Connecting Berezinskii-Kosterlitz-Thouless and BEC Phase Transitions by Tuning Interactions in a Trapped Gas.

We study the critical point for the emergence of coherence in a harmonically trapped two-dimensional Bose gas with tunable interactions. Over a wide range of interaction strengths we find excellent agreement with the classical-field predictions for the critical point of the Berezinskii-Kosterlitz-Thouless (BKT) superfluid transition. This allows us to quantitatively show, without any free parameters, that the interaction-driven BKT transition smoothly converges onto the purely quantum-statistical Bose-Einstein condensation transition in the limit of vanishing interactions.This work was supported by AFOSR, ARO, DARPA OLE, and EPSRC [Grant No. EP/K003615/1]. N. N. acknowledges support from Trinity College, Cambridge, R. P. S. from the Royal Society, and K. G. H. V. from DAAD.This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevLett.114.25530