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arXiv:cond-mat/0306627 · Physical Review Letters

Spectroscopic Insensitivity to Cold Collisions in a Two-State Mixture of Fermions

Martin W. Zwierlein, Zoran Hadzibabic, Subhadeep Gupta, Wolfgang Ketterle

Published Dec 19, 2003• 145 citations
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Abstract

We have experimentally demonstrated the absence of spectroscopic resonance shifts in a mixture of two interacting Fermi gases. This result is linked to observations in an ultracold gas of thermal bosons. There, the measured resonance shift due to interstate collisions is independent of the coherence in the system, and twice that expected from the equilibrium energy splitting between the two internal states in a fully decohered cloud. We give a simple theoretical explanation of these observations, which elucidates the effect of coherent radiation on an incoherent mixture of atoms.

DOI
10.1103/physrevlett.91.250404

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We have performed precision microwave spectroscopy on ultracold 87Rb{}^{87}\mathrm{Rb}87Rb confined in a magnetic trap, both above and below the Bose-condensation transition. The cold collision frequency shifts for both normal and condensed clouds were measured, which allowed the intrastate and interstate density correlations (characterized by sometimes controversial ``factors of 2'') to be determined. Additionally, temporal coherence of the normal cloud was studied, and the importance of mean-field and velocity-changing collisions in preserving coherence is discussed.
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We discuss the prospects for observing the characteristic features of the Efimov effect in a two-component ultracold atomic gas near an interspecies Feshbach resonance. In the ultracold regime, the Efimov effect is expected to be manifested in the three-body collision rates through the appearance of series of minima or maxima as a function of the two-body sss-wave scattering length aaa. Here, we propose the observation of this Efimov physics through measurements of the inelastic three-body rate constants near a Feshbach resonance. Our analysis suggests that boson-fermion mixtures, where the bosons are much heavier than the fermions, are the most favorable system to observe such features.
Recent progress in the field of ultracold gases has allowed for the creation of phase-segregated Bose-Fermi systems. We present a theoretical study of their collective excitations at zero temperature. As the fraction of fermion-to-boson particle number increases, the collective mode frequencies take values between those for a fully bosonic and those for a fully fermionic cloud, with damping in the intermediate region. This damping is caused by fermions which are resonantly driven at the interface.
Radio-frequency techniques were used to study ultracold fermions. We observed the absence of mean-field "clock" shifts, the dominant source of systematic error in current atomic clocks based on bosonic atoms. This absence is a direct consequence of fermionic antisymmetry. Resonance shifts proportional to interaction strengths were observed in a three-level system. However, in the strongly interacting regime, these shifts became very small, reflecting the quantum unitarity limit and many-body effects. This insight into an interacting Fermi gas is relevant for the quest to observe superfluidity in this system.
This thesis reports on experiments conducted in a single layer, quasi two-dimensional, two-component ultracold Fermi gas in the strongly interacting regime. Ultracold gases can be used to simulate key aspects of more complicated systems like for example cuprates which show high-Tc superconductivity. \nThe momentum distribution of a sample of bosonic dimers in a quasi-2D square lattice geometry was measured to obtain the coherence properties. For shallow lattices, sharp peaks in the momentum distribution, indicating coherence, were observed at zero momentum as well as at positive and negative lattice momenta along each axis. For deeper \nlattices, heating impeded the ability to prepare a Mott-insulator. \nA spatially resolved radio-frequency spectroscopy was employed for a quasi-2D Fermi gas in the normal phase throughout the BEC-BCS crossover. The interaction induced \nenergy shifts were measured in the strongly interacting region where they can be on the order of the Fermi energy and thus the local resolution is crucial. Furthermore, the onset of pairing in the strongly interacting region was measured as a function of temperature and it was shown that the fraction of free atoms decreases faster than expected from thermal non-interacting theory. At last, the pairing gap was measured using an imbalanced sample. On the BEC side it was found to be in very good agreement with two-body physics as expected. In the strongly interacting regime, however, a deviation from two-body physics indicates that here many-body effects play a role and thus further studies are required.