Bandwidth and Fermi surface of iron oxypnictides: Covalency and sensitivity to structural changes

Some important aspects of the electronic structure of the iron oxypnictides depend very sensitively on small changes in interatomic distances and bond angles within the iron-pnictogen subunit. Using first-principles full-potential electronic structure calculations, we investigate this sensitive dependence, contrasting in particular LaFeAsO and LaFePO. The width of the Fe bands is significantly larger for LaFePO, indicating a better metal and weaker electronic correlations. When calculated at their experimental crystal structures, these two materials have significantly different low-energy band structures. The topology of the Fermi surface changes when going from LaFePO to LaFeAsO, with a three-dimensional hole pocket present in the former case transforming into a tube with two-dimensional dispersion. We show that the low-energy band structure of LaFeAsO evolves toward that of LaFePO as the As atom is lowered closer to the Fe plane with respect to its experimental position. The physical origin of this sensitivity to the iron-pnictogen distance is the covalency of the iron-pnictogen bond, leading to strong hybridization effects. To illustrate this, we construct Wannier functions, which are found to have a large spatial extension when the energy window is restricted to the bands with dominant iron character. Finally, we show that the Fe bandwidth slightly increases as one moves along the rare-earth series in REFeAsO and we discuss the physical origin of this effect.

Figure 1

(Color online) Band structures of LaFeAsO (left) and LaFePO (right). The red (dark gray) and green (light gray) bands have mainly $\text{Fe}3d$ and pnictogen/oxygen $p$ characters, respectively. The colors are guides to the eyes in order to facilitate comparison of the relative positions and widths of the bands (see also the arrows).

Figure 2

(Color online) Low-energy region of LaFePnO’ s band structures with $\text{Pn}=\text{As}$ (up panel) and P (lower panel).

Figure 3

(Color online) Fermi surfaces for LaFeAsO and LaFePO calculated at their experimental crystal structures.

Figure 4

(Color online) Sensitivity of the low-energy band structure for LaFeAsO to $d_v$ (As’s vertical distance with respect to the Fe plane) for fixed lattice constants $a$ and $c$. The considered $d_v$ values are indicated in each plot.

Figure 5

(Color online) (a) $\text{Fe-}xy$ Wannier function for LaFeAsO, from the maximally localized Wannier construction performed for the restricted set of Fe-like bands ($d$ downfolding). (b) Same orbital character as in (a) but within the $dpp$ downfolding, retaining the full set of $\text{Fe}d$, $\text{As}p$, and $\text{O}p$ bands.

Figure 6

(Color online) Fe orbitals for the five-band model for LaFeAsO. First row: $xy$ orbital. Second row: $3z^2-r^2$ orbital. Third row: $x^2-y^2$ orbital. On the Fe plane (first column), on the lower As plane (second column), and on a plane parallel to the $z$ axis containing one La, O, Fe, and As per unit cell (third column). In the first two columns the $x$ axis is along the diagonal of the plots. Note that the $x^2-y^2$ orbital has nodes on the latter plane and is thus not shown in this representation.

Figure 7

(Color online) Partial density of states of the $\text{Fe}d$ orbitals (solid lines) compared to the As density of states (dashed lines; same curve in all panels).

Figure 8

Schematic ${\text{Fe}}_4\text{As}$ pyramid for LaFeAsO. The $\theta$ and ${\theta }_3$ angles are indicated.

Figure 9

(Color online) The change $\Delta W$ in the $\text{Fe}3d$ full bandwidth $W$ (solid line) and the bandwidth of its occupied part $W_{\text{occ}}$ (dashed line) with respect to the calculated value for LaFeAsO at its experimental structure (Ref. 7). These reference values are 4.25 eV for $W$ and 1.95 eV for $W_{\text{occ}}$. From the top to bottom panels, we display the results for calculations (i)–(iii) as explained in the text. Left: $\Delta W$ as a function of $d_v$ or $a$, as indicated. Right: $\Delta W$ as a function of ${\theta }_3$. The structural data for Pr and Sm compounds were taken from Refs. 32, 33, respectively.