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Résumé:

Molecular nitrogen (N2) adsorbed on graphite reveals a very rich phase diagram (Fig.1). In particular, at low temperatures a variety of structures appears, depending on surface coverage. The sequence of structural phase transitions at densities close to monolayer is already well understood and described in the literature [1-4]. Therefore, our aim is to investigate the phase transitions in multilayer systems. We want to understand whether the presence of subsequent layers modifies the mechanism and temperature of rotational phase transition and melting within a particular layer. We present here the canonical Monte Carlo simulations of full N2 monolayer absorbed on corrugated surface of graphite and its evolution as a function of the subsequent layers adsorbed on the top of the previous one. The calculations have been performed for a maximum of 4 layers. At low temperature and full monolayer coverage the N2 molecules form so called herringbone structure, commensurate with graphite. It consists of two sublattices of molecules. In solid phase, in both sublattices the molecules are parallel to each other and molecular centers of mass coincide with centers of graphite hexagons. Between the sublattices, the molecules are perpendicular (Fig.2). On the top of the first layer (sites A on Fig.3) a second one is formed, of the same structure but shifted by the 1.25 Å in x direction (sites B). The third layer can be added in two ways: by putting it above the first one (AB A hexagonal-like sequence) or above the hexagons that are still “empty” (sites C, ABC cubic sequence). The four-layer structures are, in consequence, ABAB or ABCA. Both types of molecular stacking has been investigated. Several order parameters and distributions have been analyzed to follow the phase changes as a function of the temperature. The results show that the temperature and mechanism of phase transitions vary with surface coverage and change from layer to layer.