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arXiv:cond-mat/0502507

Two Transitions in the Damping of a Unitary Fermi Gas

Joseph M. Kinast, Andrei V. Turlapov, John E. Thomas

Published Feb 21, 2005• 5 citations
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Abstract

We measure the temperature dependence of the radial breathing mode in an optically trapped, strongly-interacting Fermi gas of $^6$Li, just above the center of a broad Feshbach resonance. The frequency remains close to the unitary hydrodynamic value, while the damping rate reveals transitions at two well-separated temperatures, consistent with the existence of atom pairs above a superfluid transition.

DOI
10.48550/arxiv.cond-mat/0502507

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•Jan 1, 2004•0 citations
We measure the temperature dependence of the radial breathing mode in an optically trapped, strongly-interacting Fermi gas of 6^66Li, just above the center of a broad Feshbach resonance. The frequency remains close to the unitary hydrodynamic value, while the damping rate reveals transitions at two well-separated temperatures, consistent with the existence of atom pairs above a superfluid transition.
We measure the temperature dependence of the radial breathing mode in an optically trapped, unitary Fermi gas of 6Li, just above the center of a broad Feshbach resonance. The damping rate reveals a clear change in behavior which we interpret as arising from a superfluid transition. We suggest pair breaking as a mechanism for an increase in the damping rate which occurs at temperatures well above the transition. In contrast to the damping, the frequency varies smoothly and remains close to the unitary hydrodynamic value. At low temperature T, the damping depends on the atom number only through the reduced temperature, and extrapolates to 0 at T = 0.
We measure the magnetic-field dependence of the frequency and damping time for the radial breathing mode of an optically trapped Fermi gas of 6Li^{6}\mathrm{Li}6Li atoms near a Feshbach resonance. The measurements address the apparent discrepancy between the results of Kinast et al. [Phys. Rev. Lett. 92, 150402 (2004)] and those of Bartenstein et al. [Phys. Rev. Lett. 92, 203201 (2004)]. Over the range of magnetic field from 770to910G770\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}910\phantom{\rule{0.3em}{0ex}}\mathrm{G}770to910G, the measurements confirm the results of Kinast et al. Close to resonance, the measured frequencies are in excellent agreement with predictions for a unitary hydrodynamic gas. At a field of 925G925\phantom{\rule{0.3em}{0ex}}\mathrm{G}925G, the measured frequency begins to decrease below predictions. For fields near 1080G1080\phantom{\rule{0.3em}{0ex}}\mathrm{G}1080G, we observe a breakdown of hydrodynamic behavior, which is manifested by a sharp increase in frequency and damping rate. The observed breakdown is in qualitative agreement with the sharp transition observed by Bartenstein et al. at 910G910\phantom{\rule{0.3em}{0ex}}\mathrm{G}910G.
We study the properties of an optically‐trapped, Fermi gas of 6Li atoms, near the center of a broad Feshbach resonance, where strong interactions are observed. Strongly interacting Fermi gases exhibit universal behavior, and are of interest as models of exotic systems ranging from high temperature superconductors to neutron stars and quark‐gluon plasmas. We measure the frequency and damping rate of the radial breathing mode of the trapped gas and consider quantum viscosity as a damping mechanism. We also demonstrate that the virial theorem holds and measure the heat capacity. In the experiments, energy is precisely added to the gas and an empirical temperature is determined from the spatial profiles of the cloud. Transitions are observed in both the damping rate and the heat capacity, measured as functions of the empirical temperature. Recent theory, using a pseudogap formalism, enables the first temperature calibration, and shows that the observed transition temperatures are close to that predicted for the onset of superfluidity in a strongly attractive Fermi gas.
We observe collective oscillations of a trapped, degenerate Fermi gas of 6Li atoms at a magnetic field just above a Feshbach resonance, where the two-body physics does not support a bound state. The gas exhibits a radial breathing mode at a frequency of 2837(05) Hz, in excellent agreement with the frequency of nu(H) identical with sqrt[10nu(x)nu(y)/3]=2830(20) Hz predicted for a hydrodynamic Fermi gas with unitarity-limited interactions. The measured damping times and frequencies are inconsistent with predictions for both the collisionless mean field regime and for collisional hydrodynamics. These observations provide the first evidence for superfluid hydrodynamics in a resonantly interacting Fermi gas.
Unitary Fermi gases, where the scattering length is large compared to the interparticle spacing, can have universal properties, which are independent of the details of the interparticle interactions when the range of the scattering potential is negligible. We prepare an optically trapped, unitary Fermi gas of 6Li, tuned just above the center of a broad Feshbach resonance. In agreement with the universal hypothesis, we observe that this strongly interacting many-body system obeys the virial theorem for an ideal gas over a wide range of temperatures. Based on this result, we suggest a simple volume thermometry method for unitary gases. We also show that the observed breathing mode frequency, which is close to the unitary hydrodynamic value over a wide range of temperature, is consistent with a universal hydrodynamic gas with nearly isentropic dynamics.
In summary, we use all-optical methods with evaporative cooling near a Feshbach resonance to produce a strongly interacting degenerate Fermi gas. We observe hydrodynamic behavior in the expansion dynamics. At low temperatures, collisions may not explain the expansion dynamics. We observe hydrodynamics in the trapped gas. Our observations include collisionally-damped excitation spectra at high temperature which were not discussed above. In addition, we observe weakly damped breathing modes at low temperature. The observed temperature dependence of the damping time and hydrodynamic frequency are not consistent with collisional dynamics nor with collisionless mean field interactions. These observations constitute the first evidence for superfluid hydrodynamics in a Fermi gas.