We used high-resolution scanning tunneling spectroscopy to study the hole-doped iron pnictide superconductor Ba$_{ 0.6}$K$_{ 0.4}$Fe$_{ 2}$As$_{ 2}$ ($T_c=38$ 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 $\sim$14 meV, similar to the spin resonance measured by inelastic neutron scattering. These results indicate a strong electron-spin excitation coupling in iron pictnide superconductors, similar to that in high-$T_c$ copper oxide superconductors.
Categories: cond-mat.supr-con