Direct evidence of two superconducting gaps in FeSe$_{0.5}$Te$_{0.5}$: SnS-Andreev spectroscopy and lower critical field

We present direct measurements of the superconducting order parameter in nearly optimal FeSe Te single crystals with the critical temperature $T_C\approx14$ K. Using the intrinsic multiple Andreev reflection effect (IMARE) spectroscopy and measurements of the lower critical field, we directly determined two superconducting gaps, $\Delta_{\mathrm{L}}\approx 3.3$–$3.4$ meV and $\Delta S\approx 1$ meV, and their temperature dependences. We show that a twoband model fits well the experimental data. The estimated electron–boson coupling constants indicate a strong intraband and a moderate interband interaction.