Low-temperature specific heat in caesium borate glasses

Low-temperature specific heat measurements (1.5–25 K) performed on caesium borate glasses at different concentrations have pointed out the presence of an excess specific heat, Cp, over the Debye contribution. In a plot Cp/T3 vs. T, it appears as a bump, temperature location of which is almost unchanged whereas its intensity increases with caesium concentration. A comparison with the specific heat in sodium borate glasses has revealed highly dissimilar low-energy dynamics, despite the same structural units building up the borate matrix. The different behaviours observed have been related to the peculiar structural changes that the inclusion of alkaline cations cause in the mesoscopic length scale where the effects of the medium-range order (MRO) are usually observed. It is believed that the particularly open structure of caesium glasses assures a very low frequency for the local oscillations of Csþ ions that, consequently, rule the vibrational dynamics in the frequency region of the boson peak by their localized character.