Direct and inverse turbulent cascades in a 2D Bose gas

This thesis explores the dynamics of two-dimensional (2D) Bose gases driven far from equilibrium, as well as their subsequent relaxation dynamics, and can be divided into two sections. The first one provides the background for the novel work of the second part, which describes three experimental studies. In the first one, we provide overviews of: (i) the models used to describe ultracold Bose gases, (ii) the experimental platform, with a focus on our measurement techniques, and (iii) the theoretical background for wave turbulence in Bose gases. We then turn to our experimental work; all of which begins with a quasi-pure 39K condensate, strongly confined along one direction to realise a 2D system and trapped in a uniform in-plane potential. The first study is of the emergence of a direct cascade in a 2D Bose gas driven at a large lengthscale. By probing the gas on all relevant time- and length-scales, we observe both the establishment of statistical isotropy at high-k from an anisotropic low-k drive, and the spatiotemporal scaling of the power-law spectrum in the pre-steady-state dynamics. The second experiment probes the bidirectional relaxation dynamics of an isolated out-of-equilibrium system. We observe universal scaling in the low-k coarsening dynamics, consistent with the framework of nonthermal fixed points, and scaling in the high-k dynamics consistent with weak-wave turbulence theory. In the third and final experiment, we drive the 2D Bose gas at a scale much smaller than the system and observe a steady-state inverse cascade, towards lower k, characterised in turn by: (i) a power-law spectrum, close to that predicted in weak-wave turbulence theory, and (ii) a nonthermal spectrum reminiscent of the coarsening dynamics in the isolated Bose gas. We further show that for anisotropic driving the salient features of the cascade survive only along the drive direction, and form a complete picture of the cascade formation dynamics.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Karailiev, Andrey

Advisor dc:contributor.advisor

• Hadzibabic, Zoran

• Bose gases
• Ultracold atoms
• Far-from-equilibrium physics
• Wave turbulence

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/d67a6b68-fe27-492d-b475-59ef011ba40e/download
• https://creativecommons.org/licenses/by/4.0/

Author Identifier

0000-0002-8177-4870

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/385817