arXiv subjects

J. Boronat

Publications and source records attributed to J. Boronat.

At least 127 records · Page 7Linked to original sources

arXiv2001

Superfluidity vs Bose-Einstein condensation in a Bose gas with disorder

We investigate the phenomenon of Bose-Einstein condensation and superfluidity in a Bose gas at zero temperature with disorder. By using the Diffusion Monte-Carlo method we calculate the superfluid and the condensate fraction of the system as a function of density and strength of disorder. In the regime of weak disorder we find agreement with the analytical results obtained within the Bogoliubov model. For strong disorder the system enters an unusual regime where the superfluid fraction is smaller than the condensate fraction.

cond-mat↗

arXiv2001

Isotopic effects of hydrogen adsorption in carbon nanotubes

We present diffusion Monte Carlo calculations of D$_2$ adsorbed inside a narrow carbon nanotube. The 1D D$_2$ equation of state is reported, and the one-dimensional character of the adsorbed D$_2$ is analyzed. The isotopic dependence of the constitutive properties of the quantum fluid are studied by comparing D$_2$ and H$_2$. Quantum effects due to their different masses are observed both in the energetic and the structural properties. The influence of the interatomic potential in one-dimensional systems is also studied by comparing the properties of D$_2$ and $^4$He which have nearly the same mass but a sizeably different potential. The physics of molecular hydrogen adsorbed in the interstitial channels of a bundle of nanotubes is analyzed by means of both a diffusion Monte Carlo calculation and an approximate mean field method.

cond-mat.stat-mech↗

arXiv2000

High-momentum dynamic structure function of liquid 3He-4He mixtures: a microscopic approach

The high-momentum dynamic structure function of liquid 3He-4He mixtures has been studied introducing final state effects. Corrections to the impulse approximation have been included using a generalized Gersch-Rodriguez theory that properly takes into account the Fermi statistics of 3He atoms. The microscopic inputs, as the momentum distributions and the two-body density matrices, correspond to a variational (fermi)-hypernetted chain calculation. The agreement with experimental data obtained at $q=23.1$ Å$^{-1}$ is not completely satisfactory, the comparison being difficult due to inconsistencies present in the scattering measurements. The significant differences between the experimental determinations of the 4He condensate fraction and the 3He kinetic energy, and the theoretical results, still remain unsolved.

cond-mat.stat-mech↗

arXiv2000

Zero-temperature equation of state of quasi-one dimensional H2

We have studied molecular hydrogen in a pure 1D geometry and inside a narrow carbon nanotube by means of the diffusion Monte Carlo method. The one-dimensionality of H2 in the nanotube is well maintained in a large density range, this system being closer to an ideal 1D fluid than liquid 4He in the same setup. H2 shares with 4He the existence of a stable liquid phase and a quasi-continuous liquid-solid transition at very high linear densities.

cond-mat↗

arXiv2000

Coherent and Incoherent Dynamic Structure Function of the Free Fermi Gas

A detailed calculation of the coherent and incoherent dynamic structure functions of the free Fermi gas, starting from their expressions in terms of the one- and semi-diagonal two-body density matrices, is derived and discussed. Their behavior and evolution with the momentum transfer is analyzed, and particular attention is devoted to the contributions that both functions present at negative energies. Finally, an analysis of the energy weighted sum rules satisfied by both responses is also performed. Despite of the simplicity of the model, some of the conclusions can be extended to realistic systems.

cond-mat.soft↗

arXiv1999

Quasi one dimensional $^4$He inside carbon nanotubes

We report results of diffusion Monte Carlo calculations for both $^4$He absorbed in a narrow single walled carbon nanotube (R = 3.42 Å) and strictly one dimensional $^4$He. Inside the tube, the binding energy of liquid $^4$He is approximately three times larger than on planar graphite. At low linear densities, $^4$He in a nanotube is an experimental realization of a one-dimensional quantum fluid. However, when the density increases the structural and energetic properties of both systems differ. At high density, a quasi-continuous liquid-solid phase transition is observed in both cases.

cond-mat.stat-mech↗

arXiv1999

Finite H2 concentrations in superfluid 4He

We have studied the solubility of molecular hydrogen in bulk liquid $^4$He at zero temperature using the diffusion Monte Carlo method and realistic interatomic potentials between the different species of the mixture. Around the $^4$He equilibrium density, the H_2 molecules clusterize in liquid-like drops blocking the existence of a uniform dilution. On the contrary, at higher densities the cluster formation is less feasible and metastable dilute solutions may exist.

cond-mat.stat-mech↗

arXiv1999

Ground state of a homogeneous Bose gas: a diffusion Monte Carlo calculation

We use a diffusion Monte Carlo method to calculate the lowest energy state of a uniform gas of bosons interacting through different model potentials, both strictly repulsive and with an attractive well. We explicitly verify that at low density the energy per particle follows a universal behavior fixed by the gas parameter na^3. In the regime of densities typical for experiments in trapped Bose-condensed gases, the corrections to the mean-field energies greatly exceed the differences due to the details of the potential.

cond-mat↗

arXiv1998

A Quantum Monte Carlo Study of Static Properties of One 3He Atom in Superfluid 4He

The local environment and the energetic properties of one $^3$He atom solved in bulk superfluid $^4$He are studied by means of the diffusion Monte Carlo method. The chemical potential of the $^3$He impurity is calculated with a generalized reweighting method which allows for a reliable estimation of this quantity. Results for the chemical potential, radial distribution and structure functions, volume-excess parameter, and effective mass are given for several pressures and compared with available experimental data. An overall agreement with experiment is obtained except for the kinetic energy of the $^3$He atom which, in accordance with previous theoretical estimations, appears to be considerably larger than determinations from deep-inelastic neutron scattering.

cond-mat.stat-mech↗

arXiv1997

Momentum distributions in ^3He-^4He liquid mixtures

We present variational calculations of the one-body density matrices and momentum distributions for ^3He-^4He mixtures in the zero temperature limit, in the framework of the correlated basis functions theory. The ground-state wave function contains two- and three-body correlations and the matrix elements are computed by (Fermi)Hypernetted Chain techniques. The dependence on the ^3He concentration (x_3) of the ^4He condensate fraction $(n_0^{(4)})$ and of the ^3He pole strength (Z_F) is studied along the P=0 isobar. At low ^3He concentration, the computed ^4He condensate fraction is not significantly affected by the ^3He statistics. Despite of the low x_3 values, Z_F is found to be quite smaller than that of the corresponding pure ^3He because of the strong ^3He-^4He correlations and of the overall, large total density ρ. A small increase of $n_0^{(4)}$ along x_3 is found, which is mainly due to the decrease of ρrespect to the pure ^4He phase.

cond-mat.stat-mech↗

arXiv1996

Microscopic calculation of the phonon-roton branch in superfluid $^4$He

Diffusion Monte Carlo results for the phonon-roton excitation branch in bulk liquid $^4$He at zero temperature are presented. The sign problem associated to the excited wave function has been dealt both with the fixed-node approximation and the released-node technique. The upper bounds provided by the fixed-node approximation are shown to become exact when using the released-node method. An excellent agreement with experimental data is achieved both at the equilibrium and near the freezing densities.

cond-mat↗

arXiv1996

Vortex excitation in superfluid 4He: A Diffusion Monte Carlo study

We present a diffusion Monte Carlo study of a single vortex in two-dimensional superfluid liquid $^4$He within the fixed node approximation. We use both the Feynman phase and an improved phase which includes backflow correlations to model the nodal surface of the vortex wavefunction. Results for the particle density, core radius and excitation energies are presented.

cond-mat↗

arXiv1995

Final state effects on superfluid $^{\bf 4}$He in the deep inelastic regime

A study of Final State Effects (FSE) on the dynamic structure function of superfluid $^4$He in the Gersch--Rodriguez formalism is presented. The main ingredients needed in the calculation are the momentum distribution and the semidiagonal two--body density matrix. The influence of these ground state quantities on the FSE is analyzed. A variational form of $ρ_2$ is used, even though simpler forms turn out to give accurate results if properly chosen. Comparison to the experimental response at high momentum transfer is performed. The predicted response is quite sensitive to slight variations on the value of the condensate fraction, the best agreement with experiment being obtained with $n_0=0.082$. Sum rules of the FSE broadening function are also derived and commented. Finally, it is shown that Gersch--Rodriguez theory produces results as accurate as those coming from other more recent FSE theories.

cond-mat↗

arXiv1995

Diffusion Monte Carlo study of two-dimensional liquid $^4$He

The ground-state properties of two-dimensional liquid $^4$He at zero temperature are studied by means of a quadratic diffusion Monte Carlo method. As interatomic potential we use a revised version of the HFDHE2 Aziz potential which is expected to give a better description of the interaction between helium atoms. The equation of state is determined with great accuracy over a wide range of densities in the liquid phase from the spinodal point up to the freezing density. The spinodal decomposition density is estimated and other properties of the liquid, such as radial distribution function, static form factor, momentum distribution and density dependence of the condensate fraction are all presented.

cond-mat↗

arXiv1995

Bounds for the phonon-roton dispersion in superfluid 4He

The sum rule approach is used to derive upper bounds for the dispersion law $ω_0(q)$ of the elementary excitations of a Bose superfluid. Bounds are explicitly calculated for the phonon-roton dispersion in superfluid $^4$He, both at equilibrium ($ρ=0.02186$ Å$^{-3}$) and close to freezing ($ρ=0.02622$ Å$^{-3}$). The bound $ω_0(q) \le 2S(q)\midχ(q)\mid^{-1}$, where $S(q)$ and $χ(q)$ are the static structure factor and density response respectively, is calculated microscopically for several values of the wavevector $q$. The results provide a significant improvement with respect to the Feynman approximation $ω_F(q)= q^2(2mS(q))^{-1}$. A further, stronger bound, requiring the additional knowledge of the current correlation function is also investigated. New results for the current correlation function are presented.

cond-mat↗

arXiv1995

Unbiased estimators in Quantum Monte Carlo methods: application to liquid $^4$He

A Monte Carlo algorithm for computing quantum mechanical expectation values of coordinate operators in many body problems is presented. The algorithm, that relies on the forward walking method, fits naturally in a Green's Function Monte Carlo calculation, i.e., it does not require side walks or a bilinear sampling method. Our method evidences stability regions large enough to accurately sample unbiased pure expectation values. The proposed algorithm yields accurate results when it is applied to test problems as the hydrogen atom and the hydrogen molecule. An excellent description of several properties of a fully many body problem as liquid $^4$He at zero temperature is achieved.

cond-mat↗

arXiv1994

Effective Mass of One 4he Atom in Liquid 3he

A microscopic calculation of the effective mass of one $^4$He impurity in homogeneous liquid $^3$He at zero temperature is performed for an extended Jastrow--Slater wave function, including two-- and three--body dynamical correlations and also backflow correlations between the $^4$He atom and the particles in the medium. The effective mass at saturation density, $m_4^*/m_4=1.21$, is in very good agreement with the recent experimental determination by Edwards {\it et al}. The three-- particle correlations appear to give a small contribution to the effective mass and different approximations for the three--particle distribution function give almost identical results for $m_4^*/m_4$.

cond-mat↗

arXiv1994

Monte Carlo Calculations for Liquid $^4$He at Negative Pressure

A Quadratic Diffusion Monte Carlo method has been used to obtain the equation of state of liquid $^4$He including the negative pressure region down to the spinodal point. The atomic interaction used is a renewed version (HFD-B(HE)) of the Aziz potential, which reproduces quite accurately the features of the experimental equation of state. The spinodal pressure has been calculated and the behavior of the sound velociy around the spinodal density has been analyzed.

cond-mat↗