K. Yonemitsu et al., DOPING STATES IN THE 2-DIMENSIONAL 3-BAND PEIERLS-HUBBARD MODEL, Physical review. B, Condensed matter, 47(18), 1993, pp. 12059-12088
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.