Low-energy vibrational dynamics and connectivity in borate glasses

A study of the specific heat at low temperatures (1.5-25 K) of silver bo rate glasses has revealed the presence of an excess specific heat C p over the Debye contribution, which scales with the Ag 2 O content. This observation has been related to the presence of non-bridging oxygens in the borate network, which are assumed to be a source for additional low-energy vibrations causing the excess C p . Furthermore, comparative measurements of C p and low-frequency Raman scattering (below 100 cm -1 ) at low temperatures in the same borate glass permit one to assess the frequency dependence of the photon-vibration coupling coefficient C(v) and the spectral density of low-energy vibrational modes g(v). By comparison with the results obtained by the same procedure for vitreous SiO 2 , we show that a decreasing connectivity of the network leads to an increasing magnitude and softening of g(v).