SPIN-WAVES IN A DOPED ANTIFERROMAGNET

We have studied the doped antiferromagnet using a generalized t-J model which permits a semi-classical expansion about a magnetically ordered ground state. Of particular interest is the interplay between the spin backflow associated with holes and the coherent spin flow of the ordered magnet. A calculation of the leading-order spin-wave spectrum reveals a strong damping effect of the paramagnetic spin currents which, for small doping, renormalizes long-wavelength spin waves and causes short-wavelength spin waves to decay into particle-hole pair excitations. A correct Goldstone mode structure for the spin-wave spectrum is obtained only when the spin backflow is properly taken into account.