Very Low-Temperature Specific Heat of Submonolayer Helium Films

Abstract

levels as well as to the ground state. The attractiveness in extending this technique to rare-earth crystalline systems is evident. In crystals, the lowest level of a multiplet generally relaxes by radiative decay or by multiphonon emission to the next lowest multiplet, and can have a homogeneous linewidth of less than 1 MHz. The limitation to the spectroscopic resolution is then the inhomogeneous width, which is generally of the order of 0.1 cm" 1 . Since at low-enough concentrations the spectral diffusion is sufficiently arrested, a tunable coherent source, such as a dye laser or parametric oscillator, can be used to match the sharp rare-earth levels and thereby observe the FLN phenomenon. The possibility of using nonresonant excitation significantly extends the number of levels which can be studied by the FLN technique, with the additional experimental convenience that the emission is well separated in wavelength from the excitation. Application of Szabo's FLN technique to nonresonantly excited rare-earth systems should permit optical studies of a wide range of excited levels with resolutions of greater than 10 7 .The specific heats of He 3 and He 4 films adsorbed on exfoliated graphite have been measured between 0.04 and 4.2 K, for fractional monolayer coverages between 0.25 and 0.63. He 3 films show anomalies around 0.1 K that are absent in the He 4 results. A possible explanation is given in terms of spin ordering of the He 3 atoms.Recent experiments have 1 ' 2 shown that submonolayer coverage He 3 and He 4 films adsorbed on exfoliated graphite (Grafoil 3 ) have substantial lateral mobility, to the extent that they behave like ideal two-dimensional (2D) gases at temperatures around 4 K. In the range between \ and - § of a monolayer, published results can be summarized in the following way.For fractional monolayer coverages x of about £ of a monolayer, He 4 heat-capacity measurements show a peak around 1.2 K while He 3 measurements show no peak; both tend to C/Nk -1 at 4 K. The He 4 results have been interpreted 4 as quasi-2D Bose condensation produced by a weak inhomogeneity in the substrate adsorbing potential. The He 3 experimental curves were fitted by curves for a 2D ideal Fermi gas with high degen-eracy temperatures (for example, T F -3.4 K for x =0.26) provided that a small adjustment was made in the number of particles responsible for the signal.At higher x large heat-capacity peaks appear for both isotopes at coverages that closely correspond to the helium atoms being in registry with the graphite lattice of the substrate, the most notable being the X-type peak occuring for He 4 at x =0.6, which is equivalent to f of the hexagons of the graphite lattice being occupied by helium atoms.Substantial mobility of helium atoms adsorbed on graphite is expected theoretically. 5 The ordering transition in registry with the substrate is receiving careful treatment 6 since it resembles a 2D lattice-gas order-disorder transition. 789

Two-Dimensional Rotons

“…In Table I, I list the minimum value of EjE x (which generally occurs slightly above k 0 ). This result is in con- 9 7 who found, using the perturbation approach of Jackson and Feenberg, 8 only a minute correction to the Feynman spectrum. The present result is, however, in much better agreement with experiment.…”

“…The implication of the present work is that the real excitation spectrum lies much lower, with a consequent enhanced specific heat. In order to check this idea I calculated the entry A exp for p = 0.6p 0 in Table I, which is the roton gap estimated from the specific-heat data 9 in the vicinity of T~0.7 K. In another experiment, 12 in which the specific heat of the second layer of helium plated on Vycor glass was measured, the data were found to fit an exponential with A exp «6 K; this value is entered in Table I for the areal density p= 1.2p 0 . These results are consistent with my calculations and indicate that the accuracy of the FC method for two dimensions is comparable to that for three dimensions.…”