Optical signature of the pressure-induced dimerization in the honeycomb iridate $\alpha$-Li${}_2$IrO${}_3$

Abstract

We studied the effect of external pressure on the electrodynamic properties of $\alpha$-Li${}_2$IrO${}_3$ single crystals in the frequency range of the phonon modes and the Ir $d$-$d$ transitions. The abrupt hardening of several phonon modes under pressure supports the onset of the dimerized phase at the critical pressure $P_c$=3.8 GPa. With increasing pressure an overall decrease in spectral weight of the Ir $d$-$d$ transitions is found up to $P_c$. Above $P_c$, the local (on-site) $d$-$d$ excitations gain spectral weight with increasing pressure, which hints at a pressure-induced increase in the octahedral distortions. The non-local (intersite) Ir $d$-$d$ transitions show a monotonic blue-shift and decrease in spectral weight. The changes observed for the non-local excitations are most prominent well above $P_c$, namely for pressures $\ge$12 GPa, and only small changes occur for pressures close to $P_c$. The profile of the optical conductivity at high pressures ($\sim$20 GPa) appears to be indicative for the dimerized state in iridates.

10 pages, 6 figures; accepted for publication in Phys. Rev. B

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• Non-local features of the spin-orbit exciton in Kitaev materials
• A set-up for Hard X-ray Time-resolved Resonant Inelastic X-ray Scattering at SwissFEL
• Fluorite-related iridate Pr${}_3$IrO${}_7$: Crystal growth, structure, magnetism, thermodynamic, and optical properties
• Optical conductivity of the metallic pyrochlore iridate Pr${}_2$Ir${}_2$O${}_7$: Influence of spin-orbit coupling and electronic correlations on the electronic structure
• Pressure-dependent magnetism of the Kitaev candidate Li${}_2$RhO${}_3$