The precipitous onset step of Kosterlitz, Thouless, and Nelson has been investigated in 3 He-4 He mixture films over the temperature range 1.3 K to T\ I The constant relating the minimum value of superfluid density to the absolute temperature is found experimentally to be universal up to 30% 3 He. Comparison is made with recent mixture theories. Measurements were made at 24 MHz with a quartz microbaiance adsorptometer.Recent theoretical work has culminated in certain remarkable predictions about two-dimensional superfluids. 1 " 5 In particular the two-dimensional superfluid density
The experimental data which we have gathered on the superfluid mass content in films of 3 He-4 He mixtures yield a characteristic length which varies markedly both with temperature and 3 He concentration. The data are analyzed to present their implications on the energy gap and on other parameters characterizing the two-dimensional helium-boundary-interface excitation spectrum. i PACS numbers: 67.70. Fp, 68.15.+e, 68.25.+J, Several years ago it was demonstrated 1 ' 2 that the existence of two-dimensional helium interface excitations is clearly evidenced by data on the superfluid mass in adsorbed thin films. We deduced the quantitative features of the rotonlike dispersion curve characterizing these excitations from the results then available. We were gratified to find, in the elegant neutronscattering 3 experiments recently reported by Thomlinson, Tarvin, and Passell, substantial confirmation of our conclusions. The single and important difference between these works lies in the value of the energy gap. An energy gap of 6.3°K (0.55 meV) is deduced from the neutron-scattering data; our data yielded 4.5°K The neutron experiments, having been carried out at only one temperature, did not confirm our finding that the dispersion-curve parameters are temperature independent.
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The results of a systematic study of helium thin-film adsorption are presented. Measurements are reported, obtained with a 24-MHz crystal adsorptometer, on the adsorbed mass per unit area as a function of the three independent variables temperature, concentration, and pressure of the gas of 3He-4He atoms generating the adsorbed film. The investigations span concentrations from zero to unity, all pressures up to the dew point for each mixture, and temperatures from 1.3 to 2.1 K. From the experimental results we display the detailed form of the van der Waals attractive potential holding the helium atoms to the surface. The expected dependence on the cube of the distance from the substrate is confirmed over the range of distunces explored. The strength of the van der Waals potential is measured and reported. This attractive potential is demonstrated to be of the same form and of the same strength both for the 3He atoms and for the 4He atoms in the mixture at all concentrations. We further deduce, directly from our measurements, the detailed relationship between o', the mass per unit area of film absorbed, and d, the thickness of the film.
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