Superconductivity in Heavily Electron-Doped FeSe*

We describe heavily electron-doped FeSe by a Hubbard model over a square lattice of iron atoms with only d+ = d_xz + i d_yz and d− = d_xz − i d_yz degenerate orbitals. Nearest neighbor and next-nearest neighbor hopping parameters are chosen so that perfect nesting exists between an electron-type Fermi surface and a hole-type Fermi surface at the center and at the corner of the one-iron Brillouin zone, respectively. The latter results in an instability to a hidden spin-density wave state that exhibits two spin-1 Goldstone modes at wavenumber Q_AF=(π/a,π/a). The spectrum of magnetic excitations is obtained by a calculation of the dynamic spin susceptibility within the random-phase approximation (RPA). These results are compared to the ring of low-energy magnetic excitations observed in heavily electron-doped FeSe around Q_AF by inelastic neutron scattering [1]. The nature of Cooper pairs that result from the virtual exchange of magnetic excitations predicted by RPA is also determined. It will be compared to exact calculations that find evidence for S⁺⁻ Cooper pairing in the limit of strong on-site Coulomb repulsion[2].
[1] B. Pan et al., Nat. Comm. 8, 123 (2017).
[2] J.P. Rodriguez, Phys. Rev. B 95, 134511 (2017).