Collective modes in multiband superconductors: Raman scattering in iron selenides

TL;DR

This paper investigates Raman scattering in alkali-intercalated iron selenide superconductors, predicting collective modes related to potential d-wave pairing fluctuations, supported by experimental data.

Contribution

It provides a theoretical analysis of Raman spectra assuming s-wave symmetry, identifying collective modes associated with d-wave fluctuations, and compares predictions with experimental data.

Findings

Raman profile in B2g channel shows two collective modes.

Modes include a particle-hole exciton and a Bardasis-Schrieffer-type mode.

Experimental data supports the presence of these modes.

Abstract

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.