Collective modes in multiband superconductors: Raman scattering in iron selenides

Abstract

We study Raman scattering in the superconducting state of alkali-intercalated iron selenide materials ${A}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2}$ ($A=\text{K},\text{Rb},\text{Cs}$) in which the Fermi surface has only electron pockets. Theory predicts that both $s$-wave and $d$-wave pairing channels are attractive in this material, and the gap can have either $s$-wave or $d$-wave symmetry, depending on the system parameters. ARPES data favor $s$-wave superconductivity. We present the theory of Raman scattering in ${A}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2}$ assuming that the ground state has s-wave symmetry but $d$ wave is a close second. We argue that Raman profile in $d$-wave ${B}_{2g}$ channel displays two collective modes. One is a particle-hole exciton, another is a Bardasis-Schrieffer-type mode associated with superconducting fluctuations in $d$-wave channel. At a finite damping, the two modes merge into one broad peak. We present Raman data for ${A}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2}$ and compare them with theoretical Raman profile.