Role of short- and long-range interactions in quantum Bose fluids

MANFRED L. RISTIG, P. M. Lam

Hans-Jürgen Sommers

Thibault Scoquart, Pierre-Élie Larré, Dominique Delande +1 more

Abstract We explore the quench dynamics of a two-dimensional, weakly interacting disordered Bose gas for various relative strengths of interactions and disorder. This allows us to identify two well distinct out-of-equilibrium regimes. When interactions are smaller than the disorder, the gas experiences multiple scattering and exhibits a short-range spatial coherence. At short time this coherence is only smoothly affected by interactions, via a diffusion process of the particles' energies. When interactions are larger than the disorder, scattering ceases and the gas behaves more and more like a fluid, ultimately like a superfluid at low energy. In the superfluid regime, the gas exhibits a long-range algebraic coherence, characteristic of a pre-thermal regime in disorder.

Pavel A. Andreev

Abstract Ultracold Bose atoms is the physical system existing at the small finite temperatures, where the quantum and nonlinear phenomena play crucial role. Bosons are considered to be composed of two different fluids: the Bose–Einstein condensate and the normal fluid (the thermal component). The extended hydrodynamic models are obtained for each fluids, where the pressure evolution equations and the pressure flux third rank tensor evolution equations are obtained along with the continuity and Euler equations. It is found that the pressure evolution equation contains zero contribution of the short-range interaction. The pressure flux evolution equation contains the interaction which simplifies to the quantum fluctuations in the zero temperature limit. The structure of the third rank tensor describing this interaction is obtained in the regime of small temperature and weak interaction. The model is derived via the straightforward calculation of evolution of macroscopic functions using the microscopic many-particle Schrodinger equation in the coordinate representation. Finally, the two-fluid hydrodynamics is constructed in form of four equations for each fluid in order to give model describing the quantum fluctuations in BEC and the thermal effects in the normal fluid.

L. Reatto

Tom Kristensen, Andrea Simoni

We study the scattering and the bound-state properties of two ultracold particles interacting via a short-range and a dipolar (long-range) interaction in a strictly 1D optical lattice. Our rigorous model take naturally into account the effects of the shape of the fundamental energy band and also of the role of the excited energy bands of the lattice. We first compare our exact results for a short-range interaction with the usual Bose-Hubbard (BH) model. By doing this, we highlight the effect of the excited bands, the non on-site interaction and the tunneling between distant neighbor that are not included into the usual BH model. A key result of our model is that the effective interaction deeply depend on the particle quasi-momenta and can then present a resonant behavior even for deep lattices. As a second phase of this study, we focus on the scattering of two polar particles in a lattice. The long-range interaction and the energy band structure of the lattice induce peculiar Wigner threshold laws. Our exact two-body calculations allow us to assess the validity of an extended Bose-Hubbard model for dipolar gases. This work was supported by the Agence Nationale de la Recherche (contract no. ANR-12-BS04-0020-01)