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Vortex Dynamics in a Highly Oblate Bose-Einstein Condensate: Quantum Turbulence and Vortex Shedding : 납작한 보즈-아인슈타인 응집체에서의 양자 소용돌이 동역학: 양자 난류와 소용돌이 방출
DC Field | Value | Language |
---|---|---|
dc.contributor.advisor | 신용일 | - |
dc.contributor.author | 권우진 | - |
dc.date.accessioned | 2017-07-19T06:10:04Z | - |
dc.date.available | 2017-07-19T06:10:04Z | - |
dc.date.issued | 2016-02 | - |
dc.identifier.other | 000000132776 | - |
dc.identifier.uri | http://dcollection.snu.ac.kr:80/jsp/common/DcLoOrgPer.jsp?sItemId=000000132776 | - |
dc.description | 학위논문(박사)--서울대학교 대학원 :자연과학대학 물리·천문학부,2016. 2. 신용일. | - |
dc.description.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. | - |
dc.description.tableofcontents | Chapter 1 Introduction 1
1.1 Ultracold Bose gas and vortex physics 1 1.2 Brief reviews on Bose-Einstein condensates 2 1.3 Vortices in a BEC 4 1.3.1 Quantized vortex in a BEC 4 1.3.2 Angular momentum of a vortex 6 1.3.3 Interaction of two vortices 7 1.4 Outline of the thesis 9 Chapter 2 Quantum turbulence in a BEC 12 2.1 Introduction 12 2.1.1 Decay of a vortex in a superfluid 14 2.2 Experimental methods 15 2.2.1 Generating turbulence in a BEC 15 2.2.2 Vortex number counting 17 2.3 Relaxation of Turbulence 19 2.3.1 Nonexponential decay 19 2.3.2 Vortex pair annihilation 23 2.3.3 Decay rate measurements 26 2.4 Summary and outlook 32 Chapter 3 Mutual friction on quantized vortices in a BEC 38 3.1 Introduction 38 3.1.1 Generation of a doubly charged vortex 40 3.2 Experiments 42 3.3 Theoretical Model 47 3.3.1 Motion of vortices 47 3.3.2 Dynamics of corotating vortices in a harmonic trap 48 3.4 Determination of Mutual friction 52 3.5 Summary 54 Chapter 4 Critical velocity for vortex shedding in a BEC 61 4.1 Introduction 62 4.2 Experiment 65 4.2.1 Calibration of Gaussian beam 69 4.3 Results and Discussion 71 4.4 Summary 77 Chapter 5 Periodic vortex shedding from a moving Gaussian obstacle 82 5.1 Introduction 83 5.2 Experiment 85 5.3 Periodic vortex shedding 86 5.3.1 Penetrable obstacle 86 5.3.2 Impenetrable obstacle 90 5.4 Deterministic generation of a single vortex dipole 93 5.5 Demonstration of a vortex dipole-dipole collision 98 5.6 Possibility of a self-annihilation of a single vortex dipole? 99 5.7 Vortex dipoles vs clustered vortex street 102 5.8 Summary and Outlook 104 Chapter 6 Conclusions and Outlooks 111 Appendices 113 Appendix A Li 교체 진공작업 114 Appendix B Offset lock for 6Li imaging laser 116 Appendix C Measuring trapping frequency with parametric resonance 118 초록 120 | - |
dc.format.extent | 121 | - |
dc.language.iso | eng | - |
dc.publisher | 서울대학교 대학원 | - |
dc.subject | bose-einstein condensate, superfluid, quantum turbulence, vortex pair annihilation, critical velocity, vortex shedding, mutual friction | - |
dc.subject.ddc | 523 | - |
dc.title | Vortex Dynamics in a Highly Oblate Bose-Einstein Condensate: Quantum Turbulence and Vortex Shedding | - |
dc.title.alternative | 납작한 보즈-아인슈타인 응집체에서의 양자 소용돌이 동역학: 양자 난류와 소용돌이 방출 | - |
dc.type | Thesis | - |
dc.type | Dissertation | - |
dc.contributor.department | 자연과학대학 물리·천문학부 | - |
dc.description.degree | Doctor | - |
dc.date.awarded | 2016-02 | - |
dc.contributor.major | 원자물리 | - |
dc.identifier.holdings | 000000000027▲000000000027▲000000132776▲ | - |
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