Symmetries of the physical world have guided formulation of fundamental laws, including relativistic quantum field theory and understanding of possible states of matter. Topological defects (TDs) often control the universal behavior of macroscopic quantum systems, while topology and broken symmetries determine allowed TDs. Taking advantage of the symmetry-breaking patterns in the phase diagram of nanoconfined superfluid 3He, we show that half-quantum vortices (HQVs)—linear topological defects carrying half quantum of circulation—survive transitions from the polar phase to other superfluid phases with polar distortion. In the polar-distorted A phase, HQV cores in 2D systems should harbor non-Abelian Majorana modes. In the polar-distorted B phase, HQVs form composite defects—walls bounded by strings hypothesized decades ago in cosmology. Our experiments establish the superfluid phases of 3He in nanostructured confinement as a promising topological media for further investigations ranging from topological quantum computing to cosmology and grand unification scenarios.

The effective interaction between 3 He quasiparticles in dilute liquid 3 He-4 He mixtures affects many of its physical properties. The interaction potential is determined here from the saturation solubility and osmotic pressure data reported recently. The interactions are examined consistently over the entire pressure range of liquid-helium mixtures at very low temperatures, that is, from 0 to 25.6 bar, where the solid phase appears. To reproduce all experimental data, it was necessary to include a concentration dependence in the potential. This dependence, however, turned out to be almost the opposite to what has been proposed earlier. The deduced potential can be used to calculate, among other things, an estimate for the superfluid transition temperature of 3 He-4 He mixtures. In addition, we also find values for some less well-established parameters for helium mixtures, such as the binding energy of a 3 He atom in superfluid 4 He.

No abstract

No abstract

A common property of topological systems is the appearance of topologically protected zero-energy excitations. In a superconductor or superfluid, such states set the critical velocity of dissipationless flow v cL , proposed by Landau, to zero. We check experimentally whether stable superflow is nevertheless possible in the polar phase of p-wave superfluid 3 He, which features a Dirac node line in the energy spectrum of Bogoliubov quasiparticles. The fluid is driven by rotation of the whole cryostat, and superflow breakdown is seen as the appearance of single-or half-quantum vortices. Vortices are detected using the relaxation rate of a Bose-Einstein condensate of magnons, created within the fluid. The superflow in the polar phase is found to be stable up to a finite critical velocity v c ≈ 0.2 cm/s, despite the zero value of the Landau critical velocity. We suggest that the stability of the superflow above v cL but below v c is provided by the accumulation of the flow-induced quasiparticles into pockets in the momentum space, bounded by Bogoliubov Fermi surfaces. In the polar phase, this surface has nontrivial topology which includes two pseudo-Weyl points. Vortices forming above the critical velocity are strongly pinned in the confining matrix, used to stabilize the polar phase, and hence stable macroscopic superflow can be maintained even when the external drive is brought to zero.