Low-lying Resonant Modes in Anharmonic Crystals

Abstract

With respect to the anharmonic properties , the U-center localized mode has received till now much more attention ⁽¹⁾ than the low-lying resonant modes induced by weakly bound impurities. Nevertheless, some recent works on the stress effect ^{(2, 3)}, the sidebands⁽⁴⁾, and the temperature dependence ⁽⁵⁾ of the resonant infrared (IR) absorption give evidence that the resonant-mode properties are influenced by the local anharmonicity to a much larger extent than the local modes. This statement seems to be supported also by the fact that the harmonic theory predictions for the isotope shifts disagree with experiments; such disagreement seems indeed to be larger for resonant-modes^(5–7) than for gap or local modes⁽⁸⁾. Here, our purpose is to investigate the above properties on the basis of a simple anharmonic theory and to discuss briefly the KBr:Li isotope shift and the off-center ion problems, where anharmonicity is found to play an important role.

Work supportedby EOAR under Grant N. 67–08 with the European Office of Aerospace Research, U. S. Air Force.