Domain of Two-Dimensional Excitations in Superfluid Helium Films

Abstract

Using third-sound resonance, we find for helium film thickness less than 2.8 atomic layers that there exist excitations which are phononlike and two dimensional, obeying a $T^3$ law, that their areal density is independent of film thickness, and that their velocity (using Landau theory) is 76±2 m/sec. This velocity is slightly above the maximum thirdsound velocity, 69 m/sec, that can be attained by ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ on argon. For thicker films the areal density of these excitations becomes smaller.