Band Narrowing and Mott Localization in Iron Oxychalcogenides ${\mathrm{La}}_2{\mathbf{O}}_2{\mathrm{Fe}}_2\mathbf{O}(\mathrm{Se},\mathbf{S}{)}_2$

Bad metal properties have motivated a description of the parent iron pnictides as correlated metals on the verge of Mott localization. What has been unclear is whether interactions can push these and related compounds to the Mott-insulating side of the phase diagram. Here we consider the iron oxychalcogenides ${\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{Fe}}_2\mathrm{O}(\mathrm{Se},\mathrm{S}{)}_2$, which contain an Fe square lattice with an expanded unit cell. We show theoretically that they contain enhanced correlation effects through band narrowing compared to LaOFeAs, and we provide experimental evidence that they are Mott insulators with moderate charge gaps. We also discuss the magnetic properties in terms of a Heisenberg model with frustrating $J_1\mathrm{\text{−}}{J}_2\mathrm{\text{−}}{J}_2^{\prime }$ exchange interactions on a “doubled” checkerboard lattice.

Figure 1

(Left panel) Crystal structure of ${\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{Fe}}_2{\mathrm{OSe}}_2$; (right panel) XRD patterns for the ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{Se}}_2$ and ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{S}}_2$ samples. The cell parameter values are $a=b=4.085\AA$ and $c=18.605\AA$ for ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{Se}}_2$ and $a=b=4.042\AA$ and $c=17.929\AA$ for ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{S}}_2$. Atomic positions in the unit cell of a paramagnetic state are as follows: La ($1/2$, $1/2$, 0.184 45), Fe (0.5, 0, 0), Se (0, 0, 0.096 69), O1 ($1/2$, 0, $1/4$), O2 ($1/2$, $1/2$,0).

Figure 2

The partial DOS for paramagnetic ${\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{Fe}}_2{\mathrm{OSe}}_2$ (a) and LaOFeAs (b). Also shown in panel (a) is the Fe $3d$ partial DOS for ${\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{Fe}}_2{\mathrm{OS}}_2$. In our DFT calculation, the energy threshold separating the localized and nonlocalized electronic states is chosen to be $-6\mathrm{Ry}$. The muffin-tin radii are $2.37a_0$ (Bohr radius) for lanthanum, $2.03a_0$ for iron, $2.40a_0$ for selenium, and $1.80a_0$ for oxygen. The criterion for the number of plane waves is chosen to be $R_{\mathrm{MT}}^{\min }*K^{\max }=7$, and the number of $k$ points is $32\times 32\times 8$ for these paramagnetic-state calculations.

Figure 3

(a) The electrical resistivity ($\rho$) vs temperature ($T$) for ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{S}}_2$ and ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{Se}}_2$. Inset: $\ln \rho$ vs $1/T$; (b) The temperature dependence of susceptibility, $\chi$, measured at 3 T. The three open points, measured at 2, 50, and 150 K, respectively, are determined from the slope of the $M(H)$ curves.

Figure 4

Schematic representation of the spin building blocks for the magnetically ordered states of ${\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{Fe}}_2{\mathrm{OSe}}_2$. To be consistent with the effective model, only Fe atoms are shown. The arrows represent the spin orientation. The black solid, green dashed, and blue solid lines stand for $J_1$, $J_2$, and $J_2^{\prime }$ spin exchange paths.

Figure 5

(a): The doubled checkerboard lattice for the Hamiltonian defined in Eq. (1). It consists of two sets of interpenetrating plaquettes, indicated as (A) and (B) dashed squares. It can be decomposed into two sublattices. (b): The $T=0$ phase diagram. The occupied symbols mark the ground states with exchange couplings listed in Table , with $U=1.5\mathrm{eV}$ (triangle); $U=3.0\mathrm{eV}$ (square); $U=4.5\mathrm{eV}$ (circle).