Low-frequency Raman scattering on different types of glass formers used to test predictions of mode-coupling theory.

Abstract

We have measured low-frequency Raman scattering (RS) spectra of the glass formers ${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$, glycerol, m-tricresyl phosphate (m-TCP), and ${\mathrm{K}}_{3}$${\mathrm{Ca}}_{2}$(${\mathrm{NO}}_{3}$${)}_{7}$ in a broad temperature range above the glass transition temperature. The basic difference between the spectra is the ratio of relaxational to vibrational contributions: the more fragile the glass, the smaller is the vibrational contribution. Features of the relaxational contributions are quite similar. Testing predictions of mode-coupling theory (MCT) for m-TCP, we find that the dynamics is well described, and as the critical temperature we find ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}260 K. In the case of glycerol, due to the strong vibrational contribution to the RS spectrum, a consistent interpolation of the rescaled data is only possible if other exponents than those predicted by MCT are applied; however, the MCT exponents are obtained in a narrow frequency range, provided the contribution of the \ensuremath{\alpha} process is subtracted first. Thus, MCT describes the dynamics for \ensuremath{\nu}300 GHz even in the nonfragile glass former glycerol. We extract ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}310 K. Combining the RS results with those of NMR in the case of m-TCP, we further find (i) a change of stretching parameter close to ${\mathit{T}}_{\mathit{c}}$ and (ii) a change of the temperature dependence of the Debye-Waller factor at T\ensuremath{\sim}280 K which is close to ${\mathit{T}}_{\mathit{c}}$ for this system.