Collective Infrared Excitation in LuB12 Cage-Glass

Abstract

By measuring room temperature infrared (40–35000 cm^–1) reflectivity of metallic LuB12 single crystals with different isotopic compositions (^natB, ¹⁰B, ¹¹B), we find that to model the spectrum we had to introduce, additionally to Drude free-carrier component, a broad excitation with unusually large dielectric contribution (Δε = 8000 ± 4000), which is characterized by a non-Lorentzian lineshape. It is suggested that the origin of the excitation is connected with cooperative dynamics of Jahn–Teller active B₁₂ molecules producing quasilocal vibrations (rattling modes) of caged lutetium ions. The coupling of the Lu³⁺ rattling motions with the charge carriers of conduction band is proposed to be the reason of strongly damped character of the excitation.