Abstract

Due to thermal fluctuations, two-dimensional (2D) systems cannot undergo a conventional phase transition associated to the breaking of a continuous symmetry. Nevertheless they may exhibit a phase transition to a state with quasi-long range order via the Berezinskii-Kosterlitz-Thouless (BKT) mechanism.

A paradigm example is the 2D Bose fluid, such as a liquid helium film, 
which cannot condense at non-zero temperature although it becomes 
superfluid above a critical phase space density. The quasi-long range 
coherence and the microscopic nature of the BKT transition were recently 
explored with ultracold atomic gases. However, a direct observation of 
superfluidity in terms of frictionless flow was still missing for these 
systems.

In this talk, I will report on recent measurements of the superfluidity 
of a 2D trapped Bose gas using a moving obstacle formed by a 
micron-sized laser beam. We find a dramatic variation of the response of 
the fluid, depending on its degree of degeneracy at the obstacle 
location.