Influence of hole doping on spin dynamics in lightly doped copper oxide superconductors

The spin dynamics in the doped antiferromagnetic (AF) state of high-Tc copper oxide superconductors has been investigated within the t-t'-J model. We have shown that a small number of doped holes affects the spin dynamics in the CuO2 planes so strongly that above a very small hole concentration, nc=0.027, the two-dimensional (2D) AF ground state becomes unstable. We have also predicted a great change in the spin excitation spectrum for the doped AF state. For doping close to nc, a hydrodynamic regime of well-defined magnons collapses, and almost the whole region of long-wavelength spin waves corresponds to the overdamped regime of softened magnons in agreement with inelastic neutron-scattering experiments in YBa2Cu3O6+x reported by Rossat-Mignod et al. Analyzing the imaginary part of the spin susceptibility χ''(q,ω) we have discovered, in addition to the spin-wave part, also an incoherent band associated with electron-hole excitations. For the overdamped regime the energy ranges of coherent excitations and the incoherent band are overlapped and so it is impossible to separate them. Even for smallest q, when the magnon peak and the band are separated, the relative contribution of the band increases with increasing n and dominates near the critical point