Direct Evidence of Two Superconducting Gaps in FeSe${}_{0.5}$Te${}_{0.5}$: SnS-Andreev Spectroscopy and Lower Critical Field

Abstract

We present direct measurements of the superconducting order parameter in nearly optimal FeSe${}_{0.5}$Te${}_{0.5}$ single crystals with critical temperature $T_C\approx 14$ K. Using intrinsic multiple Andreev reflection effect (IMARE) spectroscopy and measurements of lower critical field, we directly determined two superconducting gaps, ${\Delta }_L\approx 3.3-3.4$ meV and ${\Delta }_S\approx 1$ meV, and their temperature dependences. We show that a two-band model fits well the experimental data. The estimated electron-boson coupling constants indicate a strong intraband and a moderate interband interaction.

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