Raman Response in the Nematic Phase of FeSe

Raman experiments on bulk FeSe revealed that the low-frequency part of the B1g Raman response R_B1g(Ω), which probes nematic fluctuations, rapidly decreases below the nematic transition at Tn ∼ 85 K. Such behavior is expected when a gap opens up and at a first glance is inconsistent with the fact that FeSe remains a metal below Tn. We argue that the drop of R_B1g(Ω) can be ascribed to the fact that the nematic order drastically changes the orbital content of low-energy excitations near hole and electron pockets, making them nearly mono-orbital. In this situation, the B1g Raman response gets reduced by the same vertex corrections that enforce charge conservation in the symmetric Raman channel. The reduction holds at low frequencies and gives rise to gaplike behavior of R_B1g(Ω). We also show that the enhancement of the B1g Raman response near Tn is consistent with the sign change of the nematic order parameter between hole and electron pockets.