The critical velocity behavior of superfluid 4He in a micron-size aperture at two different frequencies of oscillatory flow

Ac measurements of an effective critical velocity vsc of superfluid 4He flow through a rectangular aperture 5 μ x 0.3 μm in a foil 0.2 μm thick were made between 0.35 and 2.15 K. These measurements involved oscillatory flow in two different ranges of frequency, 70--118 Hz and 1818-1899 Hz. At temperatures T above 1.7 K, vsc(T) measured in the two frequency ranges coincided, decreasing with increasing T. Above 1.9 K, downward curvature toward vsc = 0 at Tλ occurred in proportion to (ρs/T)0.83. At temperatures below 1.7 K, differences appeared between the critical velocity behaviors in the two frequency ranges. As temperature decreased, vsc(T) at the lower frequencies continued to rise in a nearly linear fashion, while at the higher frequencies vsc(T) tended to level off. At the same time, a pronounced increase in noise and metastability in the supercritical region was seen at the higher frequencies. Possible explanations for this divergence of behavior are discussed. 24 refs., 5 figs