Spectral diffusion on ultralong time scales in low-temperature glasses

A dynamical theory is constructed to describe spectral diffusion in glasses in the temperature range near 1 K on long time scales. The theory invokes interacting tunneling centers [two-level tunneling systems (TLS's)] which provide an excess contribution to the spectral hole width which qualitatively accounts for the deviation from standard logarithmic line broadening observed by Maier et al. [Phys. Rev. Lett. 76, 2085 (1996)]. Alternative explanation schemes of the nonlogarithmic line broadening, avoiding interacting TLS's, are discussed. We devise experimental tests which could be used to access the validity of the proposed theories.