Point-contact Andreev-reflection spectroscopy in Fe(Te, Se) films: multiband superconductivity and electron-boson coupling

We report on a study of the superconducting order parameter in Fe(Te1-xSex) thin films (with different Se contents: x = 0.3, 0.4, 0.5) by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude Delta(E) similar or equal to 2.75k(B)T(c) and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude Delta(H) similar or equal to 1.75k(B)T(c). The existence of this gap and its amplitude are confirmed by PCARS measurements in Fe(Te1-xSex) single crystals. The values of the two gaps Delta(E) and Delta(H), once plotted as a function of the local critical temperature T-c(A), turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature.