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VOLUME 107 | ISSUE 2 | PAGE 108
Collective infrared excitation in the cage-glass LuB12
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 (\Delta\varepsilon=8000\pm4000) 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 quasi-local 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.