AMPLITUDE-DEPENDENT SOUND-VELOCITY INCREASE IN SUPERFLUID HE-4

We report an observation of the amplitude-dependent velocity increase of first sound in superfluid He-4. We performed a precise sound-veloci ty measurement at a frequency of 430 MHz in superfluid He-4 as a funct ion of sound amplitude. In the pressure region higher than 18 atm, we found that the velocity of sound begins to increase as the sound ampli tude is raised above the threshold power of 0.5 W/m(2) The effect beco mes more conspicuous with increasing pressure and input power density, and at 22.5 atm, delta c/c reaches as much as 3 X 10(-4) at the input power density of 50 W/m(2) In the low-pressure region less than 18 at m, the velocity change is weak and in the opposite direction. This amp litude-dependent velocity change is not explained by the third-order n onlinear effect derived from Navier-Stokes equations. Despite this str ong pressure dependence, the threshold power of the velocity change is almost constant throughout the pressure range, and is close to the on set power of vortex line growth, which suggests the phenomenon is rela ted to the turbulence.