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RPA Approach to Phonon Excitation Spectrum of MX-Compounds

The phonon spectra of doped and undoped halogen bridged transition metal (MX) complexes are studied by using a random phase approximation (RPA) for inhomogeneous mean field structures. We find the existence of both local and extended phonon modes associated with (bi)polaron or kink formation in intrinsically and extrinsically doped systems. The corresponding infrared absorption spectrum and Raman spectrum (as a function of the excitation energy) are also calculated. We find that the local phonon modes exhibit strong resonance Raman scattering for certain excitation energies corresponding to electronic gap states. These results are discussed and compared with the experimental data on MX compounds.

RPA, Phonons, IR, Raman

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