Dynamics of highly unbalanced Bose-Bose mixtures: miscible vs immiscible gases

We study the collective modes of the minority component of a highly unbalanced Bose-Bose mixtures. In the miscible case the minority component feels an effective external potential and we derive an analytical expression for the mode frequencies. The latter is independent of the minority component interaction strength. In the immiscible case we find that the ground state can be a two-domain walls soliton. Although the mode frequencies are continuous at the transition, their behaviour is very different with respect to the miscible case. The dynamical behaviour of the solitonic structure and the frequency dependence on the inter- and intra-species interaction is numerically studied using coupled Gross-Pitaevskii equations.Comment: 6 pages, 10 figure

Casimir forces and quantum friction from Ginzburg radiation in atomic BECs

We theoretically propose an experimentally viable scheme to use an impurity atom in an atomic Bose-Einstein condensate, in order to realize condensed-matter analogs of quantum vacuum effects. In a suitable atomic level configuration, the collisional interaction between the impurity atom and the density fluctuations in the condensate can be tailored to closely reproduce the electric-dipole coupling of quantum electrodynamics. By virtue of this analogy, we recover and extend the paradigm of electromagnetic vacuum forces to the domain of cold atoms, showing in particular the emergence, at supersonic atomic speeds, of a novel power-law scaling of the Casimir force felt by the atomic impurity, as well as the occurrence of a quantum frictional force, accompanied by the Ginzburg emis- sion of Bogoliubov quanta. Observable consequences of these quantum vacuum effects in realistic spectroscopic experiments are discussed.Comment: 5 pages, 2 figures. Revised version accepted in PR

Spin oscillations of the normal polarized Fermi gas at Unitarity

Using density functional theory in a time dependent approach we determine the frequencies of the compressional modes of the normal phase of a Fermi gas at unitarity as a function of its polarization. Our energy functional accounts for the typical elastic deformations exhibited by Landau theory of Fermi liquids. The comparison with the available experiments is biased by important collisional effects affecting both the {\it in phase} and the {\it out of phase} oscillations even at the lowest temperatures. New experiments in the collisionless regime would provide a crucial test of the applicability of Landau theory to the dynamics of these strongly interacting normal Fermi gases.Comment: 5 pages, 1 figur

Quadrupole oscillation in a dipolar Fermi gas: hydrodynamic vs collisionless regime

The surface quadrupole mode of an harmonically trapped dipolar Fermi gas is studied in both the hydrodynamic and collisionless regimes. The anisotropy and long range effects of the dipolar force as well as the role of the trapping geometry are explicitly investigated. In the hydrodynamic regime the frequency is always slightly smaller than the $\sqrt{2}\omega_\perp$ value holding for gases interacting with contact interactions. In the collisionless regime the frequency can be either pretty smaller or larger than the non-interacting value $2\omega_\perp$, depending on the cloud aspect ratio. Our results suggest that the frequency of the surface quadrupole oscillation can provide a useful test for studying, at very low temperatures, the transition between the normal and the superfluid phase and, in the normal phase at higher temperatures, the crossover between the collisional and collisionless regimes. The consequences of the anisotropy of the dipolar force on the virial theorem are also discussed.Comment: 8 pages, 4 figure

Andreev-Bashkin effect in superfluid cold gases mixture

We study a mixture of two superfluids with density-density and current-current (Andreev-Bashkin) interspecies interactions. The Andreev-Bashkin coupling gives rise to a dissipationless drag (or entrainment) between the two superfluids. Within the quantum hydrodynamics approximation, we study the relations between speeds of sound, susceptibilities and static structure factors, in a generic model in which the density and spin dynamics decouple. Due to translational invariance, the density channel does not feel the drag. The spin channel, instead, does not satisfy the usual Bijl-Feynman relation, since the f-sum rule is not exhausted by the spin phonons. The very same effect on one dimensional Bose mixtures and their Luttinger liquid description is analysed within perturbation theory. Using diffusion quantum Monte Carlo simulations of a system of dipolar gases in a double layer configuration, we confirm the general results. Given the recent advances in measuring the counterflow instability, we also study the effect of the entrainment on the dynamical stability of a superfluid mixture with non-zero relative velocity.Comment: 12 pages, 4 figure