Pulsed nuclear magnetic resonance measurements of the transverse frequency and magnetization of superfluid He in a 98.2% porous aerogel are observed at temperatures reduced significantly below the bulk He superfluid transition and are discussed in terms of an isotropic impurity scattering model. Magnetization measurements suggest an equal-spin pairing superfluid. For NMR tipping angles, @~4 0, the shifts drop abruptly to zero, unlike the known dependence of either the~He-A or He-B superfluid phases.

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Nuclear-magnetic-resonance measurements of the magnetization of ^He on thin ^He-'^He mixture films show discrete structure as a function of ^He coverage over the range 0.0055 < (afa < 4 layers at a "^He coverage of 44 ^mol/m^ for 30 < T < 250 mK in a weakly polarizing 2-T field. A steplike doubling of the magnetization at d3^=^0.S layer and a second less pronounced step at ^3^== 1.5 layers are ascribed to the population of higher energy levels which evolve as the ^He thickens. The magnetization as a function of temperature near the first step is fitted by a two-level model with an energy gap ei2^= 1.8 K at the step.PACS numbers: 67.70.+n, 67.50.Dg, 67.60.Fp ^He adsorbed on a substrate may adopt a variety of configurations. These depend on the nature of the substrate, the temperature, and the ^He coverage. For example, ^He adsorbed on a bare substrate such as Grafoil exhibits a complicated phase diagram for submonolayer coverages, and a monolayer is solid. ^ For temperatures near 1 mK, peaks in the magnetization^ and heat capaci-ty^ near second-layer completion are seen. At dilutionrefrigerator temperatures, the solid causes a boundarylayer enhancement of the magnetization of overlying liquid."* Creation of a mixture film by the addition of a layer or two of "^He adjacent to the substrate renders the ^He a liquid and these effects disappear.^ The ^He, with its larger zero-point motion in the van der Waals potential of the underlying substrate, for the most part resides on top of a thin "^He film, and is free to move along its nearly equipotential surface. Such ^He-'^He mixture films are an excellent system for the investigation of the ^He as a Fermi liquid. Measurements of the liquid are not obscured by the presence of solid ^He, and in the strong gradient of the van der Waals potential of the substrate one may expect to see rich behavior in the temperature and coverage dependence of the ^He as it evolves from two to three dimensions. The structure of mixture films and the Fermi-liquid properties of the ^He in them have been studied on various substrates by NMR,^'^ heat-capacity,^'^ torsional-oscillator, ^^ and third-sound ^'' ^ ^ techniques.In this work, ^-^ we present NMR measurements of the magnetization of ^He on a thin superfluid "^He film. We observe the ^He to evolve from a two-dimensional Fermi liquid to a bulklike liquid by increasing the coverage, ^3, of ^He from 0.0055 to 4 layers at a fixed ^He coverage of 44 /xmol/m^. [Used as a unit of 6^3, one layer is defined to be 1 atom/(3.93 A)^. Using in situ thirdsound techniques,^ the "^He film thickness is estimated to be 7.7 A.] These measurements, made in a weakly polarizing external field of //o = 2 T over the temperature range 30< 7"< 250 mK, show two steps in the magnetization versus t/3 and a higher-coverage linear increase of magnetization which has the slope expected for bulk ^He. We interpret the stepped structure as evidence for the occupation of ^He states of higher discrete energy levels in the film, and discuss the first step in magnetization in...

We report measurements of the NMR spin-lattice relaxation time, T\, of 3 He in 4 He films for 0.03 0.25 K and yield the energy difference, Se, between the 3 He ground state and the first excited state; Se shows substantial structure as a function of 4 He coverage.PACS numbers: 67.60.Fp, 67.70.+n, 76.60.-k 3 He atoms bound to a thin 4 He film constitute a system of remarkable richness. As in the case for bulk 4 He [1], the lowest state available to a single 3 He atom in a 4 He film is one in which the 3 He resides at the surface of the 4 He film [2,3]. This is due to the combined effect of the substrate potential and 3 He-4 He interactions. In the dilute limit the 3 He atoms in the surface state form a quasi-two-dimensional gas [4-7] which is degenerate for temperatures below the Fermi temperature. The effective potential defined by the substrate and the 4 He also produce bound states of higher energy for the 3 He in the film [4-7]. This spectrum of states is expected to be sensitive to the 4 He coverage, n* [3]. For the present work, we have used NMR to study the dynamics of the 3 He in mixture films and by this means have measured the energy separation, Se, between the ground and first excited state of the 3 He as a function of n 4 . We find that 8e shows substantial structure.In these experiments, we have carried out measurements of the spin-lattice relaxation time T\ of 3 He on a superfluid 4 He film as a function of 4 He coverage n 4 for 0.27 0.25 K, we find Tf 1 -exp( -A/T) with characteristic energy A. We interpret A to be the energy gap between the Fermi level of the 3 He in the ground state and an excited state of the 3 He in the film [4].