---

The identification of the electron-boson interaction is believed to be the key for rationalizing the Cooper pairing mechanism in a superconductor. In strong- coupling conventional superconductors, the fingerprints of the electronic band renormalization due to the electron-boson interaction are found in the fine structure of tunneling spectra, where the identification of this fine structure with the phononic density of states represents one of the hallmarks of modern condensed matters physics. In many unconventional superconductors, the unknown nature of the pairing boson, absence of momentum resolution, as well as possible inelastic tunneling effects render the interpretation of the tunneling spectra often cumbersome and ambiguous. Here we exploit a so far unnoticed boson-assisted resonant enhancement of the impurity scattering potential and thus the quasiparticle interference (QPI) amplitude in order to identify the nature of a possible pairing boson in the unconventional superconductor LiFeAs. In high-resolution Fourier transform scanning tunneling spectroscopy (FT-STS) measurements we indeed observe a resonance-like enhancement of the QPI amplitude in both the normal and the superconducting state, which implies an important coupling of electronic states to corresponding bosonic states at q~0 and energy at about 8meV. We show that these bosonic states seen in the QPI are in perfect agreement with a strong inelastic contribution on the tunneling dI/dU spectra recorded away from impurities which renders the observed small-q modes promising candidates for providing the pairing interaction in LiFeAs.

---

Christian Hess, IFW Dresden
Seminar Room of the Institute of Physics 2
Contact: P. van Loosdrecht