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postgraduate thesis: Dynamical effects in ultracold atomic gases : macroscopic quantum tunneling and bulk viscosity
Title  Dynamical effects in ultracold atomic gases : macroscopic quantum tunneling and bulk viscosity 

Authors  
Advisors  Advisor(s):Zhang, S 
Issue Date  2020 
Publisher  The University of Hong Kong (Pokfulam, Hong Kong) 
Citation  Liu, R. [劉鋭斌]. (2020). Dynamical effects in ultracold atomic gases : macroscopic quantum tunneling and bulk viscosity. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. 
Abstract  Thanks to the development of new techniques in recent decades, experimental ists have made remarkable progresses in controlling ultracold atomic gas systems. In particular, ultracold atoms confined in finely tuned trapping potential with tunable interparticle interactions provide an ideal platform for studying many interesting topics in manyparticle physics. In this thesis, we investigate some aspect of dy namical effects in ultracold atomic gases, including the manyparticle effect in the macroscopic quantum tunneling of a weakly interacting BoseEinstein condensate and the bulk viscosity of a strongly interacting Fermi gas.
Motivated by the recent experimental progresses of interactionassisted macro scopic quantum tunneling, we study the tunneling of an interacting BoseEinstein
condensate from a cubicplusquadratic well to open space. By using the Gross Pitaevskii equation and the WKB method, we investigate the effect of interparticle interactions on the tunneling rate of a condensate trapped in a cubicplusquadratic well. By applying the ThomasFermi approximation, we give the explicit depen dences on the chemical potential of the tunneling rate of the condensate. By a Magnus expansion, we obtain an effective Hamiltonian for a periodically driven sys tem and study the effect of the modulation on the tunneling rate. We find that both the repulsive interactions and an increase of modulation frequency result in an enhancement of quantum tunneling of a trapped BoseEinstein condensate in a cubicplusquadratic potential well.
Furthermore, motivated by the recent progresses of measuring the breaking of scale invariance in swave gases, we utilize the virial expansion and the kinetic theory to investigate the bulk viscosity of a strongly interacting swave Fermi gas with effective range near unitarity in the hightemperature limit. Also, we take into consideration the imaginary part of the fermion selfenergy for the correction to the bulk viscosity. We show that two parameters, relating to the scattering length and the effective range, determine the measure of scale invariance breaking in an swave Fermi gas near resonance in the hightemperature limit. We also show that the scale invariance breaking manifests itself in the bulk viscosity in terms of these two scale breaking parameters.
Subsequently, motivated by the realization of pwave resonantly Fermi gases in experiment, we generalize our calculation of the bulk viscosity to the pwave Fermi gas. We show that the bulk viscosity in the pwave Fermi gas near resonance, simi larly, is determined by two scale breaking parameters relating to the scattering length and the effective range. However, the effective range plays a very different role in the two cases. For an swave Fermi gas at resonance, the bulk viscosity vanishes, and scale invariance is recovered when the effective range is tuned to zero. Unlike the swave case, in the pwave Fermi gas, even in the limit of vanishing effective range, the scale invariance remains broken which leads to a nonzero bulk viscosity. 
Degree  Doctor of Philosophy 
Subject  Cold gases Quantum theory 
Dept/Program  Physics 
Persistent Identifier  http://hdl.handle.net/10722/290454 
DC Field  Value  Language 

dc.contributor.advisor  Zhang, S   
dc.contributor.author  Liu, Ruibin   
dc.contributor.author  劉鋭斌   
dc.date.accessioned  20201102T01:56:18Z   
dc.date.available  20201102T01:56:18Z   
dc.date.issued  2020   
dc.identifier.citation  Liu, R. [劉鋭斌]. (2020). Dynamical effects in ultracold atomic gases : macroscopic quantum tunneling and bulk viscosity. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR.   
dc.identifier.uri  http://hdl.handle.net/10722/290454   
dc.description.abstract  Thanks to the development of new techniques in recent decades, experimental ists have made remarkable progresses in controlling ultracold atomic gas systems. In particular, ultracold atoms confined in finely tuned trapping potential with tunable interparticle interactions provide an ideal platform for studying many interesting topics in manyparticle physics. In this thesis, we investigate some aspect of dy namical effects in ultracold atomic gases, including the manyparticle effect in the macroscopic quantum tunneling of a weakly interacting BoseEinstein condensate and the bulk viscosity of a strongly interacting Fermi gas. Motivated by the recent experimental progresses of interactionassisted macro scopic quantum tunneling, we study the tunneling of an interacting BoseEinstein condensate from a cubicplusquadratic well to open space. By using the Gross Pitaevskii equation and the WKB method, we investigate the effect of interparticle interactions on the tunneling rate of a condensate trapped in a cubicplusquadratic well. By applying the ThomasFermi approximation, we give the explicit depen dences on the chemical potential of the tunneling rate of the condensate. By a Magnus expansion, we obtain an effective Hamiltonian for a periodically driven sys tem and study the effect of the modulation on the tunneling rate. We find that both the repulsive interactions and an increase of modulation frequency result in an enhancement of quantum tunneling of a trapped BoseEinstein condensate in a cubicplusquadratic potential well. Furthermore, motivated by the recent progresses of measuring the breaking of scale invariance in swave gases, we utilize the virial expansion and the kinetic theory to investigate the bulk viscosity of a strongly interacting swave Fermi gas with effective range near unitarity in the hightemperature limit. Also, we take into consideration the imaginary part of the fermion selfenergy for the correction to the bulk viscosity. We show that two parameters, relating to the scattering length and the effective range, determine the measure of scale invariance breaking in an swave Fermi gas near resonance in the hightemperature limit. We also show that the scale invariance breaking manifests itself in the bulk viscosity in terms of these two scale breaking parameters. Subsequently, motivated by the realization of pwave resonantly Fermi gases in experiment, we generalize our calculation of the bulk viscosity to the pwave Fermi gas. We show that the bulk viscosity in the pwave Fermi gas near resonance, simi larly, is determined by two scale breaking parameters relating to the scattering length and the effective range. However, the effective range plays a very different role in the two cases. For an swave Fermi gas at resonance, the bulk viscosity vanishes, and scale invariance is recovered when the effective range is tuned to zero. Unlike the swave case, in the pwave Fermi gas, even in the limit of vanishing effective range, the scale invariance remains broken which leads to a nonzero bulk viscosity.   
dc.language  eng   
dc.publisher  The University of Hong Kong (Pokfulam, Hong Kong)   
dc.relation.ispartof  HKU Theses Online (HKUTO)   
dc.rights  The author retains all proprietary rights, (such as patent rights) and the right to use in future works.   
dc.rights  This work is licensed under a Creative Commons AttributionNonCommercialNoDerivatives 4.0 International License.   
dc.subject.lcsh  Cold gases   
dc.subject.lcsh  Quantum theory   
dc.title  Dynamical effects in ultracold atomic gases : macroscopic quantum tunneling and bulk viscosity   
dc.type  PG_Thesis   
dc.description.thesisname  Doctor of Philosophy   
dc.description.thesislevel  Doctoral   
dc.description.thesisdiscipline  Physics   
dc.description.nature  published_or_final_version   
dc.date.hkucongregation  2020   
dc.identifier.mmsid  991044291218703414   