Effect of next-nearest neighbor hopping on the spin dynamics in   antiferromagnets
  Recently, inelastic neutron scattering (INS) experiments on the insulating parent compounds of high-T_c materials were analyzed to extract the value of the superexchange constant $J$. Starting point of the analysis was the nearest-neighbor Heisenberg model. Motivated by recent ARPES experiments, we consider the effects of a next-nearest neighbor hopping, $t'$ in the strong coupling limit of the spin-density wave formalism, where it leads to an antiferromagnetic exchange $J'>0$ between next-nearest neighbor spins. We show that a finite $J'$ (a) changes the spin-wave velocity and thus the value of $J$ extracted from the INS data, yielding almost identical values for $J$ in La_2CuO_4 and YBa_2Cu_3O_{6.15}, and (b) leads to a considerable change in the high-frequency spin-dynamics. Finally, we calculate the 1/S quantum corrections in the presence of $J'$ and discuss their relevance for the INS data. 
Dirk. K. Morr (University of Illinois at Urbana-Champaign)