Heat Capacities of Ar, Ne, He-4-Ar, and He-4-Ne Films Adsorbed on Cu at Low Temperatures

Heat capacities of Ar and Ne monolayers adsorbed on Cu are measurable at 1-4 K. Temperature dependences are similar to the low-temperature law for harmonic oscillators, but the empirical characteristic temperatures of 11-15 K are much lower than can correspond to independent adatoms oscillating in substrate sites. The magnitudes and temperature dependences do not match those predicted for surface modes of an absorbent coated with inert impurities or for the collective modes of epitaxial films. A model involving the excitations in non-epitaxial films appears to offer an explanation of the observed magnitudes and temperature dependences. The heat capacities of He-4 adsorbed on preplated Ar or Ne show little variation with the number of Ar layers, or between Ar and Ne coatings.