Bosonic Mode in the Tunneling Spectra of Cuprate and Fe-based Superconductors
Conventional superconductors such as Pb or Nb exhibit above-gap features in the tunneling conductance that correspond to peaks in the phonon density of states thereby confirming the electron-phonon origin of superconductivity. The above-gap, SIN tunneling spectral dip feature in the bilayer cuprate Bi2
(Bi2212) is highly reproducible and its doping dependence unambiguously links it 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 higher Tc cuprates. Similar bosonic mode features are found in FeSe STM data and are linked to the neutron resonance suggesting that the dynamics of the pairing interactions are similar in cuprate and Fe based superconductors.
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