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arXiv:1402.6412 · Physical Review A

Quantum sound-cone fluctuations in cold Fermi gases: Phonon propagation

Jen-Tsung Hsiang, Da-Shin Lee, Chi-Yong Lin, R. J. Rivers

Published May 27, 2015• 3 citations
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Abstract

We examine the effect of quantum fluctuations in a tunable cold Fermi gas on phonon propagation. We show that these fluctuations can be interpreted as inducing a stochastic space-time. This effect can be displayed in the variation in the travel time of phonons, at its greatest in the crossover region between Bose-Einstein condensate and BCS regimes.

DOI
10.1103/physreva.91.051603

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We examine the effect of quantum fluctuations in a tunable cold Fermi gas on phonon propagation. We show that these fluctuations can be interpreted as inducing a stochastic space-time. This effect can be displayed in the variation in the travel time of phonons, at its greatest in the crossover region between Bose-Einstein condensate and BCS regimes.
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Abstract Second sound is an entropy wave which propagates in the superfluid component of a quantum liquid. Because it is an entropy wave, it probes the thermodynamic properties of the quantum liquid. Here, we study second sound propagation for a large range of interaction strengths within the crossover between a Bose-Einstein condensate (BEC) and the Bardeen-Cooper-Schrieffer (BCS) superfluid, extending previous work at unitarity. In particular, we investigate the strongly-interacting regime where currently theoretical predictions only exist in terms of an interpolation in the crossover. Working with a quantum gas of ultracold fermionic 6Li atoms with tunable interactions, we show that the second sound speed varies only slightly in the crossover regime. By varying the excitation procedure, we gain deeper insight on sound propagation. We compare our measurement results with classical-field simulations, which help with the interpretation of our experiments.
In this Rapid Communication we investigate the fluctuation effects on the transport properties of unitary Fermi gases in the vicinity of the superfluid transition temperature Tc{T}_{\mathrm{c}}Tc​. Based on the time-dependent Ginzburg-Landau formalism of the Bose-Einstein condensate--BCS crossover, we investigate both the residual resistivity below Tc{T}_{\mathrm{c}}Tc​ induced by phase slips and the paraconductivity above Tc{T}_{\mathrm{c}}Tc​ due to pair fluctuations. These two effects have been well studied in the weak-coupling BCS superconductor and here we generalize them to the unitary regime of ultracold Fermi gases. We find that while the residual resistivity below Tc{T}_{\mathrm{c}}Tc​ increases as one approaches the unitary limit, consistent with recent experiments, the paraconductivity exhibits nonmonotonic behavior. Our results can be verified with the recently developed transport apparatus using mesoscopic channels.
We theoretically investigate the specific heat at constant volume C V in the BCS(Bardeen-Cooper-Schrieffer)-BEC(Bose-Einstein-condensation)-crossover regime of an ultracold Fermi gas, below the superfluid phase transition temperature T c . Within the strong-coupling framework developed by Nozières and Schmitt-Rink, we show that the temperature dependence of C V drastically changes as one passes through the crossover region, and is sensitive to strong fluctuations in the Cooper channel near the unitarity limit. We also compare our results to a recent experiment on a 6 Li unitary Fermi gas. Since fluctuation effects are a crucial key in the BCS-BEC-crossover phenomenon, our results would be helpful in considering how the fermionic BCS superfluid changes into BEC with increasing the interaction strength, from the viewpoint of specific heat.