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

## TL;DR

This study investigates how external pressure affects the optical properties of $ ext{Li}_2 ext{IrO}_3$, revealing a pressure-induced dimerization transition at 3.8 GPa through phonon and electronic transition analysis.

## Contribution

It provides the first detailed optical signature of pressure-induced dimerization in $ ext{Li}_2 ext{IrO}_3$, highlighting changes in phonon modes and Ir $d$-$d$ transitions across the transition.

## Key findings

- Abrupt phonon mode hardening at 3.8 GPa indicates dimerization.
- Decrease in spectral weight of Ir $d$-$d$ transitions up to $P_c$.
- Increase in local $d$-$d$ excitations and spectral weight above $P_c$.

## 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 $\geq$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.