Thermal equilibrium polarization: a near-surface effect in dipolar-based molecular crystals

Recently, observed nonlinear optical and pyroelectric effects in channel-ty pe lattices hosting aligned dipolar moelcules have been explained by a Mark ov model of polarity formation. As a result of selective attachment process es during crystal growth, two adjacent macrodomains are formed which show o pposite net polarity. Although a state near thermal equilibrium was assumed for the attachment of dipoles, resulting crystals are metastable with resp ect to the global free energy minimum of chains coupled only in one dimensi on. In the present work we demonstrate that the free energy difference of t he two ensembles (growth vs. bulk equilibrium), Delta F, can be smaller by two orders of magnitude than realistic energies of activation for a re-orie ntation of dipoles. When linear chains are subjected to thermal equilibrium , the system develops polarity as well, however, near the surface domains a t both ends chains feature opposing net polarity. The scale length of dipol ar ordering can be as large as 20 times the molecular length. Stochastic si mulations in two dimensions indicate that both scale length and net polarit y are sensitive to even small lateral interactions (Delta E-lateral/Delta E -chain < 0.01). (C) 2000 Published by Elsevier Science B.V. All rights rese rved.