PROPER REDUCTION SCHEME TO AN EXTENDED T-J MODEL AND THE HOLE DISPERSION IN SR2CUO2CL2

The reduction of the three-band p-d model for the CuO2 plane in cuprat es with the standard set of parameters as derived by Hybertsen et al. [Phys. Rev. B 41, 11 068 (1990)] to an effective t-t'-t ''-J model wit h a three-site hopping term is discussed in detail in the framework of the cell-perturbation method. The reduction procedure is formulated t o avoid any ambiguity in obtaining the correct signs of the hopping pa rameters in the effective model, and the contributions of first and se cond order resulting from the intercell hopping are distinguished. Bas ed on the so-defined extended t-J model, the dispersion of a single ho le in an antiferromagnetic background is calculated both by a variatio nal ansatz and within the self-consistent Born approximation (SCBA). A n extension of the variational method allows us to investigate the inf luence of a three-site hopping term both on the mean-field level and b eyond this approximation. The latter corrections are shown to be small . Both methods give the coinciding result that the additional hopping terms lead to a more isotropic minimum of the dispersion at the point (pi/2, pi/2) in k space and to a slightly increased bandwidth in compa rison to the pure t-J model. These results are compared with recent ph otoemission measurements for Sr2CuO2Cl2 [Wells et al., Phys. Rev. Lett . 74, 964 (1995)], showing an improvement against the pure t-J model, but no complete agreement. In particular, a small anisotropy in the ca lculated effective masses remains. The lifetime of the hole quasiparti cles is investigated within the SCBA, and it is shown that the upper p art of the hole spectrum loses its quasiparticle character due to a st rong increase of damping if the bare hole bandwidth exceeds a certain threshold value of roughly 2J.