arXiv subjects

Bumsuk Ko

Publications and source records attributed to Bumsuk Ko.

3 recordsLinked to original sources

arXiv2019

Kibble-Zurek universality in a strongly interacting Fermi superfluid

Near a continuous phase transition, systems with different microscopic origins display universal dynamics if their underlying symmetries are compatible. In a thermally quenched system, the Kibble-Zurek mechanism for the creation of topological defects unveils this universality through a characteristic power-law exponent, which captures the dependence of the defect density on the quench rate. Here, we report the observation of the Kibble-Zurek universality in a strongly interacting Fermi superfluid. As the system's microscopic description is tuned from bosonic to fermionic, the quench formation of vortices reveals a constant scaling exponent arising from the $U(1)$ gauge symmetry of the system. For rapid quenches, destructive vortex collisions lead to the saturation of their densities, whose values can be universally scaled by the interaction-dependent area of the vortex cores.

cond-mat.quant-gas↗

arXiv2018

Critical Vortex Shedding in a Strongly Interacting Fermionic Superfluid

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 time of flight, the critical velocity $v_{\rm c}$ for vortex shedding is measured as a function of the obstacle travel distance $L$. The observed $v_{\rm c}$ increases with decreasing $L$, where the rate of increase is the highest in the unitary regime. In the deep BEC regime, an empirical dissipation model well captures the dependence of $v_{\rm c}$ on $L$, characterized by a constant value of $η= -\frac{{\rm d}(1/v_{\rm c})}{{\rm d}(1/L)}$. However, as the system is tuned across the resonance, a step increase of $η$ develops about a characteristic distance $L_{\rm c}$ as $L$ is increased, where $L_{\rm c}$ 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 $v_{\rm c}$ to the participation of pair breaking in the vortex shedding dynamics of a fermionic superfluid.

cond-mat.quant-gas↗

arXiv2017

Observation of vortex-antivortex pairing in decaying 2D turbulence of a superfluid gas

In a two-dimensional (2D) classical fluid, a large-scale flow structure emerges out of turbulence, which is known as the inverse energy cascade where energy flows from small to large length scales. An interesting question is whether this phenomenon can occur in a superfluid, which is inviscid and irrotational by nature. Atomic Bose-Einstein condensates (BECs) of highly oblate geometry provide an experimental venue for studying 2D superfluid turbulence, but their full investigation has been hindered due to a lack of the circulation sign information of individual quantum vortices in a turbulent sample. Here, we demonstrate a vortex sign detection method by using Bragg scattering, and we investigate decaying turbulence in a highly oblate BEC at low temperatures, with our lowest being $\sim 0.5 T_c$, where $T_c$ is the superfluid critical temperature. We observe that weak spatial pairing between vortices and antivortices develops in the turbulent BEC, which corresponds to the vortex-dipole gas regime predicted for high dissipation. Our results provide a direct quantitative marker for the survey of various 2D turbulence regimes in the BEC system.

cond-mat.quant-gas↗