arXiv subjects

J. Boronat

Publications and source records attributed to J. Boronat.

At least 73 records · Page 4Linked to original sources

arXiv2012

Ferromagnetic transition of a two-component Fermi gas of Hard Spheres

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 ($k_F a \sim 1$) that is too large to be compatible with the universal low-density expansion of the energy. We present new variational calculations for the hard-sphere Fermi gas, in the framework of Fermi hypperneted chain theory, that shift the transition to higher densities ($k_F a \sim 1.8$). Backflow correlations, which are mainly active in the unpolarized system, are essential for this shift.

cond-mat.quant-gas↗

arXiv2012

A microscopic description of vacancies in solid 4He

The changes that vacancies produce in the properties of hcp solid 4He are studied by means of quantum Monte Carlo methods. Our results show that the introduction of vacancies produces significant changes in the behavior of solid 4He, even when the vacancy concentration is very small. We show that there is an onset temperature where the properties of incommensurate 4He change significantly. Below this temperature, we observe the emergence of off-diagonal long range order and a complete spatial delocalization of the vacancies. This temperature is quite close to the temperature where non-classical rotational inertia has been experimentally observed. Finally, we report results on the influence of vacancies in the elastic properties of hcp 4He at zero temperature.

cond-mat.other↗

arXiv2011

Phase diagram of Rydberg atoms with repulsive van der Waals interaction

We report a quantum Monte Carlo calculation of the phase diagram of bosons interacting with a repulsive inverse sixth power pair potential, a model for assemblies of Rydberg atoms in the local van der Waals blockade regime. The model can be parametrized in terms of just two parameters, the reduced density and temperature. Solidification happens to the fcc phase. At zero temperature the transition density is found with the diffusion Monte Carlo method at density $ρ= 3.9 (\hbar^2/m C_6)^{3/4} $, where $C_6$ is the strength of the interaction. The solidification curve at non-zero temperature is studied with the path integral Monte Carlo approach and is compared with transitions in corresponding harmonic and classical crystals. Relaxation mechanisms are considered in relation to present experiments, especially pertaining to hopping of the Rydberg excitation.

cond-mat.quant-gas↗

arXiv2011

Bose-Einstein Condensation in liquid $^4$He near the liquid-solid transition line

We present precision neutron scattering measurements of the Bose-Einstein condensate fraction, n0(T), and the atomic momentum distribution, n\star(k), of liquid 4He at pressure p =24 bar. Both the temperature dependence of n0(T) and of the width of n\star(k) are determined. The n0(T) can be represented by n0(T) = n0(0)[1-(T/Tλ)γ] with a small n0(0) = 2.80\pm0.20% and large γ = 13\pm2 for T < Tλ indicating strong interaction. The onset of BEC is accompanied by a significant narrowing of the n\star(k). The narrowing accounts for 65% of the drop in kinetic energy below Tλ and reveals an important coupling between BEC and k > 0 states. The experimental results are well reproduced by Path Integral Monte Carlo calculations.

cond-mat.stat-mech↗

arXiv2011

Ewald method for polytropic potentials in arbitrary dimensionality

The Ewald summation technique is generalised to power-law 1/|r|^k potentials in three-, two- and one-dimensional geometries with explicit formulae for all the components of the sums. The cases of short-range, long-range and "marginal" interactions are treated separately. The jellium model, as a particular case of a charge-neutral system, is discussed and the explicit forms of the Ewald sums for such system are presented. A generalised form of the Ewald sums for a noncubic (nonsquare) simulation cell for three- (two-) dimensional geometry is obtained and its possible field of application is discussed. A procedure for the optimisation of the involved parameters in actual simulations is developed and an example of its application is presented.

cond-mat.quant-gas↗

arXiv2011

Phase transitions of H2 adsorbed on the surface of single carbon nanotubes

By means of Diffusion Monte Carlo calculations, we obtained the complete phase diagrams of H$_2$ adsorbed on the outer surface of isolated armchair carbon nanotubes of radii ranging from 3.42 to 10.85 Å. We only considered density ranges corresponding to the filling of the first adsorption layer in these curved structures. In all cases, the zero-temperature ground state was found to be an incommensurate solid, except in the widest tube, in which the structure with lowest energy is an analogous of the $\sqrt{3} \times \sqrt{3}$ phase found in planar substrates. Those incommensurate solids result form the arrangement of the hydrogen molecules in circumferences whose plane is perpendicular to the main axis of the carbon nanotube. For each tube, there is only one of such phases stable in the density range considered, except in the case of the (5,5) and (6,6) tubes, in which two of these incommensurate solids are separated by novel first order phase transitions.

cond-mat.mes-hall↗

arXiv2011

The elastic constants of solid 4He under pressure: a diffusion Monte Carlo study

We study the elasticity of perfect 4He at zero-temperature using the diffusion Monte Carlo method and a realistic semi-empirical pairwise potential to describe the He-He interactions. Specifically, we calculate the value of the elastic constants of hcp helium C_{ij} as a function of pressure up to 110 bar. It is found that the pressure dependence of all five non-zero C_{ij} is linear and we provide accurate parametrization of each of them. Our elastic constants results are compared to previous variational calculations and low-temperature measurements and in general notably good agreement is found among them. Furthermore, we report T = 0 results for the Gruneisen parameters, sound velocities and Debye temperature of hcp 4He. This work represents the first of a series of computational studies aimed at thoroughly characterizing the response of solid helium to external stress-strain.

cond-mat.other↗

arXiv2011

Microscopic description of anisotropic low-density dipolar Bose gases in two dimensions

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.

cond-mat.quant-gas↗

arXiv2011

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

In a recent study we have reported a new type of trial wave function symmetric under the exchange of particles and which is able to describe a supersolid phase. In this work, we use the diffusion Monte Carlo method and this model wave function to study the properties of solid 4He in two- and quasi two-dimensional geometries. In the purely two-dimensional case, we obtain results for the total ground-state energy and freezing and melting densities which are in good agreement with previous exact Monte Carlo calculations performed with a slightly different interatomic potential model. We calculate the value of the zero-temperature superfluid fraction ρ_{s} / ρof 2D solid 4He and find that it is negligible in all the considered cases, similarly to what is obtained in the perfect (free of defects) three-dimensional crystal using the same computational approach. Interestingly, by allowing the atoms to move locally in the perpendicular direction to the plane where they are confined to zero-point oscillations (quasi two-dimensional crystal) we observe the emergence of a finite superfluid density that coexists with the periodicity of the system.

cond-mat.other↗

arXiv2011

Supersolidity in quantum films adsorbed on graphene and graphite

Using quantum Monte Carlo we have studied the superfluid density of the first layer of $^4$He and H$_2$ adsorbed on graphene and graphite. Our main focus has been on the equilibrium ground state of the system, which corresponds to a registered $\sqrt3 \times \sqrt3$ phase. The perfect solid phase of H$_2$ shows no superfluid signal whereas $^4$He has a finite but small superfluid fraction (0.67%). The introduction of vacancies in the crystal makes the superfluidity increase, showing values as large as 14% in $^4$He without destroying the spatial solid order.

cond-mat.other↗

arXiv2011

Solidification of small para-H2 clusters at zero temperature

We have determined the ground-state energies of para-H$_2$ clusters at zero temperature using the diffusion Monte Carlo method. The liquid or solid character of each cluster is investigated by restricting the phase through the use of proper importance sampling. Our results show inhomogeneous crystallization of clusters, with alternating behavior between liquid and solid phases up to N=55. From there on, all clusters are solid. The ground-state energies in the range N=13--75 are established and the stable phase of each cluster is determined. In spite of the small differences observed between the energy of liquid and solid clusters, the corresponding density profiles are significantly different, feature that can help to solve ambiguities in the determination of the specific phase of H$_2$ clusters.

cond-mat.other↗

arXiv2010

Ground state of small mixed helium and spin-polarized tritium clusters: a quantum Monte Carlo stud

We report results for the ground-state energy and structural properties of small $^4$He-T$\downarrow$ clusters consisting of up to 4 T$\downarrow$ and 8 $^4$He atoms. These results have been obtained using very well-known $^4$He-$^4$He and T$\downarrow$-T$\downarrow$ interaction potentials and several models for the $^4$He-T$\downarrow$ 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 $^4$He-T$\downarrow$ interatomic potential but the stability limits remain unchanged. The only exception is the $^4$He$_2$T$\downarrow$ trimer whose stability in the case of the weakest $^4$He-T$\downarrow$ interaction potential is uncertain, while it seems stable for other potentials. The mixed trimer $^4$He(T$\downarrow$)$_2$, a candidate for Borromean state, is not bound. All other studied clusters are stable. Some of the weakest bound clusters can be classified as quantum halos, as a consequence of having high probability of being in a classically forbidden region.

cond-mat.mes-hall↗

arXiv2010

Properties of Vacancy Formation in He-4 hcp Crystals at Zero Temperature and Fixed Pressure

Equation of state of He-4 hcp crystals with vacancies is determined at zero temperature using the diffusion Monte Carlo technique, an exact ground state zero-temperature method. This allows us to extract the formation enthalpy and isobaric formation energy of a single vacancy in otherwise perfect helium solid. Results were obtained for pressures up to 160 bar. The isobaric formation energy is found to reach a minimum near 57 bar where it is equal to $10.5\pm 1.2$ K. At the same pressure, the vacancy formation volume exhibits a maximum and reaches the volume of the unit cell. This pressure coincides with the pressure interval over which a peak in the supersolid fraction of He-4 was observed in a recent experiment.

cond-mat.other↗

arXiv2010

Phase diagram of H2 adsorbed on graphene

The phase diagram of the first layer of H$_2$ adsorbed on top of a single graphene sheet has been calculated by means of a series of diffusion Monte Carlo (DMC) simulations. We have found that, as in the case of $^4$He, the ground state of molecular hydrogen is a $\sqrt3 \times \sqrt3$ 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.

cond-mat.mtrl-sci↗

arXiv2010

Quasiequilibrium supersolid phase of a two-dimensional dipolar crystal

We have studied the possible existence of a supersolid phase of a two-dimensional dipolar crystal using quantum Monte Carlo methods at zero temperature. Our results show that the commensurate solid is not a supersolid in the thermodynamic limit. The presence of vacancies or interstitials turns the solid into a supersolid phase even when a tiny fraction of them are present in a macroscopic system. The effective interaction between vacancies is repulsive making a quasiequilibrium dipolar supersolid possible.

cond-mat.other↗

arXiv2010

Quantum Monte Carlo study of large spin-polarized tritium clusters

This work expands recent investigations in the field of spin-polarized tritium (T$\downarrow$) clusters . We report the results for the ground state energy and structural properties of large T$\downarrow$ cl usters consisting of up to 320 atoms. All calculations have been performed with variational and diffusi on Monte Carlo methods, using an accurate {\it ab initio} interatomic potential. Our results for $N \le q 40$ are in good agreement with results obtained by other groups. Using a liquid-drop expression for t he energy per particle, we estimate the liquid equilibrium density, which is in good agreement with our recently obtained results for bulk T$\downarrow$. In addition, the calculations of the energy for larg e clusters have allowed for an estimation of the surface tension. From the mean-square radius of the dr op, determined using unbiased estimators, we determine the dependence of the radii on the size of the c luster and extract the unit radius of the T$\downarrow$ liquid.

cond-mat.other↗

arXiv2009

Quantum Monte Carlo simulation of spin-polarized tritium

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

cond-mat.stat-mech↗

arXiv2009

Low-dimensional weakly interacting Bose gases: non-universal equations of state

The zero-temperature equation of state is analyzed in low-dimensional bosonic systems. In the dilute regime the equation of state is universal in terms of the gas parameter, i.e. it is the same for different potentials with the same value of the s-wave scattering length. Series expansions of the universal equation of state are reported for one- and two- dimensional systems. We propose to use the concept of energy-dependent s-wave scattering length for obtaining estimations of non-universal 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 non-universal 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 non-universal terms in experiments with trapped gases is also discussed.

cond-mat.quant-gas↗