Department of Physics and Astronomy, and Institute of Applied Physics, Seoul National University, Seoul 08826, Korea.
Center for Correlated Electron Systems, Institute for Basic Science, Seoul 08826, Korea.
Phys Rev Lett. 2018 Nov 30;121(22):225301. doi: 10.1103/PhysRevLett.121.225301.
We study the critical vortex shedding in a strongly interacting fermionic superfluid of ^{6}Li across the BEC-BCS crossover. By moving an optical obstacle in the sample and directly imaging the vortices after the time of flight, the critical velocity u_{vor} for vortex shedding is measured as a function of the obstacle travel distance L. The observed u_{vor} increases with decreasing L, where the rate of increase is the highest in the unitary regime. In the deep Bose-Einstein condensation regime, an empirical dissipation model well captures the dependence of u_{vor} on L, characterized by a constant value of η=-[d(1/u_{vor})/d(1/L)]. However, as the system is tuned across the resonance, a step increase of η develops about a characteristic distance L_{c} as L is increased, where L_{c} is comparable to the obstacle size. This bimodal behavior is strengthened as the system is tuned towards the BCS regime. We attribute this evolution of u_{vor} to the emergence of the underlying fermionic degree of freedom in the vortex-shedding dynamics of a Fermi condensate.
我们研究了在玻色-爱因斯坦凝聚态（BEC）-玻色-费米凝聚态（BCS）交叉点处，^6Li 的强相互作用费米超流中的临界涡旋脱落。通过在样品中移动一个光学障碍物，并在飞行时间后直接对涡旋进行成像，我们测量了作为障碍物移动距离 L 的函数的涡旋脱落临界速度 u_{vor}。观察到的 u_{vor}随着 L 的减小而增加，在幺正态下增加的速率最高。在深玻色-爱因斯坦凝聚态下，一个经验耗散模型很好地捕捉了 u_{vor}对 L 的依赖关系，其特征是 η 的一个常数值为-[d(1/u_{vor})/d(1/L)]。然而，随着系统在共振处被调谐，当 L 增加时，η 会出现一个特征距离 L_{c}的阶跃增加，其中 L_{c}与障碍物的大小相当。随着系统向 BCS 态调谐，这种双峰行为得到了加强。我们将 u_{vor}的这种演化归因于在费米凝聚体的涡旋脱落动力学中出现的潜在费米子自由度。
