RE-ORIENTATION TRANSITION IN MOLECULAR THIN FILMS: POTTS MODEL WITH DIPOLAR INTERACTION

Danh-Tai Hoang (1) , Maciej Kasperski (2) , H. Puszkarski (3) , Hung The Diep (1)

1 LPTM - UMR 8089 - Laboratoire de Physique Théorique et Modélisation
2 Surface Physics Division, Faculty of Physics
3 Surface Physics Division, Faculty of Physics, A. Mickiewicz University, Poznan, Poland

We study the low-temperature behavior and the phase transition of a thin film by Monte Carlo simulation. The thin film has a simple cubic lattice structure where each site is occupied by a Potts parameter which indicates the molecular orientation of the site. We take only three molecular orientations in this paper which correspond to the 3-state Potts model. The Hamiltonian of the system includes: (i) the exchange interaction $J_{ij}$ between nearest-neighbor sites $i$ and $j$ (ii) the long-range dipolar interaction of amplitude $D$ truncated at a cutoff distance $r_c$ (iii) a single-ion perpendicular anisotropy of amplitude $A$. We allow $J_{ij} =J_s$ between surface spins, and $J_{ij}=J$ otherwise. We show that the ground state depends on the the ratio $D/A$ and $r_c$. For a single layer, for a given $A$, there is a critical value $D_c$ below (above) which the ground-state (GS) configuration of molecular axes is perpendicular (parallel) to the film surface. When the temperature $T$ is increased, a re-orientation transition occurs near $D_c$: the low-$T$ in-plane ordering undergoes a transition to the perpendicular ordering at a finite $T$, below the transition to the paramagnetic phase. The same phenomenon is observed in the case of a film with a thickness. We show that the surface phase transition can occur below or above the bulk transition depending on the ratio $J_s/J$. Surface and bulk order parameters as well as other physical quantities are shown and discussed.

Mots clés: Molecular Thin Films; Re-Orientation Transition; Potts Model; Monte Carlo Simulation; Phase Transition

Domaines: Mécanique statistique [cond-mat.stat-mech]

Danh-Tai Hoang, Maciej Kasperski, H. Puszkarski, Hung The Diep. RE-ORIENTATION TRANSITION IN MOLECULAR THIN FILMS: POTTS MODEL WITH DIPOLAR INTERACTION. 2012. ⟨hal-00727105⟩