Low-frequency Raman investigation of tunnelling states in glasses

Low-temperature Raman scattering measurements have been carried out in two oxide glasses. In the low-frequency range; the heavy-metal oxide glass (NbTa) exhibits a characteristic temperature dependence similar to that recently found in another glass from the same glass family (LaSF7). Indeed, an increase in the scattered intensity is again observed below 10K, suggesting a coupling of the light with tunnelling states by resonant interaction. On the contrary, no evidence of such effect has been found in the silicate lead glass (PbO). One explanation for these two distinct behaviours may originate from the existence of polar impurities or electronic defects in LaSF7 and NbTa glasses. As these heavy-metal oxide glasses present an efficient photoluminescence contribution at low temperatures related to the chemical components, we have checked that the Raman increased effect at low frequencies cannot be attributed to a residual photoluminescence contribution.