Orbiton-Phonon coupling in Ir5+(5d4) double perovskite Ba2YIrO6

TL;DR

This study investigates the orbiton-phonon interactions in Ba2YIrO6, revealing strong phonon renormalization and coupling with orbital excitations, which could explain the emergence of magnetism in this nominally non-magnetic material.

Contribution

It provides experimental evidence of orbiton-phonon coupling in Ba2YIrO6 through Raman scattering, highlighting the role of anharmonic phonons and orbital interactions in magnetic properties.

Findings

Strong phonon self-energy renormalization with temperature

Anomalous softening of phonon modes linked to IrO6 octahedra

Higher energy Raman bands show significant orbital excitation contributions

Abstract

Ba2YIrO6, a Mott insulator, with four valence electrons in Ir5+ d-shell (5d4) is supposed to be non-magnetic, with Jeff = 0, within the atomic physics picture. However, recent suggestions of non-zero magnetism have raised some fundamental questions about its origin. Focussing on the phonon dynamics, probed via Raman scattering, as a function of temperature and different incident photon energies, as an external perturbation. Our studies reveal strong renormalization of the phonon self-energy parameters and integrated intensity for first-order modes, especially redshift of the few first-order modes with decreasing temperature and anomalous softening of modes associated with IrO6 octahedra, as well as high energy Raman bands attributed to the strong anharmonic phonons and coupling with orbital excitations. The distinct renormalization of second-order Raman bands with respect to their first-order counterpart suggest that higher energy Raman bands have significant contribution from orbital excitations. Our observation indicates that strong anharmonic phonons coupled with electronic/orbital degrees of freedom provides a knob for tuning the conventional electronic levels for 5d-orbitals, and this may give rise to non-zero magnetism as postulated in recent theoretical calculations with rich magnetic phases.