Measurements of Critical Current of Superfluid 4He through An Array of Submicron Apertures very near Tλ

Far from the lambda transition the critical flow of superfluid 4 He through a small orifice is determined by thermal nucleation of quantized vortices. Between 300 mK and 2 K linearly decreasing critical flow velocity has been observed earlier. As the temperature approaches T λ the size of the vortex core increases and becomes comparable to that of the orifice. We report here measurements of the critical mass current in this temperature range. An array of 24 3 x 0.17 μm holes in parallel with a macroscopic parallel path and flexible-diaphragm Helmholtz resonator have been used. The temperature range explored was from 80 mK to 20 μK below T λ . Preliminary analysis of the data shows that for a reduced temperature t = (T λ - T)/T λ ? 1.10 -4 the critical current scales approximately as t 1.25 . Closer to T λ the critical phase difference across the array becomes comparable to 2π and the results have to be analyzed in terms of Josephson effect. The superfluid density has been measured at the same time as the critical current.