S. P. S. Porto

On linewidth of phonons associated to a disorder mechanism

Abstract An expression for the linewidth of phonons associated to a disorder mechanism in crystals is deduced. The linewidth of these ‘noise’ phonons is a function of the correlation time describing the statistical behavior of the disorder mechanism. A new view point to the application of the fluctuation-dissipation theorem for order-disorder crystals is presented. The relationship between the behavior of the linewidth of these phonons and phase-transitions mechanisms is discussed.

Analysis of the relationship between temperature dependence of the libration mode and dielectric relaxation in NaNO2

Abstract The behavior of the low frequency dielectric constant of NaNO 2 as function of frequency and temperature is consistently correlated to the temperature behavior of the frequencies and linewidths of the phonons of B 1 symmetry in the crystal. The frequency of the librational ‘soft’ mode closely related to the ferroelectric-paraelectric phase transition does not go to zero but tends to a hard core value at the transition temperature. The behavior of the dielectric constant as function of temperature and frequency is explained by assuming a dielectric relaxation mechanism proposed by Mason and not by Cochrans soft phonon model.

Hard core phonon frequency at transition temperature

Abstract A simple theoretical derivation is given to show that one may expect a hard core for the phonon frequency at the transition temperature. This approach implies that the frequency of the phonon will follow ω ∝ {[1 + γ(T − T C )] V 0 } 1 2 instead of that predicted by soft mode theory. Our expression should be valid whenever a Brownian sublattice exists in the crystal.

Debye-like diffusive central mode near the phase transition in ferroelectric lithium tantalate

Abstract We report a clear cut evidence of a temperature dependent diffusive central mode associated with the ferroelectric phase transition of LiTaO3; a detailed linshape analysis shows that this central mode is a Debye-like relaxation. We also show its influence in the dielectric constant through polariton measurements for several temperatures.

Effect of internal brownian particles on phase transition of order-disorder crystalst

The behavior of the low frequency dielectric constants of NaNO2 as a function of frequency and temperature is explained by assuming a correct balance between the Lyddane-Sachs-Teller and Mason-Debye relaxation contributions to the dielectric constant. This new approach shows that in general whenever a sublattice of brownian particles exists in a crystal, the behavior of the activation energy ΔU as a function of temperature plays even a much more important role than has been considered up to now and that the fact that ΔU ≠ 0 at the transition temperature implies a hard core frequency contrary to the soft mode theory. Further implications of these ideas to dielectric and phase transition mechanisms are discussed.