Raman response in the nematic phase of FeSe

Raman experiments on bulk FeSe revealed that the low-frequency part of $B_{1g}$ Raman response $R_{B_{1g}}$, which probes nematic fluctuations, rapidly decreases below the nematic transition at $T_n \sim 85$K. Such behavior is usually associated with the gap opening and at a first glance is inconsistent with the fact that FeSe remains a metal below $T_n$, with sizable hole and electron pockets. We argue that the drop of $R_{B_{1g}}$ in a nematic metal comes about because the nematic order drastically changes the orbital content of the pockets and makes them nearly mono-orbital. In this situation $B_{1g}$ Raman response gets reduced by the same vertex corrections that enforce charge conservation. The reduction holds at low frequencies and gives rise to gap-like behavior of $R_{B_{1g}}$, in full agreement with the experimental data.