INFLUENCE OF HOLE DOPING ON SPIN DYNAMICS IN LIGHTLY DOPED COPPER-OXIDE SUPERCONDUCTORS

The spin dynamics in the doped antiferromagnetic (AF) state of high-T- c copper oxide superconductors has been investigated within the t-t'-J model. We have shown that a small number of doped holes affects the s pin dynamics in the CuO2 planes so strongly that above a very small ho le concentration, n(c)=0.027, the two-dimensional (2D) AF ground state becomes unstable. We have also predicted a great change in the spin e xcitation spectrum for the doped AF state. For doping close to n(c), a hydrodynamic regime of well-defined magnons collapses, and almost the whole region of long-wavelength spin waves corresponds to the overdam ped regime of softened magnons in agreement with inelastic neutron-sca ttering experiments in YBa2Cu3O6+x reported by Rossat-Mignod et al. An alyzing the imaginary part of the spin susceptibility chi''(q,omega) w e have discovered, in addition to the spin-wave part, also an incohere nt band associated with electron-hole excitations. For the overdamped regime the energy ranges of coherent excitations and the incoherent ba nd are overlapped and so it is impossible to separate them. Even for s mallest q, when the magnon peak and the band are separated, the relati ve contribution of the band increases with increasing n and dominates near the critical point.