Yoshihiro Ishibashi

Note on Ferroelectric Domain Switching

A phenomenological theory is given for ferroelectric domain switching. It takes into account the initial size of a reversed nucleus and also the shape of domains. It is shown that the Avrami theorem can be easily derived if we utilize Kolmogorovs method. We solved the problems under two assumptions: one is the constant nucleation rate throughout the switching period and the order is the assumption of latent nuclei of a given definite number and no further nucleations, The i max t max / P s values of several ferroelectrics reported so far summarized and discussed in the light of the present theory.

Traffic Flow in 1D Cellular Automaton Model Including Cars Moving with High Speed

Traffic flow is simulated by one-dimensional cellular automaton (CA) model including cars moving with high speed. The simulation shows a phase transition between the moving phase and the jamming phase at p =1/( m +1) ( p : density of cars, m : the maximum number of sites by which cars advance at each time step). A mean-field theory can reproduce the average velocity found by simulations.

A theory of D-E hysteresis loop based on the Avrami model

The D - E hysteresis loop of ferroelectrics is theoretically studied on the basis of the extended Avrami theory. If the sideway velocity depends only on the instant value of the applied field, the volume fraction of the reversed area is expressed as q ( E )=1-exp (- f - d Φ( E )), where f and d are, respectively, the frequency of the applied field and the growth dimension of domains, and Φ is a function of E . This result is obtained irrespective of the waveform of the applied field and the field dependence of the sideway velocity, if the nucleation event is deterministic. For the stochastic nucleation due to thermal fluctuation, on the other hand, the above result is modified as q ( E )=1-exp (- f -( d +1) Φ( E )). The effect of the delay of wall motion is also discussed.

Self-Organized Phase Transitions in Cellular Automaton Models for Pedestrians.

Cellular automaton models for walking of pedestrians are studied in a system where one pedestrian and many pedestrians walk in the opposite direction and encounter each other on a passageway. Two models have been studied for the behavior avoiding from collision when the pedestrians meet with each other. Phase transitions on the walking behavior occur by self-organization with the critical density on the pedestrians.

Morphotropic Phase Boundary in Solid Solution Systems of Perovskite-Type Oxide Ferroelectrics

The origin of the appearance of the morphotropic phase boundary in the perovskite-type oxide solid solution systems and the increase in the dielectric susceptibilities in the vicinity of the boundary is theoretically clarified on the basis of a Landau-type free energy function. The dielectric susceptibilities are concretely expressed in terms of the model parameters, and found to diverge at the morphotropic phase boundary within the present model.

A Phenomenological Theory of the Antiferroelectric Phase Transition in Smectic Liquid Crystals

By applying the Landau theory, the appearance of the antiferroelectric phase in smectic liquid crystals and the double hysteresis loop are interpreted. Several types of phase sequences including the antiferroelectric, ferroelectric and/or ferrielectric phase are obtained phenomenologically. The D-E curves in the antiferroelectric phase are classified into three types depending on temperature and the coefficients of the fourth-order terms in the free energy.

Successive Phase Transitions in Ferroelectric NaNO2 and SC(NH2)2

The successive phase transitions from a prototype to an incommensurate phase and then to a commensurate phase are studied on the basis of a model thermodynamic potential where the instability of a one-dimensional representation is assumed. The model is suitable to a unified description of the phase transitions in SC(NH 2 ) 2 and NaNO 2 which show ferroelectricity in their commensurate phases. The incommensurate states of the model and its dielectric properties are analysed by taking harmonics into account. It has turned out that the present model can satisfactorily reproduce the transitions actually observed in SC(NH 2 ) 2 and NaNO 2 .

Switching kinetics in KNO3 ferroelectric thin-film memories

The time dependence of the current transient i(t) produced by the reversal of domains in ferroelectric potassium nitrate thin‐film memories of 75–300 nm is analyzed as a function of temperature and of thickness using the Avrami theory. For all the films the kinetics confirm the low‐dimensional nature of the system

The Origin of the Ferroelectric Phase Transition in Ammonium Sulfate

Long-wavelength lattice vibrations in ammonium sulfate are analysed group-theoretically, and symmetry coordinates of external modes belonging to the irreducible representation B 1 u , which is responsible for the phase transition from D 2 h to C 2 v , are obtained. They are composed of two groups; one is made of the purely translational and polar modes, and the other is made of the purely librational and nonpolar modes. Some peculiar properties of ammonium sulfate, including, especially, the very small value of the Curie-Weiss constant, are explained phenomenologically by a model in which a normal mode with a large component of a librational nonpolar mode admixed with a small contribution of a translational polar mode becomes soft.

Two-Sublattice Model of Ferroelectric Phase Transitions

Dielectric properties of two nonequivalent sublattice polarizations are studied. Assuming a strong coupling favoring an antiparallel alignment of polarizations, the model reveals some new features of a ferroelectric transition with no multiple change of the unit cell volume. The Curie-Weiss constant is unusually small and this fact directly accounts for a possible sign change of the spontaneous polarization by only a slight variation of model parameters with temperature. The model also can describe a para-ferroelectric phase transition followed by an isomorphous phase transition between two ferroelectric phase with no change of symmetry. Temperature dependence of susceptibility is derived with the emphasis on the critical region in which the applied pressure or electric field smears out the difference between two isomorphous phases. The model can explain some properties of ammonium sulfate, dicalcium strontium and lead propionate.