SPIN-POLARON DAMPING IN THE SPIN-FERMION MODEL FOR CUPRATE SUPERCONDUCTORS

A self-consistent, spin-rotational invariant Green's-function procedur e has been developed to calculate the spectral function of carrier exc itations in the spin-fermion model for the CuO2 plane. We start from t he mean-field description of a spin polaron in the Mori-Zwanzig projec tion method. In order to determine the spin polaron lifetime in the se lf-consistent Born approximation, the self-energy is expressed by an i rreducible Green's function. Both spin polaron and bare hole spectral functions are calculated. The numerical results show a well pronounced quasiparticle peak near the bottom of the dispersion at (pi/2,pi/2), the absence of the quasiparticle at the Gamma point, a rather large da mping away from the minimum, and an asymmetry of the spectral function with respect to the antiferromagnetic Brillouin zone. These findings are in qualitative agreement with photoemission data for undoped cupra tes. The direct oxygen-oxygen hopping is responsible for a more isotro pic minimum at(pi/2, pi/2).