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arXiv:1902.07558 · Physical Review Research

Dynamic formation of quasicondensate and spontaneous vortices in a strongly interacting Fermi gas

Xiangpei Liu, Xing-Can Yao, Youjin Deng, Yu‐Xuan Wang, Xiaoqiong Wang, Xiaopeng Li, Qijin Chen, Yu-Ao Chen, Jian-Wei Pan

Published Nov 15, 2021• 13 citations
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Abstract

We report an experimental study of quench dynamics across the superfluid transition temperature ${T}_{\mathrm{c}}$ in a strongly interacting Fermi gas by ramping down the trapping potential. The nonzero quasicondensate number ${N}_{0}$ at temperatures significantly above ${T}_{\mathrm{c}}$ in the unitary and the BEC regimes is consistent with the pseudogap physics via preformed pairs. Below ${T}_{\mathrm{c}}$, a rapid growth of ${N}_{0}$ is accompanied by the spontaneous generation of tens of vortices. We observe a power-law scaling of the vortex density versus the quasicondensate formation time, consistent with the Kibble-Zurek theory. Our work provides an example of studying emerged many-body physics by quench dynamics and paves the way for studying the quantum turbulence in a strongly interacting Fermi gas.

DOI
10.1103/physrevresearch.3.043115

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The generation of spontaneous vortices and the formation of superfluids are closely intertwined. Here, we report an experimental study of quench dynamics across the superfluid phase transition in a strongly interacting Fermi gas at unitarity, by ramping down the external trapping potential. The superfluid phase coherence is recorded, and several tens of vortices are found to be generated during the quench process. A universal growth dynamics is revealed that for different quenches, the experimental data of quasi-condensate collapse onto a single curve as a function of evolving time rescaled by characteristic coherence-formation time. Moreover, we find that for a wide range of quenches, the dependence of the vortex density on the quasi-condensate formation time, rather than the ramping time, is well described by the celebrated the Kibbble-Zurek mechanism. Our work paves the way to study the complex vortex dynamics in vortex turbulence systems.
With an eye toward the interpretation of so-called ``cosmological'' experiments performed on the low-temperature phases of 3He^{3}\mathrm{He}3He, in which regions of the superfluid are destroyed by local heating with neutron radiation, we have studied the behavior of a Fermi gas subjected to uniform variations of an attractive BCS interaction parameter $\ensuremath{\lambda}$. In 3He^{3}\mathrm{He}3He, the quenches induced by the rapid cooling of the ``hot spots'' back through the transition may lead to the formation of vortex loops via the Kibble-Zurek mechanism. A consideration of the free energy available in the quenched region for the production of such vortices reveals that the Kibble-Zurek scaling law gives at best a lower bound on the defect spacing. Further, for quenches that fall far outside the Ginzburg-Landau regime, the dynamics on the pair subspace, as initiated by quantum fluctuations, tends irreversibly to a self-driven steady state with a gap ${\ensuremath{\Delta}}_{\ensuremath{\infty}}={ϵ}_{C}{({e}^{2∕N(0)\ensuremath{\lambda}}\ensuremath{-}1)}^{\ensuremath{-}1∕2}$. In weak coupling, this is only half the BCS gap, the extra energy being taken up by the residual collective motion of the pairs.
Half a century ago, T. Kibble proposed a scenario for topological defect formation from symmetry breaking during the expansion of the early Universe. W. Zurek later crystallized the concept to superfluid helium, predicting a power-law relation between the number of quantum vortices and the rate at which the system passes through the lambda transition. Here, we report the observation of Kibble-Zurek scaling in a homogeneous, strongly interacting Fermi gas undergoing a superfluid phase transition. We investigate the superfluid transition using two distinct control parameters: temperature and interaction strength. The microscopic physics of condensate formation is markedly different for the two quench parameters, signaled by their two orders of magnitude difference in the condensate formation timescale. However, regardless of the thermodynamic direction in which the system passes through a phase transition, the Kibble-Zurek exponent is identically observed to be about 0.68 and shows good agreement with theoretical predictions that describe superfluid phase transitions. This work demonstrates the gedanken experiment Zurek proposed for liquid helium that shares the same universality class with strongly interacting Fermi gases.
We study the spontaneous formation of vortices during the superfluid condensation in a trapped fermionic gas subjected to a rapid thermal quench via evaporative cooling. Our work is based on the numerical solution of the time-dependent crossover Ginzburg-Landau equation coupled to the heat diffusion equation. We quantify the evolution of condensate density and vortex length as a function of a crossover phase parameter from BCS to BEC. The more interesting phenomena occur somewhat nearer to the BEC regime and should be experimentally observable; during the propagation of the cold front, the increase in condensate density leads to the formation of supercurrents toward the center of the condensate as well as possible condensate volume oscillations.
The unitary Fermi gas (UFG) offers a unique opportunity to study quantum turbulence both experimentally and theoretically in a strongly interacting fermionic superfluid with the highest vortex line density of any known superfluid. It yields to accurate and controlled experiments and admits the only dynamical microscopic description via time-dependent density-functional theory, apart from dilute bosonic gases, of the crossing and reconnection of superfluid vortex lines conjectured by Feynman [R. P. Feynman, Prog. Low Temp. Phys. 1, 17 (1955)] to be at the origin of quantum turbulence in superfluids at zero temperature. We demonstrate how various vortex configurations can be generated by using well-established experimental techniques: laser stirring and phase imprinting. New imaging techniques demonstrated by Ku et al. [M. J. H. Ku et al., Phys. Rev. Lett. 113, 065301 (2014)] should be able to directly visualize these crossings and reconnections in greater detail than performed so far in liquid helium. We demonstrate the critical role played by the geometry of the trap in the formation and dynamics of a vortex in the UFG and how laser stirring and phase imprint can be used to create vortex tangles with clear signatures of the onset of quantum turbulence.
We report on experiments and simulations of the growth of a Bose-Einstein condensate in which vortices are observed to form spontaneously in a non-rotating potential. Between 10% and 50% of experiments result in between one and three unambiguous vortex cores. We simulate a temperature quench of a Bose gas through the critical point using the stochastic Gross-Pitaevskii equation and find remarkable agreement with the experimental data. Faster cooling results in a higher probability of vortex formation, and we interpret these results in light of the Kibble-Zurek scenario for the formation of topological defects in continuous phase transitions.