Group seminar at MPQ and Zoom: Few- and many-body physics with box-trapped 39K Bose gases

Christoph Eigen, Cambridge University, UK
Group seminar at MPQ lecture hall and Zoom
Tuesday, April 16,  09:00 am (MEZ)

I will showcase two recent series of experiments using ultracold homogeneous 39K Bose gases, which offer widely tunable interparticle interactions using Feshbach resonances.In the first, we turn off the interactions and study the fate of a violently driven particle in a box. Already in a 1D world such a drive leads to chaotic behavior, where the system reaches a quasiequilibrium state with finite energy. In our 3D box-trapped gases, the energy keeps growing as we drive the system, exhibiting subdiffusive dynamic scaling behavior [1].
The resultant far-from-equilibrium momentum distribution is essentially isotropic, and features remarkably uniform low-momentum population, with coherence destroyed. We explain our observations in terms of a random walk that takes place in energy space [2], arising from the interplay of the underlying 1D driven chaotic state and the presence of weak disorder. In the second, we measure the spectral properties and real-time dynamics of mobile impurities injected into a homogeneous Bose-Einstein condensate [3]. We map out both attractive and repulsive branches of polaron quasiparticles resolving the repulsive polaron and the molecular state associated with the Feshbach resonance in the strongly interacting regime, and show that the latter also has a many-body character. Our measurements reveal remarkably universal behavior, controlled by the bath density and a single dimensionless interaction parameter; for near-resonant interactions the polarons are no longer well defined, but the universality persists.
[1] G. Martirosyan et al., Phys. Rev. Lett 132, 113401 (2024)
[2] Y. Zhang et al., C. R. Phys. 24 Online first (2023)
[3] J. Etrych et al., arXiv:2402.14816