Collective excitations in AFe2Se2 superconductors: application to Raman

We present studies of Raman scattering in A$_x$Fe$_{2-y}$Se$_2$ (A=K,Rb,Cs) superconductors with only electron pockets. The pairing symmetry consistent with both ARPES and neutron scattering is hybridization induced $s^{+-}$ state. Such order parameter changes sign between the hybridized pockets. The peak in calculated $B_{2g}$ Raman intensity signifies the in-gap charge excitation of a $d$-wave symmetry. A single $B_{2g}$ mode emerges as a result of strong mixing between the $d$-wave superconducting fluctuations (Bardasis-Schrieffer modes) and the charge nematic fluctuations. Increase in pocket ellipticity promotes $d$-wave ordering. As a result the $B_{2g}$ Raman mode softens, and becomes critical at the transition to an $s+id$ state with broken time reversal symmetry. The reported $B_{2g}$ excitation corresponds to the in-gap feature in experimentally observed Raman spectra.