Orbiton-phonon coupling in Ir(5d ) double perovskite BaYIrO.

BaYIrO, a Mott insulator, with four valence electrons in Ir d-shell (5d ) is supposed to be non-magnetic, with J   =  0, within the atomic physics picture. However, recent suggestions of non-zero magnetism have raised some fundamental questions about its origin. We focus 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 IrO 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.