Superconductivity and phonon self-energy effects in Fe${}_{1+y}$Te${}_{0.6}$Se${}_{0.4}$

Abstract

We study Fe${}_{1+y}$Te${}_{0.6}$Se${}_{0.4}$ multi-band superconductor with $T_c=14$K by polarization-resolved Raman spectroscopy. Deep in the superconducting state, we detect pair-breaking excitation at 45cm${}^{-1}$ ($2\Delta =5.6$meV) in the $XY$($B_{2g}$) scattering geometry, consistent with twice of the superconducting gap energy (3 meV) revealed by ARPES on the hole-like Fermi pocket with $d_{xz}/d_{yz}$ character. We analyze the superconductivity induced phonon self-energy effects for the $B_{1g}$(Fe) phonon and estimate the electron-phonon coupling constant ${\lambda }^{\Gamma }\approx 0.026$, which is insufficient to explain superconductivity with $T_c=14$K.