$f$-spin physics of rare-earth iron pnictides: Influence of $d$-electron antiferromagnetic order on the heavy-fermion phase diagram

Some of the high-$T_c$ iron pnictides contain magnetic rare-earth elements, raising the question of how the existence and tunability of a $d$-electron antiferromagnetic order influences the heavy-fermion behavior of the $f$ moments. With CeOFeP and CeOFeAs in mind as prototypes, we derive an extended Anderson lattice model appropriate for these quaternary systems. We show that the Kondo screening of the $f$ moments are efficiently suppressed by the $d$-electron ordering. We also argue that, inside the $d$-electron ordered state (as in CeOFeAs), the $f$ moments provide a rare realization of a quantum frustrated magnet with competing $J_1\text{−}{J}_2\text{−}{J}_3$ interactions in an effective square lattice. Implications for the heavy-fermion physics in broader contexts are also discussed.

Figure 1

(Color online) The lattice structure of $R\text{−}1111$ series. The small black and red (connected with solid line) circles represent Fe and O ions, respectively, and the big blue and brown and dashed gray circles are the $R$ and $X$ ions, respectively. The small solid red, large solid brown, and dotted green squares describe the Fe, $R$, and $X$ plaquettes, respectively. Left panel: $ab$ plane; right panel: $ac$ plane. The dashed and dotted lines denote $V_{pf}$ and $V_{pd}$, respectively.

Figure 2

(Color online) Mean-field Kondo parameter $b$ as a function of the $\mathbf{Q}=\left(\pi ,0\right)$ (in the notation of the one-Fe Brillouin zone) staggered magnetization $M_d$. The Kondo temperature $T_K\propto b^2$. $M_d$ is measured in ${\mu }_B/\text{Fe}$. The inset shows the $d$-electron density of states for $M_d=0$ (dotted black), 0.021 (solid red), 0.165 (dashed green), and 0.296 (dash-dotted blue).

Figure 3

(Color online) The would-be ordering patterns of the $f$ electrons due to the superexchange interactions via (a) the $R\text{−}X\text{−}R$ process alone or (b) the $R\text{-O-}R$ process alone. (c) illustrates the combined exchanges, viewed as $J_1\text{−}{J}_2\text{−}{J}_3$ interactions of an effective square lattice within an $R\text{O}$ layer, which are expected to turn the orders of (a) and (b) into a helical one. The big blue and brown circles label the rare-earth sites in the same way as in Fig. 1.