W. M. Du

Depolarized light scattering spectroscopy of glassforming liquids: Experimental tests of MCT

Abstract Light scattering investigations of supercooled liquids approaching the liquid-glass transition have revealed susceptibility spectra strikingly similar to those predicted by the mode coupling theory. The experimental procedure is described, and fits of the data to both the idealized and extended versions of the theory are reviewed.

Relaxation dynamics in orthoterphenyl: comparing βK from extended mode coupling theory and phenomenological analyses

Abstract Depolarized light scattering susceptibility spectra of orthoterphenyl (OTP) were analyzed using the idealized and extended versions of mode coupling theory (MCT). The extended MCT fits were combined with KWW fits to the α peaks to provide combined α–β fits from which the KWW stretching coefficient βK was determined. βK was found to be nearly independent of temperature, in agreement with the asymptotic prediction of MCT. The spectra were also analyzed with a five-function phenomenological model developed by Patkowski et al. [J. Non-Cryst. Solids 172–174 (1994) 52]. While this model gives excellent fits to the spectra, we found that it is not able to determine βK uniquely unless other parameters are artificially constrained. Here, and in comparing light scattering and dielectric data for salol, we find the claim that βK approaches 1 at high temperatures not to be justified by the experimental data.

Light-scattering study of β-relaxation in CaKNO3 and salol near the liquid-glass transition: idealized and extended mode coupling theory analysis☆

Abstract Depolarized light-scattering measurements have revealed the dynamics of CaKNO 3 and salol near their liquid-glass transition temperatures in the frequency range of ∼ 0.2 GHz to ∼ 4 THz. The two-step relaxation process predicted by the mode coupling theory was observed in the supercooled liquid near T c . Idealized mode coupling theory analysis for the β-relaxation region provided reasonable agreement with the scaling predictions. The crossover temperature was identified from both above and below T c . However, systematic differences between the experimental data and the idealized theoretical β-spectra were observed at low frequencies for temperatures near and below T c . An extended mode coupling theory analysis, which includes the phonon-assisted hopping transport, provided a good description of the complete experimental β-spectra. The hopping rate was quantitatively estimated from the analysis. Transition regions and trajectories were found in the temperature-hopping rate plane.

The intrinsic ⅛ lock-in improper ferroelectric phase in thiourea: dielectric susceptibility and hysteresis loop analysis

Abstract The complex dielectric constant of thiourea has been measured in the temperature range from 80 K to 300 K with different measuring-field levels and frequencies, with and without d.c. biasing fields. The temperature interval between the two peaks in the e*a(T) curve near 178 K was found to extrapolate to 0.7 K at zero measuring and biasing fields, which indicates that the ⅛ ferroelectric lock-in phase is intrinsic in a non-zero temperature range. The E-T phase diagram near the ⅛ lock-in region was explored by investigating the effects of biasing fields. D-E hysteresis loops were investigated in the vicinity of the ⅛ ferroelectric phase with measuring frequencies from 0.5 Hz to 5 mHz. The different D-E loops observed clearly demonstrate the difference between the intrinsic and “field-induced” lock-in mechanisms. The Landau free energy was analyzed and shown to produce self-consistent explanations of the experimental results and of the successive phase transitions.

Brillouin and Raman scattering spectroscopy of the liquid‐glass transition

The liquid‐glass transition in calcium potassium nitrage (CKN), salol, and aqueous LiCl solution was investigated by laser light scattering spectroscopy. Combining data obtained with a Sandercock tandem Fabry Perot interferometer and a Spex tandem Raman spectrometer provided spectra spanning more than four decades in frequency. The data was analyzed to test both qualitative and quantitative predictions of the mode coupling theory of the glass transition, and produced generally excellent agreement. For CKN, we found that the MCT crossover temperature Tc is ∼378K, 45° above Tg.

Tests of the mode-coupling theory for a molten salt

Abstract A recently reported analysis (Cummins et al., 1993, Phys. Rev. E, 47, 4223) of depolarized light and neutron scattering experiments in CaKNO3 using the mode-coupling theory is extended. The temperature dependence of the critical amplitudes found in this analysis is explained.