A calorimetric and vapor-pressure study of the behavior of 'He adsorbed on Grafoil III the temperature range 4 to 15 K is reported. These results have been combined with preexisting low-temperature heat-capacity data to form a complete thermodynamic description of the film, The Grafoil substrate evidently consists almost entlrely of basal-plane graphite, with only a small fraction of energetically distinct adsorption sites. Techniques of semiempirical modelling are introduced which make rt possible to eliminate the effects both of these inhomogeneitres and of the formation of second and higher layers, so that a detailed picture may be formed of the behavior of the 'He first layer on an ideal graphite substrate. The binding energy of a 'He atom on the graphite substrate is reported to be 143 i 2 K. with a first excited state at 89 3 K. A lattice-gas ordering transition occurs near 3 K at
We report the first results of a thermodynamic analysis of data for ^He adsorbed on Grafoil. The ^He-graphite binding energy is in excellent agreement with a prediction made on the basis of '^He-graphite atomic scattering data.
It is shown experimentally that the superfluid fraction p,/p is continuous and finite a t the point at which superflow vanishes in unsaturated helium films. It follows that there is a region of superfluidity without superflow. In addition it is shown that the behavior of the partial molar entropy may account for the disappearance of superflow without r equiring that p ,/p vanish.The onset of superfluidity in unsaturated helium films has long been the subject of study and controversy. At each temperature below the bulk lambda point, superfluid properties a r e observed to vanish at some pressure below the bulk vapor pressure. The transition curve from various measurements is shown in Fig. 1. The question of the nature of the transition has been thrown into new perspective by recent measurements of the velocity of third sound1 and of the critical velocity of persistent currents2 in unsaturated films. In these studies values of the superfluid fraction p s / p a r e deduced and in both measurements found to be finite at the onset.If one supposes the onset curve in Fig. 1 to separate the normal and superfluid phases in the film, a finite value of p s / p at the superfluid side implies a first-order phase transition. A firstorder phase transition has also been predicted by Amit3 on independent theoretical grounds. Assuming a first-order phase transition it can be shown4 that where T i s temperature, P pressure, S molar entropy, R the gas constant, N the amount adsorbed per unit area, and the subscripts I, 11, and 0 refer to normal film, superfluid film, and
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