Fig. 1: The two-dimensional quasiparticle quantum well.

a The component of the superfluid gap that corresponds to momenta perpendicular to the wall is suppressed as the container boundary is approached. This yields a potential well in which bound quasiparticles (red halo) can exist to arbitrary low energies. The gap for motion along the surface remains nonzero. At low temperatures, the density of quasiparticles in the bulk superfluid is vanishingly low. b The probe wire (cross-section shown by the grey disk) is surrounded by the bound quasiparticle potential well (red halo). The thickness of the potential well aξ (a ~ 1, see Methods) determines the bound quasiparticle mean free path as indicated by the double arrow. Here R is the radius of the wire crossbar. When the crossbar is moving, the local superflow velocity around it depends on the polar angle θ. Green notches indicate the span of θ = ± 80° where the quasiparticle escape condition v_fl > v_c is satisfied locally for the wire velocity of v = 45 mm s⁻¹.