Denis A. Semagin

Simulation of Modulated Structures in Quartz

We report the results of molecular-dynamics calculation of equilibrium modulated phases of quartz in the vicinity of pressure induced α-β transition. A numerical method suitable for simulation of modulated phases in the vicinity of transition point was developed. The phase diagram with respect to the unit cell deformations was obtained from the pairwise interatomic potentials by [Tsuneyuki et al .: Phys. Rev. Lett. 61 (1988) 869]. The results show that the modulated structures with various wavevectors appear between α- and β-phases suggesting that the softening may occur at any point on the Σ line in the Brillouin zone and that a structure with large wavevector seems to be more probable than one with a small wavevector.

Effect of weak discreteness on two-soliton collisions in nonlinear Schrödinger equation

We predict the possibility of strongly inelastic soliton collisions in a weakly discrete medium from the analysis of two-soliton solution to the nonlinear Schrodinger equation and confirm the prediction numerically. The effect turns out to be qualitatively independent on the particular type of perturbation. The conditions of mutual trapping of the colliding solitons are discussed as well.

MD Calculation for Modulated Phase in Quartz

We perform the Parinello-Rahman MD study of modulated phases in quartz. In the temperature interval of several ten degrees between f - and g -phases the three-periodic 3q-modulated phase was found. The results are used to discuss the recently proposed new interpretation of the incommensurate modulation in quartz.

Raman spectrum and the origin of phase transitions in quartz

Abstract Raman scattering and lattice dynamical calculations were performed to discuss the origin of the incommensurate(INC) phase in quartz. A broad low frequency mode was found to exist in a wide temperature range. Its intensity significantly increase in β phase on cooling towards the transition temperature. The lattice dynamical calculation suggests that this mode reflects the increase of the phonon density of TA-mode along the ∑ line. The results are consistent with the new model by Aslanyan et al. (J.Phys.Condens.Matter, 10, 4575(1998).) which claims that the INC modulation would occur first near b/3 rather than close to Γ point and that there exists another commensurate phase just above the α phase. Keywords: Quartz, Incommensurate, Phase transition, Raman scattering, Lattice dynamics

Comments on the Unsettled Problems Related to the Origin of the Incommensurate Phase of Quartz

It is shown that there are several experimental and theoretical results which are not compatible with the currently accepted model for the incommensurate (INC) phase of quartz. In this paper, we comment on the unsettled problems obtained from the light scattering spectra and computational simulations related to the new model for the INC phase.

Stable Modulated and Homogeneous Structures of Quartz (SiO 2 ): Analysis of Eigen-Modes Near Phase Transition Points

We obtain a variety of modulated and homogeneous phases of quartz, stable at zero-temperature, by means of relaxational and lattice dynamics calculation. The interatomic interaction is given by the Tsuneyuki pairwise potential. The order of appearance of the phases resembles the experimentally observed order. The essential role of the potential profile is obviated. The kinetics of phase transformations in quartz is illustrated by the analysis of dispersion curves and eigenvectors, which show some peculiarities near transition points.

Elastically hinged molecule model in physics of ferroelectric materials

Abstract The microscopic model of a crystal consisting of finite size particles is proved to be more realistic for many ferroelectric crystals than the models with pointwise particles. Two ideas naturally appeared when deriving the continuum approximations for this model. The first one is to consider a crystal as micropolar media, which is the continuum analog for the microscopic model with particles having rotational degrees of freedom. The second idea is to use the many-field approximation. A particular problem of the negative Poisson ratio, having no place in a model with pointwise particles, is also discussed. Finally, a new mechanism of the lock-in transition in a crystal supporting the incommensurate phase is offered.