BIPOLARONS AND POLAREXCITONS IN CUO3-CHAINS

Nonlinear objects as bipolarons and polarexcitons are discussed for a quasi-onedimensional doping subsystem, possibly generic for all hole doped HTSC's. Based on LCAO-deformational studies of CuO3 chains in YBa2Cu3O7 and on known phonon frequencies, an effective two band Hamiltonian with two degrees of freedom, including both intra- and intersite electron-lattice interaction, has been proposed and examined in detail for various commensurate band filling ratios (v = 1/2, 1/3, 1/4, and 1/5 for the antibonding chain band). Well localized stable bipolarons with negative formation energies and low energy biexcitons have been found. Consequences for a microscopic scenario of a boson-fermion (s-channel) approach to HTSC proposed in ref. [1] and/or an excitonic enhancement of a standard t-channel as well as available experimental data are briefly discussed.