Postdoc: Quantum vortices and inertial waves in rotating superfluid helium

Superfluidity is a fascinating and exotic state of matter that originates from quantum effects at very low temperatures. A superfluid is a liquid distinguished from a classical fluid by the absence of molecular viscosity. Consequently, an object that moves through it at low velocity does not experience any drag. Examples of superfluids are 3He and 4He, Bose-Einstein condensates (BEC) made of dilute alkaline gases, light in optical non-linear systems, and the core of neutron stars. The applications of superfluids range from cooling superconducting materials and infrared detectors to pure fundamental research in cold atoms and turbulence. The most manifest quantum effect in superfluid turbulence is the presence of quantum vortices. Such vortices are like atomic tornados, with a circulation that is quantised. In systems such as 3He and 4He and atomic BECs, quantum vortices behave as hydrodynamic vortices, reconnecting and rearranging their topology.

One of the most classical experiments with superfluids is a rotating bucket filled with superfluid helium. In a rotating superfluid, when rotation is smoothly increased, quantum vortices are nucleated one by one to match the global circulation of the system as closely as possible. The image shows how vortices arrange themselves in a very regular lattice. This picture is well understood at very low temperatures. At finite temperatures, superfluid helium is an immiscible mixture of a superfluid and a normal fluid described by the Navier-Stokes equations. The superfluid vortices and the normal fluid interact in a non-trivial manner, creating a very rich system. For example, superfluid vortices are accompanied by normal fluid vortex structures, as also shown in the image.