Evidence of a Spin Resonance Mode in the Iron-Based Superconductor Ba0.6K0.4Fe2As2 from Scanning Tunneling Spectroscopy

We used high-resolution scanning tunneling spectroscopy to study the hole-doped iron pnictide superconductor Ba0.6K0.4Fe2As2 (T-c = 38 K). Features of a bosonic excitation (mode) are observed in the measured quasiparticle density of states. The bosonic features are intimately associated with the superconducting order parameter and have a mode energy of similar to 14 meV, similar to the spin resonance measured by inelastic neutron scattering. These results indicate a strong electron-spin excitation coupling in iron pnictide superconductors, similar to that in high-T-c copper oxide superconductors.