Single 20meV boson mode in KFe2As2 detected by point-contact   spectroscopy
  We report an experimental and theoretical investigation of the electron-boson interaction in KFe2As2 by point-contact (PC) spectroscopy, model, and ab-initio LDA-based calculations for the standard electron-phonon Eliashberg function. The PC spectrum viz. the second derivative of the I - V characteristic of representative PC exhibits a pronounced maximum at about 20meV and surprisingly a featureless behavior at lower and higher energies. We discuss phonon and non-phonon (excitonic) mechanisms for the origin of this peak. Analysis of the underlying source of this peak may be important for the understanding of serious puzzles of superconductivity in this type of compounds. 
Yu.G. Naidyuk, O.E. Kvitnitskaya, N.V. Gamayunova, L. Boeri, S.   Aswartham, S. Wurmehl, B. B\"uchner, D.V. Efremov, G. Fuch...
cond-mat.supr-con