Anisotropy of incommensurate magnetic excitations in slightly overdoped   Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ probed by polarized inelastic neutron   scattering experiments
  Polarized neutron scattering experiments on the slightly overdoped superconductor Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ reveal broad magnetic resonance scattering peaking at approximately 15 meV. In spite of doping far beyond the suppression of magnetic order, this compound exhibits dispersive and anisotropic magnetic excitations. At energies below the resonance maximum, magnetic correlations polarized parallel to the layers but perpendicular to the propagation vector are reduced by a factor two compared to those in the two orthogonal directions; in contrast correlations at the peak maximum are isotropic. 
N. Qureshi, C. H. Lee, K. Kihou, K. Schmalzl, P. Steffens and M.   Braden
cond-mat.supr-con