Two-Level System Dynamics in the Long-Time Limit: A Power-Law Time Dependence

We have investigated two-level system dynamics in the time range from 10 to 10⁶ s at temperatures of 500 mK and 1 K. The experiments were performed by measuring spectral diffusion in a photochemical hole-burning system. The observed power-law time dependences can be interpreted in the framework of a system of strongly interacting spins which are, at short time scales, immobilized by their mutual coupling. With this conjecture we would be able to rationalize that the density of states is seemingly low at short time scales.