Christodoulou, Panos

Date:    Thursday, Dec 2, 2021
Time:   11:30
Place:   scheduled Zoom meeting
Host:    Tilman Esslinger

Second sound and the birth of turbulence in two dimensions

Panos Christodoulou
University of Cambridge, UK

In this talk, I present our recent experiments on driven box-trapped two-dimensional (2D) Bose gases. In the first part, I discuss our observation of first and second sound. The latter constitutes one of the most characteristic signatures of superfluidity, but its observation had remained elusive in any 2D superfluid, whose unconventional origin (compared to their three-dimensional counterparts) is described by the Berezinskii-Kosterlitz-Thouless (BKT) theory. The measured speeds of the two sounds allow the extraction of the superfluid density, which shows the expected universal jump of a BKT superfluid at the critical point. In the second part of this talk, I discuss our newest results on strongly driven 2D gases in which a direct turbulent cascade emerges: energy injected into the system at a large lengthscale flows without loss through momentum space until it is dissipated at some small lengthscale. Our ability to probe all lengthscales, from the energy-injected to the dissipation one, permits the observation of excitations to individual discrete phonon states, a spatio-temporal self-similar pre-steady-state evolution of the cascade in momentum space, and finally a fully developed steady-state turbulent cascade.