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Random-phase-approximation approach to phonon spectra in doped and undoped halogen-bridged transition-metal compounds.

We have developed a method based on a real-space random-phase approximation (RPA) to study the phonon spectra of doped and undoped halogen-bridged transition-metal-chain complexes. The method is very convenient in cases of spatially inhomogeneous mean-field structures. Thus, we are able to find the usual extended phonon modes as well as local modes associated with polaron, bipolaron, kink, exciton, or impurity structures in intrinsically or extrinsically doped systems. The corresponding infrared absorption spectra and Raman spectra (as a function of the excitation energy) are calculated. We demonstrate the method for a homogeneous charge-density-wave system, and for electron and hole polarons. We find that each type of defect has specific signatures in both infrared and Raman spectra, which can help to identify different types of defects experimentally.