Temperature dependence of optical linewidths and specific heat of rare-earth-doped silicate glasses

Optical dephasing and specific heat measurements were performed on two rare-earth-doped silicate glasses. The effective homogeneous linewidth of the optical transition, Γₕₒₘ was compared to the two-level-system (TLS) contribution to the specific heat, CᵥTLS, between 0.4 and 12 K. Both Γₕₒₘ and CᵥTLS vary as T^α with the same value of {α} for a given rare-earth/glass sample, but with different {α} for the two samples ({α}=1.3 for the Pr³⁺/glass, {α}=1.0 for Eu³⁺/glass). From the ratio of Γₕₒₘ to CᵥTLS 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.