On the nature of the orientational transition of monolayer N2 on graphite

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Abstract
The character of the orientational transition of monolayer N2 on graphite is investigated using a Monte Carlo method with deformable boundary conditions and a mean field approach involving the rotational susceptibility. The calculated specific heat and various order parameters predict a transition temperature TC=25 K, in good agreement with experiment. The results indicate, however, that the transition from static herringbone order to disorder takes place in stages over a 6–7 K temperature range, in which a local mode involving the rotation of individual molecules through a planar angle of π is activated, followed by a cooperative oscillation through π/2 in which the two herringbone sublattices are interchanged. The transition is identified by a loss of long range order. Above TC the correlation length is reduced to LC=4.2±0.5 Å at T=28 K. It is observed that quadrupolar short ranged correlations persist above TC to near melting.