Quantum zero sound in liquid helium-3

In his early paper on Fermi liquids, Landau predicted that a new regime of propagation for density waves sets in at very low temperature T when the frequency ω of the waves becomes larger not only than the inverse of the quasiparticles relaxation time (zero sound regime) but also larger than the temperature (quantum zero sound regime). In these last conditions the attenuation α is frequency dependent, thus distinguishing QZS from ZS. By measuring simultaneously the absolute values of α(ω, T) at three different frequencies (85, 250, 420 MHz) and in the 10 mK range we have verified the Landau predictions.