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Tuneable homogeneous Bose fluids close to and far from equilibrium https://doi.org/10.17863/CAM.111742 Dogra, Lena Hadzibabic, Zoran This thesis describes experiments with an ultracold weakly interacting Bose gas of 39 K in a 3- dimensional homogeneous optical trap. We expound three phenomena in different regimes from adiabatic equilibrium to ‘true’ far-from-equilibrium. We first outline a cooling effect driven by three-body recombination, which has traditionally been associated with heating and adverse effects. Three-body loss in a partially condensed three-dimensional homogeneous Bose gas, should, under certain conditions, even purify the sample, i.e. reduce the entropy per particle and increase the condensed fraction. The mechanism is a purely ideal gas effect, with the only role of weak interactions being to ensure thermalisation. We show that this ideal gas effect is robust to the influence of weak interaction energy and suggest realistic experimental conditions for the observation of the cooling and purification. Second, we describe our simultaneous observation of first and second sound, a phenomenon that is central for the concept of superfluidity. Sound waves occur close to equilibrium, as a linear response to weak perturbations. With liquid helium, the existence of two different sound modes at the same wavelength has been studied, where the modes are a pressure wave as in air, and a temperature wave. While it is known that Landau’s famous two-fluid model successfully captures the behaviour of the almost incompressible helium, we study the two modes in the opposite limit of a compressible Bose gas. By exciting center-of-mass oscillations of our homogeneous gas at different frequencies, we find two resonances out of which only one persists above the critical temperature. We further explore the microscopic structure of the two modes, which shows density oscillations dominated by, respectively, thermal and condensed atoms for first and second sound, in agreement with Landau’s hydrodynamic theory. Finally, in a normal gas above the critical temperature, we explore the crossover between the hydrodynamic and the collisionless regime by varying the interaction strength. The third subject of this thesis is turbulence, a paradigmatic example of a far-from-equilibrium phenomenon. For far-from-equilibrium states, (equilibrium) thermodynamic descriptions do not apply even locally, and it is a major challenge to find similarly concise macroscopic descriptions, in a variety of contexts also including glasses and active matter. Here we use the same excitation scheme as for our study of first and second sound, to now drive the Bose gas strongly. Under continuous drive, a turbulent cascade steady state is established, which is characterised by a power-law momentum distribution and is sustained by a constant energy flux. We find that the exponent of the distribution is essentially independent of the details of the system, and establish experimentally the cascade amplitudeand the transported energy flux as state variables that are related through a relation that is independent of the forcing and the dissipation, as well as the history of the system, in analogy to equilibrium equations of state. Finally, we show that the equations of state for a wide range of densities and interaction strengths can be scaled onto each other, giving a universal dimensionless equation of state that provides benchmarks and challenges for the theory and could also be relevant for other turbulent systems. University of Cambridge 2023-11-09 Thesis Doctoral Doctor of Philosophy (PhD) eng https://www.repository.cam.ac.uk/handle/1810/373212 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/bd0dfb3a-8a21-4953-a24c-5561d428071b/download 1b7909f7840139ae3b13d59b0dc4466a https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/cf713d1d-4e92-4973-93c6-12b20a7dc089/download 87eda9de84448d1f82354d60eee3eb5f https://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/ Turbulence Fluid dynamics Quantum physics Bose-Einstein condensation Atomic physics Non-equilibrium physics Far-from-equilibrium physics Bose gases Second sound Two-fluid model