Temperature dependence of the sound velocity in liquid4He

We have measured the temperature dependence of the sound velocity in liquid⁴He from 0.07 to 0.7 K for frequencies from 12 to 105 MHz. While the data show an approximateT⁴ temperature dependence, the frequency ordering of the curves is observed to undergo an inversion at about 0.2 K. Above this temperature we confirm that the velocity shift decreases with frequency, while below this temperature we now find that the velocity shift increases with frequency as predicted by theory.Based on work performed under the auspices of the U.S. Atomic Energy Commission.