Bosonic Mode in the Tunneling Spectra of Single Cu-O Layer 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 single Cu-O layer cuprates is discussed. 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 compounds. High Tc (94K) Hg1201 tunneling spectra exhibit a relatively small Γ and a well resolved bosonic mode feature similar to Bi2212. 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. 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 as expected from an all-electronic pairing interaction.