Dipolar and orientational glass formation in two-dimensional films of CO mixtures

Abstract

Heat-capacity and dielectric measurements have been performed to study orientational and dipolar order and disorder phenomena in submonolayer films of CO and binary mixtures of CO physisorbed on graphite and boron nitride (BN). With both methods transitions to quadrupolar and dipolar ordered low-temperature phases have been found for pure CO. Calorimetric measurements revealed that the dipolar transition of CO on graphite belongs to the universality class of the two-dimensional (2D) Ising model. It will be shown that in dilute mixtures of CO with CH 4 molecules a dramatic suppression of this phase transition occurs. This effect turned out to be much stronger than that previously observed in (CO) 1− x (N 2) x and (CO) 1− x (Ar) x mixtures, which could be successfully described by the random-field theory. From dielectric experiments first information on the dynamics of CO and (CO) 1− x (Ar) x films was gained and activation energies and attempt frequencies for reorientation processes of CO determined. Both employed methods consistently indicate that at impurity concentrations beyond 6–7% dipolar and orientational glass states are formed.