Bosonic Mode in the Tunneling Spectra of Fe-based and Cuprate Superconductors

The above-gap, SIN tunneling spectral dip feature in the bilayer cuprate Bi₂Sr₂CaCu₂O_8+δ (Bi2212) is highly reproducible and unambiguously linked to the resonance spin excitation. The search for similar bosonic mode features in the tunneling spectra of Fe-based and other cuprate superconductors is presented. The single-layer cuprate Hg1201Tc (94K) exhibit sharp spectra and a well resolved bosonic mode feature similar to Bi2212. STS data on FeSe (Tc = 8.9K) reveal a bosonic mode feature quite similar to Bi2212, also linked to the neutron resonance mode. It is demonstrated that SIS break junctions or intrinsic c-axis junctions reveal a strongly enhanced dip feature allowing the observation of a bosonic mode in strongly disordered Bi2201 with Tc values ≤ 5K. Over a wide range of Tc values the mode energy Ω remains inside the superconducting gap 2Δ as found in high Tc cuprates. The conclusion is that irrespective of Tc, composition and degree of disorder, the bosonic mode Ω retains its character as a spin exciton and appears to be a universal signature of unconventional superconductivity.