High-pressure insulator-to-metal transition in Sr${}_3$Ir${}_2$O${}_7$ studied by x-ray absorption spectroscopy

Abstract

High-pressure x-ray absorption spectroscopy was performed at the Ir $L_3$ and $L_2$ absorption edges of Sr${}_3$Ir${}_2$O${}_7$. The branching ratio of white line intensities continuously decreases with pressure, reflecting a reduction in the angular part of the expectation value of the spin-orbit coupling operator, $⟨\mathbf{L}\cdot \mathbf{S}⟩$. Up to the high-pressure structural transition at 53 GPa, this behavior can be explained within a single-ion model, where pressure increases the strength of the cubic crystal field, which suppresses the spin-orbit induced hybridization of $J_{\text{eff}}=3/2$ and $e_g$ levels. We observe a further reduction of the branching ratio above the structural transition, which cannot be explained within a single-ion model of spin-orbit coupling and cubic crystal fields. This change in $⟨\mathbf{L}\cdot \mathbf{S}⟩$ in the high-pressure, metallic phase of Sr${}_3$Ir${}_2$O${}_7$ could arise from non-cubic crystal fields or a bandwidth-driven hybridization of $J_{\text{eff}}=1/2,3/2$ states, and suggests that the electronic ground state significantly deviates from the $J_{\text{eff}}=1/2$ limit.

arXiv comment8 pages, 5 figures

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