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

TL;DR

This study uses point-contact Andreev-reflection spectroscopy to investigate multiband superconductivity and electron-boson interactions in Fe(Te,Se) films, revealing multiple superconducting gaps and their relation to spin resonance energies.

Contribution

It provides new evidence of multiple superconducting gaps and their coupling to spin resonance modes in Fe(Te,Se) films using PCARS, confirming the multiband nature of superconductivity.

Findings

Identification of two superconducting gaps, Δ_E and Δ_H.

Evidence of strong electron-boson coupling at spin-resonance energy.

Gaps' amplitudes correlate with local critical temperature T_c^A.

Abstract

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 $\Delta_E \simeq 2.75 k_B T_c$ 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 $\Delta_H \simeq 1.75 k_B T_c$. 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 $\Delta_E$ 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.