Quasiparticle interactions in two-dimensional fluidHe3films adsorbed on graphite

Abstract

The nuclear magnetic susceptibility of 3 He adsorbed on the surface of graphite, plated with a monolayer of solid 4 He, has been measured at surface densities less than 0.055 A ~2 and at temperatures down to 6 mK. A temperature-independent susceptibility is observed at low temperatures, which is strongly enhanced over the ideal-gas value, due to quasiparticle interactions. The inferred Landau-Fermi-liquid parameters allow a comparison with microscopic theories of interacting Fermi systems. PACS numbers: 67.50.-b, 67.70.+nThe fluid phase of adsorbed 3 He is potentially an important and well characterized example of a lowdimensional system of highly correlated fermions. In this Letter we show that 3 He adsorbed on graphite plated with a single atomic layer of 4 He behaves as a twodimensional Landau Fermi liquid, in which the interactions can be varied over a wide range and the crossover from degenerate to nondegenerate behavior can be clearly explored. We present the first measurements on twodimensional 3 He to show a constant low-temperature nuclear magnetic susceptibility which is strongly enhanced by quasiparticle interactions above ideal-gas values. The Fermi-liquid parameters inferred from these measurements provide a test of microscopic models of interacting Fermi systems, such as the almost localized fermion model.The two related systems that have received the most attention are 3 He adsorbed on exfoliated graphite [1] and 3 He-4 He mixture films adsorbed on heterogeneous substrates [2,3]. In the mixture films the 3 He atoms reside near the surface of a 4 He film which coats the substrate, smoothing out effects of substrate heterogeneity. The interaction of the 3 He atoms with the 4 He film gives rise to a mass enhancement of the single 3 He "impurity" due to hydrodynamic backflow. In addition the 3 He atom has available excited states, at relatively low energy, associated with motion perpendicular to the surface. These states become populated therefore at relatively low coverages -0.03 A -2 , and need to be taken into account at temperatures above a few hundred millikelvin [2,4].The system we have chosen to study is the second-layer fluid phase of 3 He on Grafoil [5l. Here the significantly higher fluid density of 0.055 A -2 is accessible before solidification commences [6,7] (the onset of promotion to the third layer occurs at 0.069 A -2 ). At these higher fluid densities strong correlation effects are observable. The binding energy of the second-layer atoms is relatively large, -25 K, and since the excitation of atoms out of this layer is negligible below 1 K the onset of degeneracy in the two-dimensional fluid can be investigated quantitatively. The more strongly bound first-layer submonolayer film also exhibits a fluid phase [1]. However, the range of

Theory ofp-wave pairing in a two-dimensional Fermi gas

“…The experimental data on the density dependence of the specific heat and magnetic susceptibility are presently available for 3 He on Grafoil [1,2,3] and for 3 He-4 He films on the Nucleopore substrate [4,5]. The latter case is more difficult to analyse because a 3 He atom occupies a surface bound state on top of 4 He and its hydrodynamic mass is substantially larger than the atomic mass of 3 He due to the interaction with underlying 4 He layers [12].…”

“…The most remarkable property of bulk 3 He is a superfluidity with nonzero angular momentum l = 1 [8]. However, in "surface" 3 He the superfluid transition has not yet been…”

“…In what follows, we analyze the possibility of superfluidity for 3 He on Grafoil on the basis of information which can be inferred from experiments on specific heat [1] and magnetic susceptibility [3]. We find that for most experimentally accessible densities, this system cannot be described by a momentum independent (i.e., s-wave) interaction.…”

“…Experiments on monolayer films of 3 He absorbed on Grafoil [1,2,3] and on 3 He- 4 He mixture films on Nucleopore substrate [4,5] provide an opportunity to study in detail the properties of 2D Fermi liquids. Interest in this subject has grown since the validity of the conventional Fermi liquid theory in 2D was questioned in the context of the high-T c superconductivity [6].…”

“…In the present approach, we focus on the 2D Fermi liquid at sufficiently low densities where perturbative calculations [7] do not show any divergencies which might signal the breakdown of the Fermi liquid description. Accordingly, we assume that the Fermi liquid picture is valid for 3 He in two dimensions.…”