Nonlinear dynamics of ferroelectrics with three-well local potential

For Sn$_2$P$_2$S$_6$ ferroelectrics, the appearance of spontaneous polarization is related to stereoactivity of tin cations and valence fluctuations of phosphorous cations. Here, the continuous phase transition and its behavior under pressure is determined by thee-well local potential and can be described in an anharmonic quantum oscillator model. For such a model, the spectrum of pseudospin fluctuations at different temperatures and pressures has been calculated and compared with the data of Raman spectroscopy. It was revealed that the ferroelectric lattice instability is related to several low energy optic modes.Comment: 8 pages, 8 figure

Highly efficient acoustooptic diffraction in Sn2P2S6 crystals

We have studied the acoustooptic (AO) diffraction in Sn2P2S6 crystals and found that they manifest high values of AO figure of merit. The above crystals may therefore be used as highly efficient materials in different AO applications.Comment: 5 pages, 1 figur

Lattice models and Landau theory for type II incommensurate crystals

Ground state properties and phonon dispersion curves of a classical linear chain model describing a crystal with an incommensurate phase are studied. This model is the DIFFOUR (discrete frustrated phi4) model with an extra fourth-order term added to it. The incommensurability in these models may arise if there is frustration between nearest-neighbor and next-nearest-neighbor interactions. We discuss the effect of the additional term on the phonon branches and phase diagram of the DIFFOUR model. We find some features not present in the DIFFOUR model such as the renormalization of the nearest-neighbor coupling. Furthermore the ratio between the slopes of the soft phonon mode in the ferroelectric and paraelectric phase can take on values different from -2. Temperature dependences of the parameters in the model are different above and below the paraelectric transition, in contrast with the assumptions made in Landau theory. In the continuum limit this model reduces to the Landau free energy expansion for type II incommensurate crystals and it can be seen as the lowest-order generalization of the simplest Lifshitz-point model. Part of the numerical calculations have been done by an adaption of the Effective Potential Method, orginally used for models with nearest-neighbor interaction, to models with also next-nearest-neighbor interactions.Comment: 33 pages, 7 figures, RevTex, submitted to Phys. Rev.