Nanomechanical probes of superfluidity

Superfluid 3He is among the most fascinating manifestations of collective quantum behaviour. Many behaviours that appear fundamental to our universe, such as gauge invariance and the Higgs mechanism, have emergent analogues in the superfluid. Other superfluid features mimic important properties of condensed matter, such as topological defects. Understanding superfluidity is also important for astronomy, because the cores of neutron stars appear to exist in a superfluid phase. If this is true, then fermionic superfluids account for a large fraction of all the condensed matter in the universe – far more than exists as an ordinary liquid!

On earth, superfluids can be found only in specialised cryostats. There is at present no experimental tool to measure superfluids on the mesoscopic scale, i.e. between the size of atoms and the scale of the superfluid coherence length. A vibrating nanotube, working as a tiny moving-wire viscometer, would be able to probe this unexplored regime. In these experiments, we do not need to optimise the nanotube devices for the very highest frequencies, but will still benefit from unsurpassed sensitivity and resolution. Furthermore, the nanotube resonant frequency could be tuned over a much higher range than present-day viscometers, right up to the pair-breaking energy of ~70 MHz.

We are preparing to probe liquid 3He-B using an immersed nanotube resonator. These experiments require extremely low temperatures; however, we have access to the Lancaster Advanced Microkelvin Refrigerator, which uses a powerful nuclear demagnetization stage to cool superfluid and mechanical devices. To measure the resonator, we will use the optomechanical techniques, which allow ohmic heating to be reduced if necessary to an extremely low level. In this way, we aim to measure the intricate energy levels of the superfluid quasiparticles.

Figure: A nanomechanical probe of superfluid collective excitations. With the vibrating nanotube immersed in helium, it couples to the many exotic modes available in the superfluid, which change its damping and effective mass.