DOPING STATES IN THE 2-DIMENSIONAL 3-BAND PEIERLS-HUBBARD MODEL

Doping states in a two-dimensional three-band Peierls-Hubbard model fo r the copper oxides are investigated with inhomogeneous Hartree-Fock ( HF) and random-phase approximations. The doping states are sensitive t o small changes of interaction parameters because they easily change l ocal energy balance between different interactions around added holes. For the parameter values derived from constrained-density-functional methods for the copper oxides, added holes form isolated small ferroma gnetic polarons. When the parameters are varied around these values, d ifferent types of doping states are obtained: For stronger on-site rep ulsion at Cu sites, larger ferromagnetic polarons are formed, which ar e qualitatively different from the small polarons; for stronger neares t-neighbor Cu-O repulsion, polarons are clumped or there occurs phase separation into Cu- and O-hole-rich regions; intersite electron-lattic e coupling rapidly changes the small polarons by quenching a Cu magnet ic moment and locally distorting the lattice in an otherwise undistort ed antiferromagnetic background. This is regarded as a rapid crossover from a Zhang-Rice singlet to a covalent molecular singlet, and occurs substantially below a critical strength for destruction of the stoich iometric antiferromagnetic state. However intrasite electron-lattice c oupling, in contrast to the intersite coupling, does not dramatically affect the hole-doping states. Doping induces modes in magnetic, optic al, and vibronic response functions. Local infrared-active phonon mode s are induced in infrared absorption spectra for finite electron-latti ce coupling. They are correlated with doping-induced particle-hole exc itations observed in optical absorption spectra and in magnetic excita tion spectra. These doping-induced particle-hole excitations are assoc iated with the local HF eigenstates in the charge-transfer gap. Each d oping state has distinctive excitation spectra in the magnetic, optica l, and vibronic channels. In particular, the hole-doping states with s mall polarons have doping-induced, infrared absorption peaks on the lo w-frequency side of the stoichiometric peak, while the electron-doping states have them on the high-frequency side.