The critical-velocity behavior of oscillatory superfluid 4He flow through a 2 × 2-μm aperture in a 0.1-μm-thick metal foil has been studied from 0.36 to 2.10K at frequencies from less than 50 Hz up to above 1880 Hz. The pressure remained less than 0.5 bar. In early runs during which the frequency remained below 400 Hz, the critical velocity was a nearly-linearly decreasing function of increasing temperature throughout the region of temperature studied. In runs at the lowest frequencies, isolated 2π phase slips could be observed at the onset of dissipation. In runs with frequencies higher than 400 Hz, downward curvature was observed in the decrease of critical velocity with increasing temperature. In addition, above 500 Hz an alteration in supercritical behavior was seen at the lower temperatures, involving the appearance of large-energy-loss events. These irregular events typically lasted a few tens of half-cycles of oscillation and could involve hundreds of times more energy loss than would have occurred in a single complete 2π phase slip at maximum flow. The temperatures at which this altered behavior was observed rose with frequency, from ∼0.6K and below at 500 Hz, to ∼1.0K and below at 1880 Hz.
Bibliographical noteFunding Information:
We wish to acknowledge the support of this work by the National Science Foundation through grants DMR 90-02890, 94-03522, and 96-31703. In addition, we would like to thank James Kakalios for discussions regarding avalanche behavior.
- Aperture flow
- Critical velocity
- Large-energy-loss events
- Phase slip
- Superfluid He