Effect of Dipolar Interaction in Molecular Crystals

TL;DR

This paper studies how dipolar interactions influence phase transitions in molecular crystals using a Potts model, revealing that the first-order transition persists even with strong dipolar effects.

Contribution

It introduces a model incorporating dipolar interactions into the Potts framework and demonstrates their impact on the ground state and phase transition nature.

Findings

Dipolar interactions lead to layered ground states with specific periodicity.

The phase transition remains first order at large cutoff distances.

Ground state configurations depend on dipolar amplitude and cutoff distance.

Abstract

We investigate in this paper the ground state and the nature of the transition from an orientational ordered phase at low temperature to the disordered state at high temperature in a molecular crystal. Our model is a Potts model which takes into account the exchange interaction $J$ between nearest-neighbor molecules and a dipolar interaction between molecular axes in three dimensions. The dipolar interaction is characterized by two parameters: its amplitude $D$ and the cutoff distance $r_c$. If the molecular axis at a lattice site has three orientations, say the $x$, $y$ or $z$ axes, then when D=0, the system is equivalent to the 3-state Potts model: the transition to the disordered phase is known to be of first order. When $D\neq 0$, the ground-state configuration is shown to be composed of two independent interpenetrating layered subsystems which form a sandwich whose periodicity depends on $D$ and $r_c$. We show by extensive Monte Carlo simulation with a histogram method that the phase transition remains of first order at relatively large values of $r_c$.