Abstract

Superfluidity, condensation and coherence are intimately connected. In 3D systems such as bulk 4He and conventional superconductors, frictionless flow emerges as the bosonic atoms or Cooper pairs condense into a macroscopic wavefunction exhibiting true long range order. In the first part of the talk I will present an experiment in which we map out the critical velocity of a flat three-dimensional superfluid in the crossover from Bose-Einstein condensation to Bardeen-Cooper-Schrieffer superfluidity.

In the second part, I will focus on the coherence in two-dimensional superfluids, which show a strikingly different behavior than their 3D counterpart: While true long-range order in 2D systems is precluded by thermal fluctuations, Berezinskii-Kosterlitz-Thouless (BKT) theory predicts that 2D systems can still become superfluid with a first order correlation function g1(r) that decays algebraically with distance r with a temperature-dependent scaling exponent tau. I will present experimental results of a local coherence measurement of a strongly interacting 2D Bose gas, which allows us to determine the scaling exponent as a function of phase space density and extract the superfluid density.