Ac Micro-calorimetry Study Of Helium-4 Film On Single Crystal Graphite.

We studied the heat capacity of $\sp4$He monolayer films on the substrate of single crystal graphite, which possesses excellent surface uniformity; A single leaf of graphite was used for the calorimeter and a Ge metal film resistance thermometer was fabricated by vacuum deposition through a Cu-mesh mask. The Ge resistance thermometer sensitivity was controlled and optimized by varying the deposition speed and by thermal processing. For a weak link, half mil diameter NbTi wires were used with electrical contacts by silver epoxy bonding. Coverages could be controlled by equilibrium with an adjacent exfoliated graphite ballast cell. The Ge metal film resistor and weak link allowed the cell to be baked up to 300C in a low temperature environment. Several measurements of film heat capacity were made around the ordering transition. The critical exponent of the tallest peak is consistent to previous measurements of the three state Potts model when the in situ baking temperature was 250C. The shape of the curve is symmetric and along the critical line the exponents decrease as the coverage moves away from the critical density. After in situ baking at 300C a strong heat capacity asymmetry was observed around the order-disorder transition while the phase boundary remained quite sharp. The critical exponents, $\alpha$, is less than the expected exponent of the three state Potts model, and a new feature of stronger divergence around the critical temperature could be seen.PhDPhysicsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128084/2/8801294.pd