Fermi Surface Pockets in Heavily Electron-Doped FeSe by Hidden Magnetic Order

We study the Hubbard model over a square lattice of iron atoms, each containing only d+ = d_xz + i d_yz and d− = d_xz − i d_yz degenerate orbitals. Super-exchange interactions via the chalcogenide atoms are also added. Nearest neighbor (1) and next-nearest neighbor (2) hopping parameters are chosen so that perfect nesting exists between an electron-type and a hole-type Fermi surface at the center and at the corner of the one-iron Brillouin zone, respectively. A hidden spin-density wave (hSDW) instability exists for super-exchange coupling constants J₂ > 0.5 J₁. Quasi-particle excitation energies disperse along Dirac cones at the intersections of the Fermi surfaces with the principal axes. The hSDW state also exhibits two spin-1 Goldstone modes at wavenumber Q_AF=(π/a,π/a). Virtual emission/absorption of the latter by hSDW quasiparticles results in a migration of the Dirac cones towards the corner of the two-iron Brillouin zone. This agrees with Schwinger-boson-slave-fermion mean field theory and exact calculations at the limit of strong on-site Coulomb repulsion, which find electron-type Fermi surface pockets at the corner of the two-iron Brillouin zone[1].
[1] J.P. Rodriguez, Phys. Rev. B 95, 134511 (2017).