Exact Calculations for Ultracold Rotating Bose Gases

Volker Schweikhard

J Sabin Del Valle, Enrico Bodo, Franco Antonio Gianturco

The behaviour of the rotationally inelastic cross sections for ultracold positrons injected in molecular gases is examined computationally by considering the ambient molecules to be rotationally 'hot' ( j = 10) and by letting the relative translational energies tend to zero. The size of the corresponding state-to-state transitions are obtained from an exact quantum calculation carried out in the space-frame representation, using non-empirical model interactions between e + and the target test molecules (H 2 , N 2 , O 2 , Li 2 ). The structural features of the molecules are related, via the dynamical coupling effects, to the size and efficiency of the cooling process along the above series of target gases. It is shown that positron projectiles can provide a possible route to the cooling of molecular rotations for simple homonuclear diatomics within the framework of ultracold dynamical conditions.

Ming-Guang Hu

An ultracold Bose-Fermi gas mixture of 40K and 87Rb atoms has tunable interspecies interactions and therefore provides a fantastic platform for exploring not only few-body physics such as Feshbach molecule formation and Efimov trimers, but also many-body physics including Bose polarons, quantum Hall physics and so on. In this thesis, I present experimental evidence of Bose polarons in cold atoms obtained using radio-frequency spectroscopy to measure the excitation spectrum of fermionic 40K impurities resonantly interacting with a BEC of 87Rb atoms. These Bose polarons originate from the dressing of an impurity coupled to its environment, which is an important paradigm in quantum many-body physics. I also present initial work that launches an exciting new direction for our experiment, which is exploring rotating quantum gases. Goals for this work include studying both rotating Bose and Fermi superfluids with tunable interactions as well as working toward rapidly rotating quantum gases in the quantum Hall regime. For these goals, a new all-optical trap for rotating gases was designed, implemented, and tested using a 87Rb Bose-Einstein condensate.

Soheil Baharian

有希 遠藤

Alexis G. Morris, David L. Feder

The energy spectrum for an ultracold rotating Bose gas in a harmonic trap is calculated exactly for small systems, allowing the atoms to occupy several Landau levels. Two vortexlike states and two strongly correlated states (the Pfaffian and Laughlin) are considered in detail. In particular, their critical rotation frequencies and energy gaps are determined as a function of particle number, interaction strength, and the number of Landau levels occupied (up to three). For the vortexlike states, the lowest-Landau-level (LLL) approximation is justified only if the interaction strength decreases with the number of particles; nevertheless, the constant of proportionality increases rapidly with the angular momentum per particle. For the strongly correlated states, however, the interaction strength can increase with particle number without violating the LLL condition. The results suggest that, in large systems, the Pfaffian and Laughlin states might be stabilized at rotation frequencies below the centrifugal limit for sufficiently large interaction strengths, with energy gaps a significant fraction of the trap energy.