We study a ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.6}{\mathrm{Se}}_{0.4}$ multiband superconductor with $T_c=14\mathrm{K}$ by polarization-resolved Raman spectroscopy. Deep in the superconducting state, we detect pair-breaking excitation at $2\mathrm{\Delta }=5.6\mathrm{meV}$ in the $XY(B_{2g}$) scattering geometry, consistent with the fundamental gap energy $\mathrm{\Delta }=3\mathrm{meV}$ revealed by ARPES on the holelike Fermi pocket with $d_{xz}/d_{yz}$ character. We analyze the superconductivity-induced phonon self-energy effects for the $B_{1g}$(Fe) phonon mode and derive the electron-phonon coupling constant ${\lambda }^{\mathrm{\Gamma }}\approx 0.026$, which is insufficient to explain superconductivity with $T_c=14\mathrm{K}$.

• Received 17 November 2019
• Revised 13 February 2020
• Accepted 10 March 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.013373

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Published by the American Physical Society