Rattling ions and the anomalous vibrational dynamics of glasses

We present low-temperature specific heat and low-frequency Raman measurements in borate glasses and crystals. We provide evidence of the existence of localized modes due to guest atoms that overlap significantly with the Boson peak. A model to evaluate the frequency of these local excitations is proposed, and its application to the investigated glasses is discussed. Our results show that the contribution of weakly coupled atoms needs to be considered for an accurate investigation of the relationship between the boson peak and elastic properties.