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(Te_{1−x}Se_{x}) 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 Δ_E≃2.75kBTc 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 Δ_H≃1.75k_BTc. The existence of this gap and its amplitude are confirmed by PCARS measurements in Fe(Te_{1−x}Se_{x}) single crystals. The values of the two gaps Δ_E and Δ_H, once plotted as a function of the local critical temperature Tc^A, turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature.