Observation of the Efimovian expansion in scale-invariant Fermi gases

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Science  22 Jul 2016:
Vol. 353, Issue 6297, pp. 371-374
DOI: 10.1126/science.aaf0666

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Steps to ultracold gas expansion

Cold atomic gases are often studied while confined in parabolic traps, with the largest atomic density at the center of the trap. When the trap is made shallower, the gas radially expands as the energy cost for atoms that are farther from the trap center decreases. Deng et al. observed an interesting effect when they reduced the characteristic frequency of the parabolic trap so that it was at any moment inversely proportional to the elapsed time. Instead of expanding continuously, a strongly interacting Fermi gas held in such a trap stalled at certain time points. These time points formed a geometric progression, a consequence of scale invariance in the strongly interacting limit.

Science, this issue p. 371


Scale invariance plays an important role in unitary Fermi gases. Discrete scaling symmetry manifests itself in quantum few-body systems such as the Efimov effect. Here, we report on the theoretical prediction and experimental observation of a distinct type of expansion dynamics for scale-invariant quantum gases. When the frequency of the harmonic trap holding the gas decreases continuously as the inverse of time t, the expansion of the cloud size exhibits a sequence of plateaus. The locations of these plateaus obey a discrete geometric scaling law with a controllable scale factor, and the expansion dynamics is governed by a log-periodic function. This marked expansion shares the same scaling law and mathematical description as the Efimov effect.

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