Fig. 1.

(A) Schematic of the Raman scattering process with incoming and scattered photons of frequency ${\omega }_{i/s}$ and polarization ${\mathbf{e}}_{i/s}$, respectively. The Raman shift is defined as the frequency shift between the incoming and scattered photon frequencies. (B) FeSe ab plane with Se atoms alternating above and below the plane defined by the Fe atoms. The 1 Fe unit cell, which neglects the alternating Se atoms, is drawn in dotted lines. In the tetragonal phase above $T_S$, a = b, and the crystal structure of FeSe has a fourfold symmetry axis. The $B_{1g}$ symmetry is obtained using crossed incoming and scattered photon polarizations at 45 degrees of the Fe–Fe bonds. (C, D) Fermi surface deformation associated to a d-wave Pomeranchuk order for (C) an isotropic Fermi liquid and (D) the multiband Fe SC showing d-wave-like deformations with global $B_{1g}$ symmetry which break the fourfold symmetry axis. The deformations shown are consistent with ARPES measurements in the orthorhombic phase of FeSe (32): and the hole pocket (red) expand along one direction, the elliptical electron pockets (blue) shrink (expand) along the same (other) direction. The 1 Fe unit cell is used.