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

TL;DR

This study investigates the insulator-to-metal transition in Sr$_3$Ir$_2$O$_7$ under high pressure using x-ray absorption spectroscopy, revealing changes in spin-orbit coupling and electronic structure beyond simple models.

Contribution

It provides new insights into the high-pressure electronic behavior of Sr$_3$Ir$_2$O$_7$, highlighting deviations from the $J_{ ext{eff}}=1/2$ state in the metallic phase.

Findings

Branching ratio decreases with pressure, indicating reduced spin-orbit coupling.

Structural transition occurs at 53 GPa, affecting electronic states.

High-pressure metallic phase shows deviations from single-ion spin-orbit models.

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, $\left\langle {\bf L} \cdot {\bf S} \right\rangle$. 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 $\left\langle {\bf L} \cdot {\bf S} \right\rangle$ 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.