Doping Dependence of Resonant Spin Excitations in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

The first spectroscopic evidence of unconventional s$_\pm$-symmetry in the iron-based superconductors was provided by inelastic neutron scattering on Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ with the observation of a resonant spin excitation at the wavevector, Q, that connected the hole and electron Fermi surfaces, centered at the zone center and zone boundary, respectively. Subsequent measurements as a function of hole doping showed that the resonant excitations split into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes and the resonant enhancement below T$_c$ falls to zero as the magnetic interactions weaken, in good agreement with RPA theory. We have now extended these measurements to investigate the doping dependence of BaFe$_2$(As$_{1-x}$P$_x$)$_2$, whose nominal isovalence has been the subject of debate, from $x=0.2$ to 0.6. We find no evidence of a splitting of the resonance with increasing $x$, but see a monotonic decrease in the resonant enhancement across the superconducting dome falling to zero close to $x=0.6$.