  We have used Raman scattering to investigate the magnetic excitations and
lattice dynamics in the prototypical spin-orbit Mott insulators Sr2IrO4 and
Sr3Ir2O7. Both compounds exhibit pronounced two-magnon Raman scattering
features with different energies, lineshapes, and temperature dependencies,
which in part reflect the different influence of long-range frustrating
exchange interactions. Additionally, we find strong Fano asymmetries in the
lineshapes of low-energy phonon modes in both compounds, which disappear upon
cooling below the antiferromagnetic ordering temperatures. These unusual phonon
anomalies indicate that the spin-orbit coupling in Mott-insulating iridates is
not sufficiently strong to quench the orbital dynamics in the paramagnetic
state.

Strongly Correlated Electrons;Materials Science