We study Raman scattering in the superconducting state of alkali-intercalated iron selenide materials AxFe2-ySe2 (A=K,Rb,Cs) in which 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 AxFe2-ySe2 assuming that the ground state has s-wave symmetry but d− wave is a close second. We argue that Raman profile in d-wave B2g 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 AxFe2-ySe2 and compare them with theoretical Raman profile.
@article{arxiv.1405.6246,
title = {Collective modes in multiband superconductors: Raman scattering in iron selenides},
author = {M. Khodas and A. V. Chubukov and G. Blumberg},
journal= {arXiv preprint arXiv:1405.6246},
year = {2014}
}