Two-level-system dynamics in doped polymer glasses below 1 K: hole burning as an optical analog to heat-release experiments

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Abstract
We have performed optical hole-burning experiments on a long time scale at very low temperatures (T = 50 mK). In accordance with the tunneling model, we have observed a logarithmic broadening behavior of the optical transitions. Also, we have found different magnitudes of hole broadening, depending on the age of the spectral holes. This effect reflects a relaxation of two-level systems after the rapid cooling procedure on a long time scale. Thus we have found an optical analog to heat-release experiments.