Free and spin-polaron states in high Tc superconductors

Hole states in the antiferromagnetic ally ordered CuO2 planes of cuprate perovskites are calculated within the three-band Hubbard model and with a self-consistent calculation of the Cu spin polarization. It is found that in the low doping limit the spin-polaron and the free-hole states are separated by a barrier of energy similar to 0.05eV. At finite dopings the spin-polaron states become less favorable and at high doping they turn out to be metastable. In case of high mobility of donors a phase separation of the charge carriers into domains with a local hole concentration c similar to 0.5 takes place. This explains experimentally observed formation of stripe domains with local hole concentrations c = 0.5.