Transverse acoustic impedance of He II under pressure

The transverse acoustic impedance Z of bulk liiquid He II has been measured at a frequency of 20.5 MHz at temperatures from 30 mK to 2.2 K, at pressures up to 25 bar. Results at SVP have been published previously. The impedance was found from the temperature dependence of the quality factor and resonant frequency of an AT-cut quartz crystal resonator immersed in the liquid. Below 0.5 K the crystal also acts as a microbalance and a change in frequency associated with a loss peak at 10 bar is interpreted as the localization of a second layer of 4He atoms. At higher pressures no further growth of solid 4He on the crystal surfaces was observed. Between 0.6 and 1.6 K we interpret Z as being primarily due to rotons and we have determined the roton relaxation time at all pressures at tau = (8.5 +- 0.4) x 10-14 T/sup -1/3/ exp (Δ/kT) sec, using the theory by Roberts and Donnelly, where Δ is the roton energy gap. The transition from hydrodynamic viscous behavior to the nonhydrodynamic regime was investigated in detail. The data were consistent with completely diffuse scattering of the rotons at the gold-plated electrodes on the crystal. Measurements near the lambda-point, for T/T/sub lambda/>0.99, were analyzed to determine the superfluid healing length as a (T) = (0.10 +- 0.02)epsilon/sup -2/2/ nm, where epsilon = 1-T/T/sub lambda/,, at all pressures. An analysis of data for epsilon< or =0.1 gave a slightly larger value, equivalent to a (T) = (0.13 +- 0.02)epsilon/sup -2/3/ nm. Below 1.6 K the excess normal fluid density associated with the healing layer is similar to that found in thin films