arXiv subjects

Jordi Boronat

Publications and source records attributed to Jordi Boronat.

60 records · Page 4Linked to original sources

arXiv2012

Onset temperature of Bose-Einstein condensation in incommensurate solid 4He

The temperature dependence of the one-body density matrix in 4He crystals presenting vacancies is computed with Path Integral Monte Carlo. The main purpose of this study is to estimate the onset temperature T_0 of Bose-Einstein condensation in these systems. We see that T_0 depends on the vacancy concentration X_v of the simulated system, but not following the law $T_0 \sim X_v^{2/3}$ obtained assuming non-interacting vacancies. For the lowest X_v we have studied, that is X_v = 1/256, we get T_0 = (0.15 \pm 0.05) K, close to the temperatures at which a finite fraction of non-classical rotational inertia is experimentally observed. Below T_0, vacancies do not act as classical point defects becoming completely delocalized entities.

cond-mat.other↗

arXiv2011

Microscopic approach to the bcc phase of solid 4He

The coexistence between the hcp and bcc phases of solid 4He at fixed pressure, and at three different temperatures, is studied by means of the path integral Monte Carlo method. Microscopic results for the main energetic and structure properties of the two phases are reported. The differences between hcp and bcc 4He are shown to be small with the obvious exception of the static structure factor. When crossing the phase transition line, the most significant changes are observed in the kinetic energy per particle and in the Lindemann ratio, both pointing to a less correlated quantum solid in the bcc side. No off-diagonal long-range order is observed in the the hcp and bcc perfect lattices.

cond-mat.other↗

arXiv2011

Condensate fraction in liquid 4He at zero temperature

We present results of the one-body density matrix (OBDM) and the condensate fraction n_0 of liquid 4He calculated at zero temperature by means of the Path Integral Ground State Monte Carlo method. This technique allows to generate a highly accurate approximation for the ground state wave function Psi_0 in a totally model-independent way, that depends only on the Hamiltonian of the system and on the symmetry properties of Psi_0. With this unbiased estimation of the OBDM, we obtain precise results for the condensate fraction n_0 and the kinetic energy K of the system. The dependence of n_0 with the pressure shows an excellent agreement of our results with recent experimental measurements. Above the melting pressure, overpressurized liquid 4He shows a small condensate fraction that has dropped to 0.8% at the highest pressure of p = 87 bar.

cond-mat.other↗

arXiv2010

Path Integral Monte Carlo calculation of momentum distribution in solid \^4\He

We perform calculations of the momentum distribution n(k) in solid \^4\He by means of path integral Monte Carlo methods. We see that, in perfect crystal, n(k) does not depend on temperature T and that is different from the classical Gaussian shape of Maxwell-Boltzmann distribution, even though these discrepancies decrease when the density of the system increases. In crystal presenting vacancies, we see that for T \ge 0.75K, n(k) presents the same behavior as in the perfect crystal, but, at lower T, it presents a peak when k\to0.

cond-mat.other↗

arXiv2009

Thermal and quantum fluctuations in chains of ultracold polar molecules

Ultracold polar molecules, in highly anisotropic traps and interacting via a repulsive dipolar potential, may form one-dimensional chains at high densities. According to classical theory, at low temperatures there exists a critical value of the density at which a second order phase transition from a linear to a zigzag chain occurs. We study the effect of thermal and quantum fluctuations on these self-organized structures using classical and quantum Monte Carlo methods, by means of which we evaluate the pair correlation function and the static structure factor. Depending on the parameters, these functions exhibit properties typical of a crystalline or of a liquid system. We compare the thermal and the quantum results, identifying analogies and differences. Finally, we discuss experimental parameter regimes where the effects of quantum fluctuations on the linear - zigzag transition can be observed.

cond-mat.mes-hall↗

arXiv1999

Progress in Monte Carlo calculations of Fermi systems: normal liquid 3He

The application of the diffusion Monte Carlo method to a strongly interacting Fermi system as normal liquid $^3$He is explored. We show that the fixed-node method together with the released-node technique and a systematic method to analytically improve the nodal surface constitute an efficient strategy to improve the calculation up to a desired accuracy. This methodology shows unambiguously that backflow correlations, when properly optimized, are enough to generate an equation of state of liquid $^3$He in excellent agreement with experimental data from equilibrium up to freezing.

cond-mat.stat-mech↗