Probing superfluidity via stirring and Reducing phase fluctuations via driving
I this talk I will present two recent studies. In the first we investigate the superfluid properties of a Li-6 cloud across the BEC-BCS transition probed via stirring with a laser beam, as reported in PRL 114, 095301 (2015). We demonstrate a quantitative understanding of the induced heating. Using both numerical and analytical methods, we identify the non-zero temperature, the circular motion of the stirrer and the geometry of the cloud as key factors influencing the magnitude of the critical velocity. A direct comparison to the experimental data shows excellent agreement. In the second study, we investigate the dynamics of a layered superconductor with alternating inter-layered coupling, driven by an external potential that models a light pulse coupling to an optical phonon mode. Using a Langevin and Fokker-Planck description of a thermally fluctuating pairing field, we first consider a toy model of two driven, coupled, non-linear oscillators which represent two neighboring pairs of layers and their two plasmons. We demonstrate that the external driving leads to a reduction of phase fluctuations of the low-energy oscillator, an effect which is amplified via the resonance of the high energy oscillator. We find that this suppression is even more pronounced for the bulk system, and occurs for the low-energy fluctuations, as visible in the power spectrum.