SciPost Thesis Link
Title:  Implementation and Applications of the Stochastic Projected GrossPitaevskii Equation  
Author:  Samuel James Rooney  
As Contributor:  (not claimed)  
Type:  Ph.D.  
Field:  Physics  
Specialties: 


Approach:  Theoretical  
URL:  http://hdl.handle.net/10523/5460  
Degree granting institution:  University of Otago  
Supervisor(s):  Ashton Bradley, Blair Blakie, Uli Zuelicke  
Defense date:  20150212 
Abstract:
Providing a complete description of dissipative superfluid dynamics is one of the major challenges of manybody quantum field theory. In this thesis we make a fundamental step towards this goal by implementing the stochastic projected GrossPitaevskii equation (SPGPE) in complete form for the first time. The SPGPE is a hightemperature theory of BoseEinstein condensate dynamics, providing a classicalfield description of a lowenergy subspace in contact with a thermal reservoir. The reservoir interaction terms account for dissipation and noise from thermal interactions, and arise from two distinct processes described as numberdamping and energydamping. This work advances previous applications of the SPGPE theory, which have only included numberdamping processes, by implementing the energydamping processes. We describe the properties of the deterministic and noise terms corresponding to the energydamping process, and develop a novel algorithm to accurately and efficiently evaluate the energydamping terms in the SPGPE. We apply the SPGPE to a range of experimentally accessible systems, considering both nonequilibrium and quasiequilibrium dynamics. We model the experiment of Neely et al. [Phys. Rev. Lett. 111, 235301 (2013)], where stirring of a toroidally trapped BoseEinstein condensate generates a disordered array of quantum vortices that decay, via thermal dissipation, to form a macroscopic persistent current. We perform numerical simulations of the experiment using the numberdamping SPGPE and ab initio determined reservoir parameters. We quantitatively reproduce both the formation time and size of the persistent current, as measured in the experiment. In the first application of the full SPGPE, we consider the nonequilibrium dynamics of a condensate excited into a largeamplitude breathing mode. We find that in such nonequilibrium regimes, the energydamping dominates over the numberdamping process, leading to qualitatively different system dynamics. In particular, energy damping causes the system to rapidly reach thermal equilibrium without greatly depleting the condensate, showing that energy damping provides a highly coherent dissipation mechanism. Finally, we apply the SPGPE to the quasiequilibrium dynamics of singlevortex decay. Energydamping processes have previously been neglected for this system. SPGPE simulations show that in fact energydamping has a dominant effect on the lifetime of a single vortex, with lifetimes less than half those predicted by the numberdamping SPGPE. In contrast to the breathing mode decay, we observe little qualitative difference between the energydamping and numberdamping descriptions of vortex decay. Our findings show that while energydamping processes are important to quantitatively describe quasiequilibrium dynamics, the system behavior may be described by the numberdamping SPGPE with a suitably modified dissipation rate.