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Spontaneous vortices in the formation of fermionic superfluid

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

Published Feb 20, 2019• 0 citations
OpenAlex

Abstract

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.

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•Mar 12, 2021•0 citations
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.
We report an experimental study of quench dynamics across the superfluid transition temperature Tc{T}_{\mathrm{c}}Tc​ in a strongly interacting Fermi gas by ramping down the trapping potential. The nonzero quasicondensate number N0{N}_{0}N0​ at temperatures significantly above Tc{T}_{\mathrm{c}}Tc​ in the unitary and the BEC regimes is consistent with the pseudogap physics via preformed pairs. Below Tc{T}_{\mathrm{c}}Tc​, a rapid growth of N0{N}_{0}N0​ 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.
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.
We present an experimental study of a two component Fermi gas following an interaction quench into the superfluid phase. Starting with a weakly attractive gas in the normal phase, interactions are ramped to unitarity at a range of rates and we measure the subsequent dynamics as the gas approaches equilibrium. Both the formation and condensation of fermion pairs are mapped via measurements of the pair momentum distribution and can take place on very different timescales, depending on the adiabaticity of the quench. The contact parameter is seen to respond very quickly to changes in the interaction strength, indicating that short-range correlations, based on the occupation of high-momentum modes, evolve far more rapidly than the correlations in low-momentum modes necessary for pair condensation.
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.
Phase transitions abound in the physical world, from the subatomic length scales of quark condensation to the decoupling forces in the early universe. In the Bose-Einstein condensation phase transition, a gas of trapped bosonic atoms is cooled to a critical temperature. Below this temperature, a macroscopic number of atoms suddenly starts to occupy a single quantum state; these atoms comprise the Bose-Einstein condensate (BEC). The dynamics of the BEC phase transition are the focus of this dissertation and the experiments described here have provided new information on the details of BEC formation. New theoretical developments are proving to be valuable tools for describing BEC phase transition dynamics and interpreting new experimental results. With their amenability to optical manipulation and probing along with the advent of new microscopic theories, BECs provide an important new avenue for gaining insight into the universal dynamics of phase transitions in general.Spontaneous symmetry breaking in the system's order parameter may be one result of cooling through a phase transition. A potential consequence of this is the spontaneous formation of topological defects, which in a BEC appear as vortices. We experimentally observed and characterized the spontaneous formation of vortices during BEC growth. We attribute vortex creation to coherence length limitations during the initial stages of the phase transition. Parallel to these experimental observations, theory collaborators have used the Stochastic Gross-Pitaevski Equation formalism to simulate the growth of a condensate from a thermal cloud. The experimental and theoretical statistical results of the spontaneous formation of vortex cores during the growth of the condensate are in good quantitative agreement with one another, supporting our understanding of the dynamics of the phase transition. We believe that our results are also qualitatively consistent with the Kibble-Zurek mechanism, a universal model for topological defect formation.Ultimately, our understanding of the dynamics of the BEC phase transition may lead to a broader understanding of phase transitions in general, and provide new insight into the development of coherence in numerous systems.
Non-equilibrium phenomena in many-body quantum systems involves the interplay of correlations and dynamics spanning different spatio-temporal scales. We present an experimental study of these dynamics in a two-component Fermi gas following a quench from weak to strong interactions. Starting with a weakly attractive gas in the normal phase, interactions are ramped to the unitarity limit at a range of rates, which involves crossing the normal to superfluid phase transition. We measure the subsequent dynamics of the pair momentum distribution which reveals the timescales for both the formation and condensation of pairs. Furthermore, we show that the contact parameter responds very rapidly to changes in the interaction strength, demonstrating that short-range correlations, based on the occupation of high-momentum modes, evolve far more rapidly than correlations in low-momentum modes necessary for pair condensation.