Experimental Difference between First and Zero Sound in KBr

Long-wavelength acoustic phonons have been studied for each of the [00ζ]T, [00ζ]L, [0ζζ]T₁, and [0ζζ]L branches in KBr at 95 and 463^∘{}K by means of inelastic neutron scattering. This study has shown that the elastic constants of KBr determined by inelastic neutron scattering (zero-sound regime) exhibit a different temperature dependence from those determined by ultrasonic techniques (first-sound regime). For example, the change in the zero-sound elastic constant for the [0ζζ]T₁ branch between 95 and 463^∘{}K is (23±{}4)%, while the change in the first-sound elastic constant determined by Hauss\"uhl using ultrasonic techniques is (35.1±{}1.2)%. Cowley's theory for first and zero sound shows reasonable agreement with experiment.