U utorak 09. septembra 2008. godine u 12:30 sati,
u sali 10 Instituta za fiziku, Pregrevica br. 118, Zemun
Priv-Doz. Dr. Axel Pelster
University of Duisburg-Essen, Germany
Bose-Einstein Condensation in a Random Potential
The talk reviews recent progress on understanding both thermodynamic and dynamic properties of ultracold bosonic atoms in potentials with quenched disorder. Although, originally, this `dirty boson problem` arose in the context of superfluid Helium in Vycor, modern experiments have led to a strong revival of investigating the delicate interplay between interaction and disorder in dilute Bose gases. Today, disorder appears either naturally as, e.g., in magnetic wire traps or it may be created artificially and controllably as, e.g., by the use of laser speckle fields.
At first, we determine perturbatively the shift of the critical temperature Tc which characterizes the onset of Bose-Einstein condensation for an ultracold dilute atomic gas in a harmonic trap due to weak disorder. To this end we treat both a Gaussian and a Lorentzian spatial correlation for the quenched disorder potential.
Then we study how the collective mode frequencies of a condensate in a harmonic trap are shifted by the presence of additional weak quenched disorder. To this end we apply the Bogoliubov theory for dirty bosons, which was first developed by Huang and Meng in 1992, to an inhomogeneous condensate in the Thomas-Fermi approximation. This approach describes how local condensates in the minima interfere with the superfluid property of the condensate. In case of a Gaussian disorder correlation we find that the negative shifts of the collective frequencies for the monopole and the dipole mode decrease rapidly with increasing correlation length. Thus, our theory makes
it possible to experimentally test the predictions of the Huang-Meng theory.
Finally, we develop a Hartree-Fock mean-field theory for a homogeneous disordered Bose gas by invoking the replica method. Within this non-perturbative approximation scheme we find the phase boundaries for the gas, the superfluid phase as well as the Bose-glass phase.