SUB-HARMONIC RESPONSES IN LIQUID-HELIUM

We report measurements of the subharmonic responses of liquid helium to ultrasound in the temperature range 1.3K<T<2.5K at applied frequencies of 0.3, 1.0, 2.8, and 10 MHz. Experimental results are compared in detail to the oscillating bubble model of Eller and Flynn and to predictions based upon parametric amplification. We show that the subharmonic response in liquid helium, below the superfluid transition, is directly associated with bulk turbulence. Visualization of the subharmonic response by light diffraction demonstrates that it can be a sustained and coherent bulk effect. © 1979 Acoustical Society of America