Description:

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 Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) is highly reproducible and its doping dependence rules out a phonon origin and unambiguously links it to the resonance spin excitation. The observation of similar bosonic mode features in the tunneling spectra of Fe-based superconductors and 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 single-layer cuprates. 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 the STM data of Fe-based superconductors and are linked to the neutron resonance suggesting that the dynamics of the pairing interactions are similar.

Professor John Zasadzinski, Illinois Institute of Technology

Host: James Sauls

Keywords: Physics, Astronomy, Condensed Matter