NONRIGID HOLE BAND IN THE EXTENDED T-J MODEL

The dispersion of one hole in an extended t-J model with additional ho pping terms to second and third nearest neighbours and a frustration t erm in the exchange part has been investigated, Two methods, a Green's function projection technique describing a magnetic polaron of minima l size and the exact diagonalization of a 4 4 lattice, have been app lied, showing reasonable agreement among each other, Using additional hopping integrals which are characteristic for the CuO2 plane in cupra tes we find in the nonfrustrated case an isotropic minimum of the disp ersion at the point (pi/2, pi/2) in k-space in good coincidence with r ecent angle-resolved photoemission results for the insulating compound Sr2CuO2Cl2, Including frustration or finite temperature which shall s imulate the effect of doping, the dispersion is drastically changed su ch that a flat region and an extended saddle point may be observed bet ween (pi/2, 0) and (pi, 0) in agreement with experimental results for the optimally doped cuprates.