Relaxations, low-energy excitations and specific heat in phosphate glasses

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Abstract
We have obtained low-frequency Raman spectra of samarium phosphate glasses (Sm2O3)x (P2O5)]1-x in a wide temperature range from 11 K up to the glass transition temperature (T g ≈ 910 K) and measured their low-temperature specific heat, in an attempt to test the predictions of some recent models, the soft-potential (SP) model and the mode-coupling (MC) model, addressing low-energy modes in glasses. The temperature and frequency dependence of the boson peak (BP) and the quasielastic scattering (QS), both features characteristic of glasses in the low-frequency range, were determined. In addition, at high temperatures (above 600 K) we have detected a strong structureless background, which is attributed to higher-order Raman scattering. Taking the background into account, the BP changes with temperature in accordance with the Bose-Einstein population factor over the investigated temperature range. The temperature dependence of the QS is found to be consistent with the suggestions of the SP and MC models. Comparison of low-temperature Raman and heat capacity data indicates the presence of excess vibrational modes which, if assumed to be distinctly different in nature from phonons, coexist with Debye-like modes of comparable spectral density. The frequency dependence of the estimated excess density of states is compatible with the SP predictions. However, the light-vibration coupling coefficient is found to be approximately linearly dependent on frequency, in contrast with recent assumptions of SP.