Mechanism of enhanced optical second-harmonic generation in the conducting pyrochlore-type Pb$_{2}$Ir$_{2}$O$_{7-x}$ oxide compound

TL;DR

This study investigates the structural and electronic origins of enhanced second-harmonic generation in a noncentrosymmetric pyrochlore oxide, revealing that hybridization induces local inversion symmetry breaking in Ir 5d bands.

Contribution

It demonstrates that strong hybridization between Ir 5d and O' 2p orbitals induces local inversion symmetry breaking, leading to enhanced optical second-harmonic generation in Pb$_{2}$Ir$_{2}$O$_{7-x}$.

Findings

Gigantic second-harmonic generation signals observed.

Inversion symmetry breaking mainly due to Pb-O' network distortion.

Hybridization induces noncentrosymmetricity in Ir 5d bands.

Abstract

The structural, electronic, and optical properties of pyrochlore-type Pb$_{2}$Ir$_{2}$O$_{6}$O'$_{0.55}$, which is a metal without spatial inversion symmetry at room temperature, were investigated. Structural analysis revealed that the structural distortion relevant to the breakdown of the inversion symmetry is dominated by the Pb-O' network but is very small in the Ir-O network. At the same time, gigantic second-harmonic generation signals were observed, which can only occur if the local environment of the Ir 5$d$ electrons features broken inversion symmetry. First-principles electronic structure calculations reveal that the underlying mechanism for this phenomenon is the induction of the noncentrosymmetricity in the Ir 5$d$ bands by the strong hybridization with O' 2$p$ orbitals. Our results stimulate theoretical study of inversion-broken iridates, where exotic quantum states such as a topological insulator and Dirac semimetal are anticipated.