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.