Continuous orientational ordering transition of carbon monoxide on graphite

Abstract

In the compressed incommensurate phase at low temperatures, the molecular axes of CO on graphite are ordered with respect to each other, forming a pinwheel structure. A sharp heat capacity peak is found upon warming through the (orientational) order-disorder transition of this pin wheel phase. Critical analysis of the peak indicates that this transition is continuous and belongs to the two-dimensional Ising universality class, in agreement with theoretical expectations.