Slave Boson Approach to The Neutron Scattering in YBCO Superconductors
  The evolution of the so-called `41meV resonance' in the magnetic response of YBCO cuprates is studied with slave-boson theory for the t-t'-J-model. The resonance appears as a collective spin fluctuation in the d-wave superconducting (SC) state. It is undamped at optimal doping due to a threshold in the excitation energies of particle--hole pairs with relative wave vector (pi,pi). When hole filling is reduced, the resonance moves to lower energies and broadens. Below the resonance energy we find a crossover to an incommensurate response in agreement with a recent experiment on YBa_2CU_3O_6.6. We show that dynamic nesting in the d-wave SC state causes this effect. 
Jan Brinckmann and Patrick A. Lee