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SUMMARY:Superfluid behaviour of a two dimensional Bose gas - Christof Weit
 enberg (Laboratoire Kastler Brossel-CNRS)
DTSTART:20121210T153000Z
DTEND:20121210T170000Z
UID:TALK41969@talks.cam.ac.uk
CONTACT:6270
DESCRIPTION:Due to thermal fluctuations\, two-dimensional (2D) systems can
 not \n undergo a conventional phase transition associated to the breaking 
 of a \n continuous symmetry. Nevertheless they may exhibit a phase transit
 ion to \n a state with quasi-long range order via the \n Berezinskii-Koste
 rlitz-Thouless (BKT) mechanism.\n\n A paradigm example is the 2D Bose flui
 d\, such as a liquid helium film\, \n which cannot condense at non-zero te
 mperature although it becomes \n superfluid above a critical phase space d
 ensity. The quasi-long range \n coherence and the microscopic nature of th
 e BKT transition were recently \n explored with ultracold atomic gases. Ho
 wever\, a direct observation of \n superfluidity in terms of frictionless 
 flow was still missing for these \n systems.\n\n In this talk\, I will rep
 ort on recent measurements of the superfluidity \n of a 2D trapped Bose ga
 s using a moving obstacle formed by a \n micron-sized laser beam. We find 
 a dramatic variation of the response of \n the fluid\, depending on its de
 gree of degeneracy at the obstacle \n location.\n
LOCATION:Rutherford building\, Seminar Room B
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