Superfluid-vortex thresholds in a rotating annulus were detected with second sound in the temperature range $0.01 \mathrm{K}<T_{\lambda }-T<\mathrm{}0.2\mathrm{} \mathrm{K}$. A rapid decrease of the threshold angular velocity ${\omega }_{c1\mathrm{}}$ was observed as $T\to T_{\lambda }$. The data can only be fitted to the theory of Fetter if a rapidly diverging vortex-core parameter is assumed.

• Received 20 May 1977

DOI:https://doi.org/10.1103/PhysRevLett.39.470