Solid Phase ofHe4Monolayers: Debye Temperatures and "Melting" Anomalies

Specific heats of ${\mathrm{He}}^4$ monolayers on graphite indicate two-dimensional solid phases. Debye temperatures $\Theta$ range from 17.6° to 56° systematically with coverage. Anomalies are identified with melting, but the melting process is a continuous transition. Films and bulk solid ${\mathrm{He}}^4$ at equal values of interatomic spacing have nearly the same $\Theta$'s and melting temperatures.