Excess of low-energy excitations in glasses

Low-frequency Raman spectra of metaphosphate glasses (Sm₂{O}₃)ₓ(P₂{O}₅)_1-x, (Eu₂{O}₃)ₓ(P₂{O}₅)_1-x, (Gd₂{O}₃)ₓ(P₂{O}₅)_1-x, and GeO₂ have been measured in a wide temperature range from T{}10 K to the respective glass transition temperatures T{}1000 K. Analysis of the Raman data in comparison with complementary low-temperature specific heat, and ultrasonic measurements reveals that the temperature and frequency dependences of the quasielastic scattering and boson peak, as well as anomalous low-temperature specific heat, are in qualitative agreement with the predictions of soft potential model (SPM). In the case of the spectral density of ``excess'' soft modes, which are assumed to coexist and interact with ordinary phonons, the agreement can even be made quantitative under the restricted assumptions that the low-frequency Raman scattering reflects the total (phonon + excess) density of states, and the photon-vibration coupling constant is approximately linearly dependent upon frequency. However neither of these assumptions are compatible with recent formulations of SPM.