Breaking the Roton Barrier: An Experimental Study of Motion Faster than the Landau Critical Velocity for Roton Creation in He II

We report the first observation of objects moving through He II with equilibrium drift velocities $\overline{v}$ beyond the Landau critical velocity $v_c$ for roton creation. With $T\sim 0.4$ K, $P=25\mathrm{}$ bar, $F=2\mathrm{}$ kV ${\mathrm{cm}}^{-1\mathrm{}}$, $\overline{v}-v_c$ for negative ions is larger than a theoretical prediction by a factor of ∼${10}^5$. The vortex-ring nucleation rate is found to decrease with $F$ above 300 V ${\mathrm{cm}}^{-1\mathrm{}}$, thus resolving apparent inconsistencies between earlier experiments.