Disordered bosons: Condensate and excitations

Thermodynamic evidence for nanoscale bose-einstein condensation in (4)He confined in nanoporous media

We report the measurements of the heat capacity of (4)He confined in nanoporous Gelsil glass that has nanopores of 2.5-nm diameter at pressures up to 5.3 MPa. The heat capacity has a broad peak at a temperature much higher than the superfluid transition temperature obtained using the torsional oscillator technique. The peak provides definite thermodynamic evidence for the formation of localized Bose-Einstein condensates on nanometer length scales. The temperature dependence of the heat capacity is described well by the excitations of phonons and rotons, supporting the existence of localized Bose-Einstein condensates.

Anderson localization of Bogolyubov quasiparticles in interacting Bose-Einstein condensates

We study the Anderson localization of Bogolyubov quasiparticles in an interacting Bose-Einstein condensate (with a healing [corrected] length xi) subjected to a random potential (with a finite correlation length sigma(R)). We derive analytically the Lyapunov exponent as a function of the quasiparticle momentum k, and we study the localization maximum k(max). For 1D speckle potentials, we find that k(max) proportional variant 1/xi when xi>>sigma(R) while k(max) proportional variant 1/sigma(R) when xi<<sigma(R), and that the localization is strongest when xi approximately sigma(R). Numerical calculations support our analysis, and our estimates indicate that the localization of the Bogolyubov quasiparticles is accessible in experiments with ultracold atoms.

Routes towards Anderson-like localization of Bose-Einstein condensates in disordered optical lattices

We investigate, both experimentally and theoretically, possible routes towards Anderson-like localization of Bose-Einstein condensates in disordered potentials. The dependence of this quantum interference effect on the nonlinear interactions and the shape of the disorder potential is investigated. Experiments with an optical lattice and a superimposed disordered potential reveal the lack of Anderson localization. A theoretical analysis shows that this absence is due to the large length scale of the disorder potential as well as its screening by the nonlinear interactions. Further analysis shows that incommensurable superlattices should allow for the observation of the crossover from the nonlinear screening regime to the Anderson localized case within realistic experimental parameters.