Superfluid helium-3 and quantum solids

6 problems

• Is second-layer 3He on graphite a gapless spin liquid

  Open

  A layer of helium-3 atoms adsorbed on graphite forms a triangular solid in which nuclear spins interact through atoms exchanging places in rings. Down to the lowest temperatures reached, these spins show no order, suggesting a quantum spin liquid.

• Why the 3He A-to-B transition nucleates far faster than predicted

  Open

  When superfluid helium-3 is cooled, the A phase should turn into the B phase, but standard nucleation theory says the needed seed bubble of B costs so much energy that this should essentially never happen. Yet it happens in every experiment.

• Direct detection of Majorana surface states in superfluid 3He-B

  Contested

  Theory says the B phase of superfluid helium-3 is topological, so its surfaces should carry Majorana states (particles that are their own antiparticles). Experiments see surface states, but none has shown the unusual magnetic response that would identify them as Majorana.

• Majorana modes in half-quantum vortices of polar-distorted 3He

  Partially resolved

  In helium-3 inside nematic aerogel (a porous solid of aligned strands), vortices carrying half of the usual circulation were observed in 2016. Theory says that in some of these phases each such vortex core holds a Majorana zero mode, which has not been detected.

• Spatially modulated superfluid phase of 3He in thin slabs

  Contested

  When helium-3 is confined in a slab about one ten-thousandth of a centimeter thick, theory predicts a superfluid phase whose order varies periodically along the slab, like stripes. Experiments disagree on whether this phase exists and what form it takes.

• Superclimb of dislocations and mass flow through solid 4He

  Partially resolved

  Solid helium-4 lets helium atoms flow through it, and it is far softer than expected because its line defects (dislocations) glide almost freely. Whether the flow runs along dislocation lines with superfluid cores is not proven.