Dr Emil Varga (Charles University, Prague) gives a webinar on “Physics of superfluid 4He in two-dimensional confinement”.

Superfluid helium-4 confined to quasi-2D channels offers a rich physics playground for topics ranging across topological phase transitions, finite-size effects, and two-dimensional turbulence.

In this seminar, I will discuss an experimental approach based on nanofluidic Helmholtz resonators that allows us to study all the above-mentioned phenomena. We show that flows in sub-micron quasi-2D channels can become turbulent with multiple stable states of large-scale dissipation, which can be explained in terms of a predator-prey model as an interplay between spontaneously and extrinsically generated vorticity polarization. Reducing the confinement further to less than 50 nanometers, suppression of the superfluid density due to finite-size of the system becomes observable in a wide range of temperatures.

We show that the suppression can be fully accounted for by surface-bound rotonlike excitation with energy gap significantly reduced compared to the bulk. Close to the superfluid phase transition, the strongly confined channels exhibit an abrupt loss of superfluid response consistent with the prediction based on the Kosterlitz-Thouless topological phase transition, albeit with as-yet unexplained pressure dependence.