Browsing by Author "Werner, Jörg"

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Observation of Feshbach resonances in an ultracold gas of 52Cr
(2006) Werner, Jörg; Pfau, Tilman (Prof. Dr.)
The first realization of Bose-Einstein condensation (BEC) in dilute atomic gases in the year 1995 has pioneered the exploration of an exciting new form of matter: the macroscopic quantum state. In many of the more recent experiments the interatomic interaction is responsible for the astonishing variety of observed phenomena. The exact knowledge of the interaction potentials between two atoms is one of the prerequisites for these kind of experiments. A very precise method for determining the interaction potentials exploits the existence of magnetically induced Feshbach resonances. For certain values of the applied external magnetic field the interaction can be arbitrarily tuned. From the positions of these Feshbach resonances one can deduce many details of the acting interaction potentials. The topic of this thesis is the first observation of fourteen magnetically induced Feshbach resonances in collisions between optically trapped ultracold chromium atoms. The search for Feshbach resonances was performed for magnetic fields between 4 and 600 G. A calibration of the magnetic field was done for each observed resonance slightly above and below the resonance using rf-spectroscopy. This allowed us to determine the positions of the resonances with an accuracy of below 100 mG. Up to now, not much was known about the interaction potentials between two chromium atoms. Due to a close collaboration with Andrea Simoni and Eite Tiesinga of the National Institute of Standards and Technology, Gaithersburg, USA we succeeded in identifying thirteen of the fourteen experimentally observed resonances and to assign the relevant quantum numbers. The eleven strongest Feshbach resonances build a complete set of all possible resonances up to second-order in the magnetic dipole-dipole coupling for the deca-triplett s-wave entrance channel. The two remaining identified resonances are resonances with a d-wave entrance channel and are thus much weaker. From the experimental resonance positions and knowing the corresponding quantum numbers allowed us to calculate a set of parameters describing the Born-Oppenheimer potentials with unprecedent precision. The average difference between theoretical and experimental resonance positions is only 0.6 G. Neglecting the spin-spin dipole interaction in our calculations leads to an average deviation of about 10 G. This is a clear sign, that the dipole-dipole interaction plays an important role in collisions between ultracold chromium atoms. The precise knowledge of the molecular potentials opens up new vistas. Based on the measured molecular potentials we can predict the collisional properties in other entrance channels and even for other chromium isotopes. Maybe these new results will lead to a better understanding of the bonding mechanisms of the chromium dimer, which is not yet fully understood in detail. Concerning further experiments with ultracold chromium atoms, the possibility to vary the interaction strength by using a Feshbach resonance is of interest. This will allow to change the strength of the isotropic contact interaction in relation to the anisotropic dipole-dipole interaction. The observed Feshbach resonances can even be used to create ultracold molecules.