Tuning J$_{eff}$ = 1/2 Insulating State via Electron Doping and Pressure in Double-Layered Iridates

Sr$_3$Ir$_2$O$_7$ exhibits a novel J$_{eff}$=1/2 insulating state featuring a splitting between J$_{eff}$=1/2 and 3/2 bands due to spin-orbit interaction. We report that a metal-insulator transition can be induced by either dilute electron (La$^{3+}$) doping for Sr$^{2+}$ ions in Sr$_3$Ir$_2$O$_7$ or via application of high pressure. The following constitutes the central findings of our recent study of single-crystal Sr$_3$Ir$_2$O$_7$ and (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$: (1) application of high hydrostatic pressure P results in a drastic drop in the electrical resistivity by four orders of magnitude at a critical pressure, P$_C$ = 13.2 GPa, suggesting a significantly reduced splitting between J$_{eff}$=1/2 and 3/2 bands, but further increasing P up to 35 GPa produces no fully metallic state at low temperatures; (2) however, slight doping of La$^{3+}$ ions for Sr$^{2+}$ ions in Sr$_3$Ir$_2$O$_7$ readily induces a robust metallic state that follows no Fermi liquid behavior; and (3) the magnetic ordering temperature is significantly suppressed from 285 K for x=0 but remains finite for (Sr$_{0.94}$La$_{0.06}$)$_3$Ir$_2$O$_7$ where the metallic state occurs. The results will be discussed along with comparisons drawn with Sr$_2$IrO$_4$, a prototype of the J$_{eff}$ = 1/2 insulator.