Proximity of antiferromagnetism and superconductivity in ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$: Effective Hamiltonian from ab initio studies

We report density functional theory calculations for the parent compound LaFeAsO of the recently discovered 26 K Fe-based superconductor ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$. We find that the ground state is an ordered antiferromagnet, with staggered moment of about $2.3{\mu }_B$, on the border with the Mott insulating state. We fit the bands crossing the Fermi surface, derived from Fe and As, to a tight-binding Hamiltonian using maximally localized Wannier functions on $\text{Fe}3d$ and $\text{As}4p$ orbitals. The model Hamiltonian accurately describes the Fermi surface obtained via first-principles calculations. Due to the evident proximity of superconductivity to antiferromagnetism and the Mott transition, we suggest that the system may be an analog of the electron-doped cuprates, where antiferromagnetism and superconductivity coexist.

Figure 1

(Color online) Undoped LaFeAsO band structure of (a) AFM state and (b) PM state. The red lines represent DFT calculation results; blue lines are band structures reconstructed from the tight-binding model using maximally localized Wannier functions (MLWF). Since spin-up and spin-down bands are degenerate for the AFM state, we plot only spin-up bands here. For both figures, Fermi energies are indicated by the green line at 0 eV.

Figure 2

(Color online) DOS (top panel) and PDOS (FeAs and LaO planes: middle panel and bottom panel, respectively) of undoped LaFeAsO. Since the spin-up and spin-down states are degenerate for the AFM state, we plot spin-up states only. The Fermi level is aligned to 0.0 eV.

Figure 3

(Color online) Diagrams for (a) atomic-type MLWFs and (b) strongest hoppings in the system. The diagram is presented on the x-y plane with the most important hoppings labeled. Iron atoms and arsenic atoms are located at the vertices and centers of the square lattice, respectively. Note that (b) depicts an irreducible subset of hoppings, and the $z$-displacement of As atoms is not shown.

Figure 4

(Color online) DOS calculated from $\text{GGA}+U$ with different $U$’s, 2.0, 3.0, and 4.5 eV. $E_F$ is aligned to 0.0 eV in all cases. Due to degeneracy, only the $\alpha$-spin DOS is plotted.

Figure 5

(Color online) DOS of ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ with $x=0.125$ projected onto FeAs layers. The $E_F$ is aligned to 0.0 eV.