Spatially modulated superfluid phase of 3He in thin slabs
In plain words
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.
Precise statement
In $3\mathrm{He}$ confined to slabs of thickness $D \sim 1e-4\,\mathrm{cm}$ with pair-breaking walls, quasiclassical theory predicts a stripe phase (Vorontsov and Sauls 2007) of periodic domain walls in a planar-distorted B order parameter, between the A and B phases. Determine whether a modulated phase exists, its period and its region in $\left(D, P, T\right)$, and distinguish a stripe phase from a pair-density-wave state and from a polar-distorted B phase with pinned domain walls. An answer is a phase diagram with direct evidence of the modulation.
What would settle it
NMR line shapes or flow measurements in slabs of several thicknesses whose features match the predicted modulation period and its D dependence.
Status in the literature
NMR in a 1.1e-4 cm slab (Levitin et al. 2019) gave evidence for a stripe phase; a nanofluidic resonator experiment (Shook et al. 2020) claimed a pair-density-wave state, and a 2020 Comment by Levitin et al. disputed that interpretation.