Vortex Dynamics in a Highly Oblate Bose-Einstein Condensate: Quantum Turbulence and Vortex Shedding

Abstract

Quantized vortices which are one of elementary excitations in a super uid have long been investigated in Bose condensed systems. Since the first observation of a vortex in a BEC in 1998, experimental researches on vortices have continued including formation of vortices, vortex lattice dynamics and super uid turbulence, etc. Although many issues on vortices already have been investigated due to long history of studies on vortices, there still remain many interesting problems on quantized vortices in a Bose-Einstein condensate. Quantum turbulence (super uid turbulence) has been one of great issues in super uid helium systems. In particular, vortex tangle and its decay dynamics which accompany vortex reconnections are still ongoing issues. Dilute gas of Bose condensed system is a good and new platform to study them. In this thesis, highly oblate BECs have been employed to investigate two dimensional (2D) quantum turbulence, generated by a fastly moving repulsive Gaussian beam, and its decay is interpreted with vortex pair annihilation which is 2D version of vortex reconnection. Our experimental results show that relaxation of super uid turbulence is highly dependent on thermal part of the system. i And this lead us to investigate thermal friction on vortices in finite temperature BECs. Dissipative motion of a vortex in a super uid is described by the interaction between vortex core and thermal component. This is the famous concepts of mutual friction and we measure dimensionless coefficient of mutual friction a using thermal dissipative motion of corotating two vortices. Finally, we study the problem of vortex shedding generated by a ow past a repulsive Gaussian beam. Systematic studies have long been missed on critical velocity vc of a BEC superfuid and we present the detailed measurement on vc for vortex shedding by changing height V0 and radius σ of the Gaussian obstacle. vc shows minimum near V0 ≈ μ, where μ is the chemical potential of the BEC, and monotonically increases for decreasing σ for fixed V0 when V0 >> μ. The results can be understood qualitatively by considering the soft boundary effect of the Gaussian beam. Moreover, we investigate the characteristics of vortex shedding depending on V0 and observe that penetrable beam (V0 < μ) generates vortex dipoles in a periodic way while impenetrable beam (V0 > μ) is much more prone to irregular vortex shedding. Using this periodic behavior of penetrable beam, we demonstrate a deterministic generation of a single vortex dipole which will enable us to do experiments on such as vortex pair-pair collisions.