Raman Response in the Nematic Phase of FeSe

Raman experiments on bulk FeSe revealed that the low-frequency part of the B_{1g} Raman response R_{B1g}(Ω), which probes nematic fluctuations, rapidly decreases below the nematic transition at T_{n}∼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 T_{n}. 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 B_{1g} 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 B_{1g} Raman response near T_{n} is consistent with the sign change of the nematic order parameter between hole and electron pockets.