Dipolar order and disorder phenomena in pure CO and dilute (CO) 1− x(Ar) x mixtures physisorbed on graphite

Abstract

A detailed heat capacity study has been performed to characterize the properties of pure CO and (CO) 1− x (Ar) x mixture films physisorbed on the basal planes of graphite. The phase diagram of CO monolayers will be presented. At T c=5.18 K a phase transition to a novel low temperature phase is found with head-tail (dipolar) order of the molecules in an orientationally ordered herringbone structure. The phase transition is shown to belong to the 2D Ising universality class with anisotropic interactions between the molecules. Diluting CO weakly with Ar impurities leads to a dramatic destruction of the phase transition. This effect can be quantitatively described by the 2D random field Ising model. It is demonstrated that random fields are the source of head-tail disorder for weakly diluted CO–Ar mixtures. An anomalous shift of the transition temperatures to higher values with growing Ar composition is observed, which may be attributed to the special interactions between both components of the mixture. At Ar concentrations above 7% the system gradually freezes into a 2D dipolar glassy phase at low temperatures. Pure CO and (CO) 1− x (Ar) x mixture films can be regarded as experimental realizations of the 2D Ising model and the 2D random field Ising model respectively.