Conserving approximation for the three-band Hubbard model: flat   quasiparticle dispersion
  It is shown that the low-energy single-particle excitation-spectrum of the three-band Hubbard model at hole-dopings away from half-filling agrees remarkably well with Quantum Monte Carlo data and spectroscopic experiments within the framework of a conserving approximation that includes self-consistently the interaction with charge, spin, and two-particle fluctuations. The dispersion of the quasiparticle-peak obtained from the spectral-weight function is flat around the $(\pi,0)$ and $(0,\pi)$ points as has been observed in recent angle-resolved photoemission measurement. The significant reduction of the quasiparticle-dispersion near the Fermi-energy is due to spin-fluctuations, rather than being induced by band effects. 
R. Putz, R. Preuss, A. Muramatsu and W. Hanke