J. Boronat

Equation of state of a Fermi gas in the BEC-BCS crossover: a quantum Monte Carlo study

We calculate the equation of state of a two-component Fermi gas with attractive short-range interspecies interactions using the fixed-node diffusion Monte Carlo method. The interaction strength is varied over a wide range by tuning the value a of the s-wave scattering length of the two-body potential. For a>0 and a smaller than the inverse Fermi wave vector our results show a molecular regime with repulsive interactions well described by the dimer-dimer scattering length a(m)=0.6a. The pair correlation functions of parallel and opposite spins are also discussed as a function of the interaction strength.

Unbiased estimators in quantum Monte Carlo methods: Application to liquid He 4

A Monte Carlo algorithm for computing quantum-mechanical expectation values of coordinate operators in many-body problems is presented. The algorithm, which relies on the forward walking method, fits naturally in a Greens 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 such as the hydrogen atom and the hydrogen molecule. An excellent description of several properties of a fully many-body problem such as liquid

Quantum phase transition in a two-dimensional system of dipoles.

^{4}\mathrm{He}

Effective mass of one 4He atom in liquid 3He.

at zero temperature is achieved.

Beyond the Tonks-Girardeau Gas: Strongly Correlated Regime in Quasi-One-Dimensional Bose Gases

The ground-state phase diagram of a two-dimensional Bose system with dipole-dipole interactions is studied by means of a quantum Monte Carlo technique. Our calculation predicts a quantum phase transition from a gas to a solid phase when the density increases. In the gas phase, the condensate fraction is calculated as a function of the density. Using the Feynman approximation, the collective excitation branch is studied and the appearance of a roton minimum is observed. The results of the static structure factor at both sides of the gas-solid phase are also presented. The Lindemann ratio at the transition point becomes gamma=0.230(6). The condensate fraction in the gas phase is estimated as a function of the density.

Quasi-one-dimensional 4 He inside carbon nanotubes

A microscopic calculation of the effective mass of one [sup 4]He impurity in homogeneous liquid [sup 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 [sup 4]He atom and the particles in the medium. The effective mass at equilibrium density, [ital m][l angle][ital main][r angle][sub 4][sup *]/[ital m][sub 4]=1.21, is in very good agreement with the recent experimental determination by Edwards [ital et] [ital 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 [ital m][sub 4][sup *]/[ital m][sub 4].

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

We consider a homogeneous 1D Bose gas with contact interactions and a large attractive coupling constant. This system can be realized in tight waveguides by exploiting a confinement induced resonance of the effective 1D scattering amplitude. By using the diffusion Monte Carlo method we show that, for small densities, the gaslike state is well described by a gas of hard rods. The critical density for cluster formation is estimated using the variational Monte Carlo method. The behavior of the correlation functions and of the frequency of the lowest breathing mode for harmonically trapped systems shows that the gas is more strongly correlated than in the Tonks-Girardeau regime.

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

We report results of diffusion Monte Carlo calculations for both

Effective mass of one He 4 atom in liquid He 3

{}^{4}\mathrm{He}

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

absorbed in a narrow single walled carbon nanotube