[en] The authors review the relationship between superfluidity, Bose condensation and vortices in two dimensional superfluids. The theory of the dynamic response of superfluid films, due to Ambegaokar et al., is reviewed and then extended to describe the non-linear response of both helium and superconducting films

[en] The authors have studied the non-linear response of a helium film to an external AC drive near the Kosterlitz Thouless superfluid transition by using the theory of Ambegaokar et al. Previously they have investigated the weakly non-linear response, corresponding to a small ratio of the diffusion length r_D to the length scale r_V = h/mv_n where v_n is the velocity of the substrate. Here the authors extend the calculation to describe the opposite regime where the ratio r_D/r_V is large. The period shift and Q factor of a torsional oscillator containing the film are calculated for the strongly non-linear regime. The expected features of the response in the intermediate regime where r_D ∼ r_V are obtained by smoothly interpolating the results obtained in the two limiting cases

No abstract available

[en] The authors have calculated the first non-linear corrections to the period shift and Q factor of a torsional oscillator containing a film of helium. To do so they have solved the Fokker Planck equation for the distribution of vortex pairs in the presence of an oscillating superfluid velocity field. The non-linear response is predicted to be proportional to v_n² for low values of the normal fluid velocity, v_n, which is in good agreement with the experimental results of Adams and Glaberson. For temperatures above the Kosterlitz Thouless transition temperature, T_KT, the ratio of the correlation length ξ₊ to the diffusion length becomes important in describing the temperature dependence of the non-linear response. The authors predict how the non-linear response depends on this ratio in the region where the ratio is large. 10 refs., 9 figs

[en] We discuss the effects of a weak random external potential on the properties of the dilute Bose gas at zero temperature. The results recently obtained by Huang and Meng for the depletion of the condensate and of the superfluid density are recovered. Results for the shift of the velocity of sound as well as for its damping due to collisions with the external field are presented. The damping of phonons is calculated also for dense superfluids

[en] We discuss the effect of an external AC drive on the motion of vortices near the vortex unbinding transition in two-dimensional superfluids, and on the motion of the interface between a crystal and its melt at the roughening transition. Both belong to the Kosterlitz Thouless class of phase transitions. There are three length scales in the problem: the correlation length, a diffusion length which varies as ω^-1/2 and a length inversely related to the strength of the drive. A possible resonance experiment for helium crystals is discussed. (orig.)

[en] The roton contribution to the temperature dependence of the condensate density is discussed. The analysis is carried out through the inclusion of a gauge symmetry breaking field. Under the hypothesis that the thermal excitation of rotons is the main responsible for the depletion of the condensate, the authors obtain an explicit result for the temperature dependence of the condensate density which rather well agrees with the available experimental data. 11 refs., 1 fig

[en] The ground-state correlation properties of a one-dimensional Bose system described by the Lieb-Liniger Hamiltonian are investigated by using exact quantum Monte Carlo techniques. The pair distribution function, static structure factor, one-body density matrix, and momentum distribution of a homogeneous system are calculated for different values of the gas parameter ranging from the Tonks-Girardeau to the mean-field regime. Results for the momentum distribution of a harmonically trapped gas in configurations relevant to experiments are also presented

[en] We investigate the phase diagram of asymmetric two-component Fermi gases at zero temperature as a function of polarization and interaction strength. The equations of state of the uniform superfluid and normal phase are determined using quantum Monte Carlo simulations. We find three different mixed states, where the superfluid and the normal phase coexist in equilibrium, corresponding to phase separation between (a) the polarized superfluid and the fully polarized normal gas, (b) the polarized superfluid and the partially polarized normal gas, and (c) the unpolarized superfluid and the partially polarized normal gas

[en] We investigate the crossover from three to one dimension in a Bose gas confined in highly anisotropic traps. By using quantum Monte Carlo techniques, we solve the many-body Schroedinger equation for the ground state and obtain exact results for the energy per particle and the mean square radii of the cloud in the transverse and longitudinal direction. Results are compared with the predictions of mean-field theory obtained from the Gross-Pitaevskii equation and with the one-dimensional Lieb-Liniger equation of state. We explicitly prove the occurrence of important beyond mean-field effects, including the appearance of Fermi-like properties as the system enters the Tonks gas regime