How does 2D quantum superfluid feel to touch?

The bulk superfluid ³He comprises triplet-paired Cooper pairs and exists in the B phase at the lowest temperatures. The pure superfluid regime, where the thermal quasiparticle density is minimal, can be reached by cooling the B phase of superfluid ³He. 

At the container’s edges, a quantum well encloses the bulk superfluid, keeping a sea of quasiparticles with energies lower than those of the bulk inside. These states can be produced in a non-equilibrium distribution inside the quantum well, allowing us to witness the dynamics of their mobility indirectly.

A new study by the Lancaster University scientists shows how the superfluid helium ³He would feel if you could put your hand into it. They show that the induced quasiparticle currents flow diffusively in the two-dimensional system.

Dr Samuli Autti, the lead author of the research, said, “Nobody has been able to answer this question during the 100-year history of quantum physics. We now show that, at least in superfluid ³He, this question can be answered.”

The studies used a mechanical resonator the size of a finger to probe the extremely cold superfluid while being conducted in a customized refrigerator at a temperature roughly 10000th of a degree above absolute zero.

Superfluid ³He transfers the heat it generates along the container’s surfaces when agitated with a rod. The superfluid’s bulk exhibits vacuum-like behavior and complete passivity.

Dr Autti said: “This liquid would feel two-dimensional if you could stick your finger into it. The bulk of the superfluid feels empty, while heat flows in a two-dimensional subsystem along the edges of the bulk – in other words, along your finger.”

The bulk of superfluid ³He, the scientists conclude, is encircled by an independent two-dimensional superfluid that interacts with mechanical probes rather than the bulk superfluid itself; the bulk superfluid is only accessible to the independent superfluid in the event of an abrupt energy burst.

In other words, superfluid 3He is thermo-mechanically two-dimensional at the lowest temperatures and applied energies.

Dr Autti said, “This also redefines our understanding of superfluid ³He. For the scientist, that may be even more influential than hands-in quantum physics.”

“Our work shows that this two-dimensional quantum condensate and the dynamics of the surface bound states are experimentally accessible, opening the possibility of engineering two-dimensional quantum condensates of arbitrary topology.”

Journal Reference:

1. Autti, S., Haley, R.P., Jennings, A. et al. Transport of bound quasiparticle states in a two-dimensional boundary superfluid. Nat Commun 14, 6819 (2023). DOI: 10.1038/s41467-023-42520-y