Critical velocities and phase slips in superfluid ⁴He flow through a 2 x 2 /μm aperture

Ac measurements of the critical velocity behavior of superfluid ⁴Ile flow through a 2 × 2 pm square aperture in a 0.1 (tm thick titanium foil have been made at temperatures between 0.36 and 2.10 K at frequencies which vary with temperature from 19 to 11 IIz. Single 2ir phase slips are clearly resolved over the entire temperature range. These slips define a critical velocity that decreases nearly linearly with increasing temperature. Whereas below 1.0 K the width of the observed critical velocity distribution is relatively temperature independent, above 1.0 K it decreases linearly with temperature toward zero near Ta-We also observe a smaller, less temperature-dependent, transient critical velocity induced by high drive levels.