Optical responses in a one-dimensional two-band Peierls-Hubbard model near 5/8 filling (i.e., near 1/4 filling in the effective one-band model) are studied within inhomogeneous Hartree-Fock and random-phase approximations. Low-energy collective excitations are produced by a sliding motion of a spin-density wave. In the commensurate case (i.e., at 5/8 filling) or near this filling with moderate strength of intersite electron-lattice coupling, the spin-density wave (with polaronic defects in the latter case) is pinned by commensurability with the lattice, and the lowest-frequency optical response lies in the infrared regime. Otherwise, the spin-density wave is incommensurate and moves freely unless it is pinned by extrinsic impurities. Implications for midinfrared absorptions observed in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ compounds are discussed. With sufficiently weak hopping between the Cu and O sites, a ferromagnetic phase is found.

• Received 14 April 1993

DOI:https://doi.org/10.1103/PhysRevB.48.3349