Boson peak in alkaline borate glasses: Raman spectroscopy, neutron scattering, and specific-heat measurements

The dependence of the boson peak on the alkaline ion in modified borate glasses (M2O)(0.14)(B2O3)(0.86) (M+=Li+,Na+,K+,Cs+) was analyzed by performing Raman spectroscopy, inelastic neutron scattering, and low-temperature specific-heat measurements. It is found that the distribution of vibrations merging into the boson peak shifts to higher frequency by going from cesium to lithium. A linear correlation between the boson peak frequency and the transverse sound velocity is evidenced. The dependence on the polarizing power of the metallic cation is analyzed, stemming from considerations about elastic moduli. These findings suggest a mainly transverse character of the excess vibrational modes in glasses.