Spectroscopy of Disordered Materials

Abstract

<p>Mixed crystals of (KBr)₁_ᵪ(KCN)ᵪ show a complicated structural behavior at low temperatures which includes an intriguing orientational glass state. Better understanding of these relatively simple disordered systems may improve our understanding of the glassy state in general.</p> <p>We have used the Brillouin spectroscopy to measure the acoustic phonon properties in the KBr-KCN alloys. All aspects of the spectra, such as Brillouin shift, linewidth and intensity, were found to qualitatively agree with Michel's microscopic theory. From the frequency and the width of the acoustic phonon, we determined the reorientation rate of CN‾ quadrupoles as a function of temperature, which can be described by an Arrhenius law.</p> <p>Quasi-elastic light scattering in (KBr)₁_ᵪ(KCN)ᵪ was measured using a high resolution tandem Fabry-Perot interferometer. The width of the quasi-elastic peak was found to have a strong temperature dependence. We suggest that this peak is dynamic in origin and is related to the reorientation of the cyanide dipoles.</p> <p>We observed the appearance of symmetry forbidden scattering peaks in the Brillouin spectra of (KBr)₁_ᵪ(KCN)ᵪ in the presence of a uniaxial stress. This effect can be explained as a stress-induced birefringence altering the polarization state of the scattering light. There are no observable changes in Brillouin shift and width induced by such a stress.</p> <p>The dielectric loss and capacitance of an x=0.5 sample was determined using a high precision bridge. Only one loss peak appears at "30 K in the temperature interval from 1.2 K to 294 K, which indicates that there is no coupling between quadrupole and dipole moments at around quadrupolar freezing temperature ("80 K).</p> <p>A calorimetric technique was used to investigate thermal properties of (KBr)₀.₅(KCN)₀.₅ at T≤1 K. The specific heat of the sample is in good agreement with reported values. Samples which have undergone different thermal treatments show different heat drift behavior. Analysis of the time-decay of temperature after application of a heat pulse reveals that the decay curve follows a stretched exponential form.</p> <p>Finally, the acoustic sound wave in propylene glycol was studied using a Brillouin scattering technique. The relaxation rates obtained in the gigahertz frequency range were compared with the low frequency specific heat data in terms of the Vogel-Tamman-Fulcher law and the scaling law. The latter was predicted by recent hydrodynamic theories for the glass transition.</p>