Tuning the $J_{\mathrm{eff}}=\frac{1}{2}$ insulating state via electron doping and pressure in the double-layered iridate Sr${}_3$Ir${}_2$O${}_7$

Sr${}_3$Ir${}_2$O${}_7$ exhibits a novel $J_{\mathrm{eff}}=\frac{1}{2}$ insulating state that features a splitting between $J_{\mathrm{eff}}=\frac{1}{2}$ and $\frac{3}{2}$ bands due to spin-orbit interaction. We report a metal-insulator transition in Sr${}_3$Ir${}_2$O${}_7$ via either dilute electron doping (La${}^{3+}$ for Sr${}^{2+}$) or application of high pressure up to 35 GPa. Our study of single-crystal Sr${}_3$Ir${}_2$O${}_7$ and (Sr${}_{1-x}$La${}_x$)${}_3$Ir${}_2$O${}_7$ reveals that application of high hydrostatic pressure $P$ leads to a drastic reduction in the electrical resistivity by as much as six orders of magnitude at a critical pressure $P_C$ $=$ 13.2 GPa, manifesting a closing of the gap; but further increasing $P$ up to 35 GPa produces no fully metallic state at low temperatures, possibly as a consequence of localization due to a narrow distribution of bonding angles θ. In contrast, slight doping of La${}^{3+}$ ions for Sr${}^{2+}$ ions in Sr${}_3$Ir${}_2$O${}_7$ readily induces a robust metallic state in the resistivity at low temperatures; the magnetic ordering temperature is significantly suppressed but remains finite for (Sr${}_{0.95}$La${}_{0.05}$)${}_3$Ir${}_2$O${}_7$ where the metallic state occurs. The results are discussed along with comparisons drawn with Sr${}_2$IrO${}_4$, a prototype of the $J_{\mathrm{eff}}=\frac{1}{2}$ insulator.

Figure 1

Temperature dependence of (a) the magnetization and Ir-O-Ir bond angle θ (right scale) for Sr${}_2$IrO${}_4$, (b) the magnetization M and Ir-O-Ir bond angle θ (right scale) for Sr${}_3$Ir${}_2$O${}_7$, (c) the $c$ axis resistivity ${\rho }_c$ for Sr${}_2$IrO${}_4$ and Sr${}_3$Ir${}_2$O${}_7$, and (d) the specific heat $C$ for Sr${}_2$IrO${}_4$ and Sr${}_3$Ir${}_2$O${}_7$.

Figure 2

Temperature dependence of (a) the $a$ axis resistivity ${\rho }_a$, and (b) the $c$ axis resistivity ${\rho }_c$ for (Sr${}_{1-x}$La${}_x$)${}_3$Ir${}_2$O${}_7$. (c) Temperature dependence of ${\rho }_a$, ${\rho }_c$, and $d{\rho }_a$/$dT$ and $d{\rho }_c$/$dT$ (right scale) for $x$ $=$ 0.05.

Figure 3

Temperature dependence of (a) the $a$ axis magnetic susceptibility ${\chi }_a$ and (b) the $c$ axis magnetic susceptibility ${\chi }_c$ for (Sr${}_{1-x}$La${}_x$)${}_3$Ir${}_2$O${}_7$. Note that no magnetic anomalies are observed in ${\chi }_c$.

Figure 4

(a) Temperature dependence of the $a$ axis resistivity ${\rho }_a$ at representative pressures $P$ for Sr${}_3$Ir${}_2$O${}_7$. Inset: ${\rho }_a$ versus $T$ for 25.5 GPa. (b) The activation gap Δ (left scale) and ratio ${\rho }_a$ (2 K)/${\rho }_a$ (300 K) (right scale) versus $P$ for Sr${}_3$Ir${}_2$O${}_7$. Inset: Schematic evolution of the density of states under pressure.

Figure 5

The $a$ axis resistivity ${\rho }_a$ at 2 K as a function of $x$ for (Sr${}_{1-x}$La${}_x$)${}_3$Ir${}_2$O${}_7$ and pressure (upper axis) for Sr${}_3$Ir${}_2$O${}_7$.