The new academic year started with a webinar session on Cooper pair density waves in superfluid 3He. The speakers are Dr Lev Levitin from Royal Holloway University of London, and Prof. Takeshi Mizushima from Osaka University, Japan.

Dr. Levitin “Search for Cooper pair density waves in superfluid helium 3”

Confinement on a lengthscale comparable to the superfluid coherence length is a powerful tool for modifying the order parameter of the p-wave superfluid 3He via anisotropic surface pairbreaking. A stripe phase has been predicted in slab geometry, comprised of domains of Balian-Werthamer phase with alternating sign of one of the order parameter components. This superfluid with a broken translational symmetry is both a Cooper pair density wave (PDW) and a supersolid. I report on nuclear magnetic resonance experiments in superfluid 3He confined to 1 micron slab, probing the spatially-averaged order parameter amplitudes, sensitive to such domains. The stripe phase with equal amount of + and - domains was not observed, instead there is evidence for a spatially modulated phase with unequal amounts of the two types of domains. I will discuss the possible origins of such imbalance, consistent with a “polka dot” PDW, and review the phase diagram of confined superfluid 3He.

Prof. Mizushima (from 32min) “Spatially-modulated superfluid phases in confined 3He: nucleation and crystallization of polka-dots”

Superfluid 3He has provided a platform for studying new states of matter. It has been predicted that under strong confinement the planar-distorted B phase of superfluid 3He becomes unstable towards pair-density-wave (PDW), that is, crystalline-ordered superfluid. Here we study the possible PDW states in confined superfluid 3He. Using the Ginzburg-Landau theory incorporating strong coupling corrections, we find that two-dimensional PDW state is energetically competitive to one-dimensional PDW state. The two-dimensional PDW consists of a periodic array of two degenerate Balian-Werthamer domains. We find that as temperature increases, the periodic pattern changes from a brick-type texture to a polka-dot texture. Recent NMR measurement of superfluid confined to a nanoscale cavity have found evidence for a spatially modulated phase, which is neither the one-dimensional PDW phase nor the planar-distorted B phase [Levitin et al., PRL 122, 085301 (2019)]. In this talk, I will discuss a possible scenario to understand the experimental observations from the aspect of two-dimensional PDWs.