We report inelastic neutron scattering experiments on single crystals of superconducting Ba_0.67K_0.33Fe₂As₂ (T_c = 38 K). In addition to confirming the resonance previously found in powder samples, we find that spin excitations in the normal state form longitudinally elongated ellipses along the Q_AFM direction in momentum space, consistent with density functional theory predictions. On cooling below T_c, while the resonance preserves its momentum anisotropy as expected, spin excitations at energies below the resonance become essentially isotropic in the in-plane momentum space and dramatically increase their correlation length. These results suggest that the superconducting gap structures in Ba_0.67Ka_0.33Fe₂As₂ are more complicated than those suggested from angle resolved photoemission experiments.

© 2011 Macmillan Publishers Limited. All rights reserved