Spin-orbit coupling in iridium-based 5 d compounds probed by x-ray absorption spectroscopy

We have performed x-ray absorption spectroscopy (XAS) measurements on a series of Ir-based 5$d$ transition metal compounds, including Ir, IrCl${}_{3}$, IrO${}_{2}$, Na${}_{2}$IrO${}_{3}$, Sr${}_{2}$IrO${}_{4}$, and Y${}_{2}$Ir${}_{2}$O${}_{7}$. By comparing the intensity of the ``white-line'' features observed at the Ir L${}_{2}$ and L${}_{3}$ absorption edges, it is possible to extract valuable information about the strength of the spin-orbit coupling in these systems. We observe remarkably large, nonstatistical branching ratios in all Ir compounds studied, with little or no dependence on chemical composition, crystal structure, or electronic state. This result confirms the presence of strong spin-orbit coupling effects in novel iridates such as Sr${}_{2}$IrO${}_{4}$, Na${}_{2}$IrO${}_{3}$, and Y${}_{2}$Ir${}_{2}$O${}_{7}$, and suggests that even simple Ir-based compounds such as IrO${}_{2}$ and IrCl${}_{3}$ may warrant further study. In contrast, XAS measurements on Re-based 5$d$ compounds, such as Re, ReO${}_{2}$, ReO${}_{3}$, and Ba${}_{2}$FeReO${}_{6}$, reveal statistical branching ratios indicative of negligible spin-orbit coupling effects.