"Non-equilibrium and emergent phenomena in superfluid 3He"

In this project we will study a newly discovered phenomenon - existence of superfluidity at fluid velocities by far exceeding the critical value. We will work at the limit of what is technically possible in the field of low temperature physics, in the microkelvin region. At these temperatures superfluid 3He is in deep quantum regime with only few normal excitations, which enables us to study its properties emerging far from its thermal equilibrium. One of them is an ability to extend the superfluidity region to velocities far beyond the Landau critical value.
We will explore a vast parameter space to determine the region where superfluidity survives above the Landau velocity. One of the parameters will be the surface specularity of the scatterer moving in the superfluid: this can be tuned by adding 4He on the surface. We will study whether the superflow can exist in all three superfluid states available for experiments: the A phase, the B phase and the recently discovered polar phase. The latter can be stabilised in the strongly anisotropic nanostructured medium consisting of thin (10 nm) parallel strands - nafen. We will collaborate with theoreticians world-wide to develop a suitable theory that would explain the observed phenomena.
The project will start with the feasibility studies, namely determining how covering the surfaces of experimental cell with a few monolayers of 4He will affect the thermal Kapitza resistance between the refrigerant (copper and silver powder) and the superfluid. These experiments will define our strategy of designing the experiment with a piece of nafen moving through the superfluid at a uniform velocity.
We will need to design and build an instrument for the 3He-4He isotope separation in order to purify the 3He gas recovered after the experiments from the contaminant 4He.
The experiment with nafen will require NMR measurements in order to determine the structure of the superfluid state. We will need to develop and build a suitable NMR spectrometer including superconducting vector-field magnets. The measurements of the drag force on the nafen sample moving in the superfluid will require designing a suitable quasiparticle bolometer.
The developed experimental setup will allow the measurement of the critical velocity as well as observation of other phenomena. One of them is the possible quantum criticality - a phase transition at absolute zero of temperatures. The new experiment will also allow measuring the superfluid density (using small-amplitude mechanical oscillations) and the strength of the spin-orbital interaction (using NMR frequency shift data) in all superfluid states. This will allow us to characterise the influence of impurities (nafen) on superfluidity and will have wider implications, including better understanding of p-wave superconductivity.