Nuclear Magnetic Resonance Investigation of ³He Surface States in Adsorbed ³He-⁴He Films

Abstract

We have made nuclear susceptibility measurements of ³He atoms adsorbed on the free surface of a ⁴He film. Apart from providing a unique opportunity for studying the magnetic properties of a two-dimensional Fermi system, this experiment is of interest in relation to recent theoretical and experimental investigations of “surface states” in ³He-⁴He mixtures.^1–3 Specific heat measurements² carried out with a Vycor substrate similar to that used in this work have shown that there is a strong dependence on whether or not a free surface is present. With a free surface the specific heat rises almost linearly as expected for a degenerate Fermi gas, while at full pores the limiting low-temperature specific heat is independent of temperature, indicating that the ³He atoms are behaving as a nondegenerate gas. The latest surface tension data³ have confirmed Andreev’s predictions¹ that ³He atoms are bound on the free ⁴He surface with a binding energy ε _o equal to 1.8°K and that if the surface concentration is sufficiently dilute, they constitute a two-dimensional gas of quasiparticles with effective mass m* = 1.7m ₃, where m ₃ is the bare ³He mass. For higher surface densities, however, there appears to be an attractive interaction V ₀ between the surface quasiparticles, and Guo et al.³ estimate that this alters the chemical potential by approximately -0.7°K for a complete layer.

The work was carried out with the support of the Science Research Council (Grant Nos. 3/9/1832 and B/SR/4261, and a Research Studentship), and the U.S. Air Force Office of Scientific Research through the European Office of Aerospace Research under Grant EOOAR 68-0023.