Raman experiments on bulk FeSe revealed that the low-frequency part of the B1g Raman response RB1g(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 RB1g(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 RB1g(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.
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We are thankful to B. Andersen, A. Bohmer, G. Blumberg, M. Christensen, R. Fernandes, P. Hirschfeld, Y. Gallais, J. Kang, A. Klein, A. Kreisel, I. Paul, A. Sacuto, and H. Yamase for useful discussions. This work has been supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under Award No. DE-SC0014402, by the Italian MAECI under the Italian-India collaborative Project No. SUPERTOP-PGR04879, by the Italian MIUR project PRIN 2017 No. 2017Z8TS5B, and by Regione Lazio (L. R. 13/08) under project SIMAP. A. V. C. thanks for hospitality the Sapienza University of Rome, where this work was initiated.
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