Glass-like two-level systems in minimally disordered mixed crystals
  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. 
J. P. Wrubel, B. E. Hubbard, N. I. Agladze, A. J. Sievers, P. P.   Fedorov, D. I. Klimenchenko, A. I. Ryskin, J. A. Campbell