Optical dephasing and specific heat measurements were performed on two rare-earth-doped silicate glasses. The effective homogeneous linewidth of the optical transition, I h, was compared to the two-level-system (TLS) contribution to the specific heat, C"",between 0.4 and 12 K. Both I'1, and C"" vary as T with the same value of n for a given rare-earth/glass sample, but with difFerent n for the two samples (n = 1.3 for the Pr +/glass, n = 1.0 for Eu +/glass). From the ratio of I'h to C"" for each glass, it appears that the coupling strengths to the TLS for each rare-earth optical transition are similar, but that the density of TLS differ due to their different compositions.
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