Landau critical velocity for a macroscopic object moving in superfluidB3: Evidence for gap suppression at a moving surface

We have observed that the motion of a vibrating-wire resonator in superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$ at temperatures below ${\mathrm{T}}_{\mathrm{c}}$/8 is associated with a sharp critical velocity, ${\mathrm{v}}_{\mathrm{c}}$, above which very high dissipation sets in. We identify this velocity with the Landau critical velocity for pair breaking. That the observed value of ${\mathrm{v}}_{\mathrm{c}}$ is a factor of 2 lower than the critical velocity expected from the BCS value of the gap we take as evidence that the gap is suppressed by about 50% of its bulk value near a moving boundary.