Quantum lattice effects in mixed-valence transition-metal chain complexes

Inspired by the recent observation of intrinsically localized vibrational modes in halide-bridged transition-metal chain complexes [Swanson et al. Phys. Rev. Lett. 82, 3288 (1999)], we study strong-coupling effects between electronic and lattice degrees of freedom on the basis of a two-band, 3/4-filled Peierls-Hubbard model. Combining a very efficient Jacobi-Davidson algorithm with the maximum entropy method, the low-energy physics of the Peierls-Hubbard model is analyzed in finite chains with high accuracy, preserving the full dynamics of the Raman- and infrared-active phonon modes. Results for several experimental observables, including the valence disproportionation, local magnetic moments, lattice distortions, spin and charge structure factors, and optical response are discussed. The redshift of the overtone resonance Raman spectrum is calculated to be in quantitative agreement with the experimental data found for isotopically pure ${\mathrm{Pt}}^{37}\mathrm{Cl}.$ Most significantly, the numerical results provide clear evidence of the existence of spatially localized multiphonon bound states in quasi-one-dimensional charge-density-wave systems with strong electron-lattice coupling.