Effect of Nematicity In FeSe On Fermionic Excitations

In this work we study the effect of nematicity in FeSe on the temperature variation of the energies of fermions on $d_xz/d_yz$ and $d_{xy}$ orbitals at the M point ($=(\pi,\pi)$ in 2Fe zone). In the absence of nematicity, the energies of $d_{xz}$ and $d_{yz}$ orbitals are degenerate at $M$ even in the presence of spin-orbit coupling, and $d_{xy}$ excitation is also doubly degenerate. A nematic order at $T < T_s$ should split these degeneracies. Recent ARPES experiments demonstrated that, besides splitting, nematic order causes a prominent change of the temperature dependence of excitations on $d_{xz}/d_{yz}$ orbitals, whose energy was increasing by magnitude with decreasing $T$ at $T > T_s$, but changes trend and decreases below $T_s$, while d_xy orbital does not exhibit such a behavior (A. Fedorov et al., Scientific Reports 6, 36834(2016); A. Coldea et al., arXiv:1706.00338(2017)). We attribute this phenomenon to the influence of $B_{1g}$ orbital order at the hole pockets on the dispersion at $M$ via the self-energy effect. We argue that the self-energy predominantly affects d_xz/d_yz orbitals at M because hole Fermi pockets are made of the same orbitals.