Ordered Helium Films on Highly Uniform Graphite—Finite-Size Effects, Critical Parameters, and the Three-State Potts Model

The heat capacity of the ordering transition of helium films is studied on a new graphite substrate whose microcrystallite allignment and lateral size are ten times that for Grafoil. Transition peaks are considerably sharper than those observed on Grafoil allowing determination of critical parameters $\alpha \cong 0.36$ and $\frac{A^{+}}{A^{-}}\cong 0.65$. The $\alpha$ is substantially larger than recent three-state Potts-model calculations in two dimensions (of which the ordering transition is the only example), raising the possibility that the ordered state belongs to a new university class.