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We have performed x-ray absorption spectroscopy (XAS) measurements on a series of Ir-based 5d transition metal compounds, including Ir, IrCl₃, IrO₂, Na₂IrO₃, Sr₂IrO₄, and Y₂Ir₂O₇. By comparing the intensity of the ``white-line'' features observed at the Ir L₂ and L₃ 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 Sr₂IrO₄, Na₂IrO₃, and Y₂Ir₂O₇, and suggests that even simple Ir-based compounds such as IrO₂ and IrCl₃ may warrant further study. In contrast, XAS measurements on Re-based 5d compounds, such as Re, ReO₂, ReO₃, and Ba₂FeReO₆, reveal statistical branching ratios indicative of negligible spin-orbit coupling effects.
Clancy et al. (Mon,) studied this question.