Frequency dependence of superfluid onset temperature of4He films on oscillating substrates

We have studied the frequency dependence of the superfluid onset for⁴He films on Mylar substrates using a two-torsional-oscillators technique. A peak of the dissipation accompanied by the superfluid onset appears at higher temperatures for 2009 Hz than for 530 Hz. The temperature difference T_p between the two peaks is 2.0 ± 0.5 mKfor coverages with the onset temperature T_c of 1.4 K, and increases by a factor of three with decreasing T_c from 1.4 to 0.95 K. In the temperature dependence of the dissipation, the dissipation width also increases by a factor of three with decreasing T_c from 1.4 to 0.95 K. The T_c dependencies of T_p and the dissipation width indicate that an intrinsic parameter D/r₀² of a vortex decreases with the superfluid onset temperature, where D is a diffusion constant and r₀ a core radius.