Glass-like two-level systems in minimally disordered mixed crystals

摘要

THz spectroscopy is used to identify a broad distribution of two-levelsystems, characteristic of glasses, in the substitutional monatomic mixedcrystal systems, Ba_{1-x}Ca_xF_2 and Pb_{1-x}Ca_xF_2. In these minimallydisordered systems, two-level behavior begins at a specific CaF_2concentration. The concentration dependence, successfully modeled using thestatistics of the impurity distribution in the lattice, points to a collectivedopant tunneling mechanism.