Similarities in the Tunneling Spectral Dip Feature of FeAs-based and Cuprate Superconductors
Recent scanning tunneling spectroscopy (STS) measurements on stoichiometric LiFeAs (Chi et al) and the hole-doped pnictide, Ba0.6K0.4Fe2As2 (Shan et al) reveal a spectral dip/hump feature in the tunneling conductance at bias voltages above the superconducting gap voltage (e) that weakens with increasing T and ultimately disappears at Tc. We argue that this is a strong-coupling type effect quite similar to that observed in cuprate superconductors such as Bi2212.
In all cases, the estimated boson lies inside the superconducting gap 2 as expected from a spin exciton and is linked to the resonance spin excitation observed in neutron scattering. It is found that ~ 5kBTc over wide range of Tc for both classes of superconductors. Quantitative fits of Bi2212 tunneling data over a wide range of doping using Eliashberg theory show that 2F() is dominated by coupling to this mode. These results indicate that in both classes of superconductors the pairing mechanism is due to a spin fluctuation spectrum that is renormalized by the superconductivity.