A first-principle perspective on electronic nematicity in FeSe

Electronic nematicity is an important order in most iron-based superconductors, and FeSe represents a special example, in which nematicity disentangles from spin ordering. A first-principle description of this order remains elusive. Here, we show that by carefully searching the paramagnetic energy landscape within the density functional theory, a nematic solution stands out at either the +U or hybrid functional level with the lowest energy. The band structure and Fermi surface can be well compared with the recent experimental results. Symmetry analysis assigns the dominant order parameter to the E_u irreducible representations of the D_4h point group. Distinct from the B_1g Ising nematicity as widely discussed in the context of vestigial stripe antiferromagnetic order, the two-component E_u vector order features mixing of the Fe d-orbitals and inversion symmetry breaking, which lead to striking experimental consequences, e.g., missing of an electron pocket.