Berlin 2014 – scientific programme

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Q: Fachverband Quantenoptik und Photonik

Q 4: Quantum gases: Fermions

Q 4.3: Talk

Monday, March 17, 2014, 11:00–11:15, DO24 Reuter Saal

The critical velocity in the BEC-BCS crossover — •Kai Morgener, Niclas Luick, Wolf Weimer, Jonas Siegl, Klaus Hueck, and Henning Moritz — Institut für Laserphysik, Universität Hamburg, D-22761 Hamburg

Ultracold fermionic gases are an ideal model system for the study of quantum many-body phenomena. Of particular interest is superfluidity due to the open questions surrounding high temperature superconductors in solids. One hallmark property of superfluid systems is the critical velocity below which obstacles can move through the fluid without friction.

The broad Feshbach resonance of the fermionic 6Li quantum gases provides the unique possibility to investigate superfluidity over a wide range of interactions. We stir the gas with a red detuned laser beam as a local density perturbation. Above the critical velocity heating can be observed. We present high precision measurements of the critical velocity along the BEC-BCS crossover. Our measurements are in excellent agreement with theoretical predictions which assume that the nature of excitations ranges from Bogoliubov sound waves in the BEC on the repulsive side of the resonance to Cooper-pair breaking for attractive interactions in the BCS regime. The role of vortex excitations is negligible small.