Observation of random-field behavior in (CO${)}_{1\mathrm{-}\mathit{x}}$(${\mathrm{N}}_2$${)}_{\mathit{x}}$ mixtures physisorbed on graphite

Heat-capacity studies have been carried out on submonolayers of (CO${)}_{1\mathrm{-}\mathit{x}}$(${\mathrm{N}}_2$ ${)}_{\mathit{x}}$ mixtures physisorbed on graphite. We find that pure CO undergoes an end-to-end ordering transition at ${\mathit{T}}_{\mathit{c}}$=5.18 K, which is shown to belong to the two-dimensional Ising universality class. In diluting CO with ${\mathrm{N}}_2$ the heat-capacity peaks become progressively rounded until the phase transition is completely suppressed at a ${\mathrm{N}}_2$ concentration of only x=0.07. The analysis of the data shows that this behavior is in quantitative agreement with predictions of the random-field theory applied to adsorbed films.