THz spectroscopy is used to identify a broad distribution of two-level systems, characteristic of glasses, in the substitutional monatomic mixed crystal systems, Ba_{ 1-x}Ca_xF_2 and Pb_{ 1-x}Ca_xF_2. In these minimally disordered systems, two-level behavior begins at a specific CaF_2 concentration. The concentration dependence, successfully modeled using the statistics of the impurity distribution in the lattice, points to a collective dopant tunneling mechanism.
Categories: cond-mat.dis-nn