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

The above-gap, SIN tunneling spectral dip (bosonic mode) feature in the bilayer cuprate Bi₂Sr₂CaCu₂O_8+δ (Bi2212) is highly reproducible and its doping dependence links it to the resonance spin excitation. SIS junctions on Bi2212 reveal a strongly enhanced dip strength and symmetric, decreasing conductance backgrounds, indicating the elastic tunneling origin of the bosonic mode and the connection to the pairing self-energy. The observation of similar bosonic mode features in the tunneling spectra of Fe-based superconductors and other cuprate superconductors is discussed. Scanning tunneling spectroscopy (STS) on FeSe reveals a similar background shape to Bi2212 indicating the symmetric bosonic mode features also are dominated by elastic tunneling. Disorder effects which show up as a depairing rate, Γ, in the fitting of the density of states (DOS) become increasingly important for lower Tc single-layer cuprates. It is demonstrated that SIS break junctions and intrinsic c-axis junctions allow the observation of a bosonic mode in strongly disordered Bi2201 with Tc values ≤ 5K. The scaling of Ω with Tc over nearly two decades, and over two different classes of superconductors suggests that the bosonic mode is a universal signature of unconventional superconductivity.