori; Giorgini, Stefano; Boronat Medico, Jordi
Physical review B: condensed matter and materials physics
Date of publication: 201211
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A microscopic description of vacancies in solid He4
Rota, Riccardo; Lutsyshyn, Yaroslav; Cazorla, C; Boronat Medico, Jordi
Journal of low temperature physics
Date of publication: 201208
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Ewald method for polytropic potentials in arbitrary dimensionality
Osychenko, O.N.; Astrakharchik, Grigori; Boronat Medico, Jordi
Molecular physics
Date of publication: 2012
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Ferromagnetic transition of a twocomponent Fermi gas of hard spheres
Arias de Saavedra, F.; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi; Polls, A.
Physical review A
Date of publication: 20120312
Journal article
Read the abstract Access to the full text Share Reference managersWe use microscopic manybody 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 lowdensity expansion of the energy. We present variational calculations for the hardsphere 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. 
Superfluidity of metastable glassy bulk parahydrogen at low temperature
Osychenko, O.N.; Rota, Riccardo; Boronat Medico, Jordi
Physical review B: condensed matter and materials physics
Date of publication: 20120613
Journal article
Read the abstract Access to the full text Share Reference managersMolecular parahydrogen 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 pH2 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 onebody 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. 
Zerotemperature phase diagram of Yukawa bosons
Osychenko, O.N.; Astrakharchik, Grigori; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
Physical review A
Date of publication: 2012
Journal article
Read the abstract Access to the full text Share Reference managersWe study the zerotemperature 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 heavylight mass ratio M/m, which might lead to crystallization in a twodimensional geometry if the mass ratio of heavylight fermions exceeds a certain critical value. In the present work we do a thorough study of the quantum threedimensional 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 twocomponent fermionic system, this strength corresponds to a heavylight mass ratio of M/m ∼ 180, so that it is impossible to realize the gascrystal 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. 
4He adsorbed outside a single carbon nanotube
Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
Physical review B: condensed matter and materials physics
Date of publication: 20121008
Journal article
Read the abstract Access to the full text Share Reference managersThe 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. 
Condensate Fraction in Liquid (4)He at Zero Temperature
Rota, Riccardo; Boronat Medico, Jordi
Journal of low temperature physics
Date of publication: 201201
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Onset Temperature of BoseEinstein Condensation in Incommensurate Solid 4He
Rota, Riccardo; Boronat Medico, Jordi
Physical review letters
Date of publication: 2012
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Zerotemperature phase diagram of the second layer of 4He adsorbed on graphene
Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
Physical review B: condensed matter and materials physics
Date of publication: 20120525
Journal article
Read the abstract Access to the full text Share Reference managersThe 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. 
Monte Carlo Study of Quantum Phase Transitions at Zero Temperature
Osychenko, Oleg
Defense's date: 20121220
Department of Applied Physics, Universitat Politècnica de Catalunya
Theses
Read the abstract Access to the full text Share Reference managersThe 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 manybody systems with nontrivial 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 dipoledipole, 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 shortranged, longranged and "marginal" forces, and for a jellium model. It is argued that in the case of some shortrange 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 quasineutral Coulomb system. We have obtained the zerotemperature 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 groundstate wave function obtained by solving the corresponding EulerLagrange 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 BoseEinstein 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 parahydrogen atoms at low temperatures below the point of crystallization by means of QMC methods. The zerotemperature simulation was performed in order to investigate the properties of a metastable liquid phase and to find the fraction of the BoseEinstein condensate in the relevant range of densities. The methods of choice for the zerotemperature simulations of the paraH2 system were VMC and DMC techniques. The results of the zerotemperature simulations suggest that the metastable liquid parahydrogen is a strongly correlated liquid, which again serves as an evidence of high instability of this hypothetical system. The calculation of the BoseEinstein 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 dipolodipolo, 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 сuasineutro. 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 EulerLagrange 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 BoseEinstein. 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. 
Microscopic approach to the bcc phase of solid 4He
Rota, Riccardo; Boronat Medico, Jordi
Molecular physics
Date of publication: 2011
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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: 20110104
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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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Path Integral Monte Carlo and BoseEinstein condensation in quantum fluids and solids
Rota, Riccardo
Defense's date: 20111220
Department of Applied Physics, Universitat Politècnica de Catalunya
Theses
Read the abstract Access to the full text Share Reference managersSeveral microscopic theories point out that BoseEinstein condensation (BEC), i.e., a macroscopic occupation of the lowest energy single particle state in manyboson 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 manyboson 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 highorder 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 onebody 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 parahydrogen 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. 
Microscopic description of anisotropic lowdensity dipolar Bose gases in two dimensions
Macia Rey, Adrian; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi; Zillich, Robert E.
Physical review A
Date of publication: 20110919
Journal article
Read the abstract Access to the full text Share Reference managersA microscopic description of the zeroenergy twobody ground state and manybody 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 dipoledipole interaction, on the energy per particle, comparing the results with meanfield 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 zeroenergy twobody 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 groundstate properties of the manybody 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: 20110317
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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: 201103
Journal article
Read the abstract View Share Reference managersWe 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. Vacancyvacancy 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 fourvacancy clusters are unbound, or are bound too weakly for the temperatures of the supersolid experiments. 
Solidification of small pH(2) clusters at zero temperature
Sola, E.; Boronat Medico, Jordi
Russian journal of physical chemistry A
Date of publication: 20110630
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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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Path integral Monte Carlo calculation of momentum distribution in solid He4
Rota, Riccardo; Boronat Medico, Jordi
Journal of low temperature physics
Date of publication: 201102
Journal article
Read the abstract Access to the full text Share Reference managersWe 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 MaxwellBoltzmann 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 spinpolarized tritium clusters: A quantum Monte Carlo study
Stipanovic, P.; Markic, L. Vranjes; Boronat Medico, Jordi; Kezic, B.
Journal of chemical physics
Date of publication: 20110207
Journal article
Read the abstract View Share Reference managersWe report results for the groundstate 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 wellknown 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. 
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.
BoseEinstein Condensation
Presentation's date: 20110915
Presentation of work at congresses
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Twocomponent Fermi gas of unequal masses at unitarity.
Astrakharchik, Grigori; Boronat Medico, Jordi; Giorgini, Stefano
Workshop on Frontiers in Ultracold Fermi Gases
Presentation's date: 20110606
Presentation of work at congresses
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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: 201002
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Quasiequilibrium supersolid phase of a twodimensional dipolar crystal
Kurbakov, I. L.; Lozovik, Yu. E.; Astrakharchik, Grigori; Boronat Medico, Jordi
Physical review B: condensed matter and materials physics
Date of publication: 20100709
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Highorder time expansion path integral ground state
Rota, Riccardo; Casulleras Ambros, Joaquin; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
Physical review E: statistical, nonlinear, and soft matter physics
Date of publication: 201001
Journal article
Read the abstract Access to the full text Share Reference managersThe feasibility of path integral Monte Carlo ground state calculations with very few beads using a highorder shorttime Green’s function expansion is discussed. An explicit expression of the evolution operator which provides dramatic enhancements in the quality of groundstate wave functions is examined. The efficiency of the method makes possible to remove the trial wave function and thus obtain completely modelindependent 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 shadowtype wave function, whose precise content is provided by the highorder algorithm employed. 
Lowdimensional weakly interacting Bose gases: nonuniversal equations of state
Astrakharchik, Grigori; Boronat Medico, Jordi; Kurbakov, I L; Lozovik, Y E; Mazzanti Castrillejo, Fernando Pablo
Physical review A
Date of publication: 201001
Journal article
Read the abstract Access to the full text Share Reference managersThe zerotemperature equation of state is analyzed in lowdimensional bosonic systems. We propose to use the concept of energydependent swave scattering length for obtaining estimations of nonuniversal terms in the energy expansion. We test this approach by making a comparison to exactly solvable onedimensional problems and find that the generated terms have the correct structure. The applicability to twodimensional 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. 
Groundstate properties and superfluidity of two and quasitwodimensional solid He4
Cazorla, C; Astrakharchik, Grigori; Casulleras Ambros, Joaquin; Boronat Medico, Jordi
Journal of physics: condensed matter
Date of publication: 20100428
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Phase diagram of H2 adsorbed on graphene
Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
Physical review B: condensed matter and materials physics
Date of publication: 20100415
Journal article
Read the abstract Access to the full text Share Reference managersThe 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. 
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: 20100809
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Organización Conferencia/'Correlations in Quantum Gases/'
Boronat Medico, Jordi
Participation in a competitive project
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Quantum Monte Carlo simulation of spinpolarized tritium
Vranjes, L; Beslic, Ivana; Boronat Medico, Jordi
Physical review B: condensed matter and materials physics
Date of publication: 2009
Journal article
Read the abstract Access to the full text Share Reference managersThe groundstate properties of spinpolarized 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 liquidsolid phase transition has been determined using the doubletangent 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: 200911
Journal article
Read the abstract View Share Reference managersWe 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 onedimensional 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 quasionedimensional confinement where to study quantum fluids in extremely reduced dimensionality. The results obtained show that in the narrowest configurations the system is nearly onedimensional reinforcing the interest on the physics of onedimensional 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. 
He4 on a Single Graphene Sheet
Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi
Physical review letters
Date of publication: 200902
Journal article
Read the abstract View Share Reference managersThe 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 CHe 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 twodimensional 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: 200907
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