Microcalorimetry study of the monolayer 4 He ordering transition on single crystal graphite

Abstract

We have developed a low-temperature, ac microcalorimeter for exploring adsorption on a single 2.25 mm 2 graphite leaf that is capable of 10 picoJoule/degree resolution. The microcalorimeter was used to determine the phase diagram and heat capacity critical exponents α of monolayer 4 He films at the commensurate ordering transition. After in situ baking at 600 K, we reproduced the narrow-ordered phase region (≃1% in coverage) reported by Campbell and Bretz for HOPG, but find quasilogarithmic, rather than power law, heat capacity divergences. We argue that the disappearance of the Potts-like exponent in the heat capacity is attributable to the geometry of nucleation along cleavage edge planes present on single crystal graphite surfaces.