Jahn-Teller effect in systems with strong on-site spin-orbit coupling

When strong spin-orbit coupling removes orbital degeneracy, it would at the same time appear to render the Jahn-Teller mechanism ineffective. We discuss such a situation, the $t_{2g}$ manifold of iridates, and show that, while the Jahn-Teller effect does indeed not affect the $j=1/2$ antiferromagnetically ordered ground state, it leads to distinctive signatures in the $j=3/2$ spin-orbit exciton. It allows for a hopping of the spin-orbit exciton between the nearest neighbor sites without producing defects in the $j=1/2$ antiferromagnet. This arises because the lattice-driven Jahn-Teller mechanism only couples to the orbital degree of freedom, but is not sensitive to the phase of the wave function that defines isospin $j_z$. This contrasts sharply with purely electronic propagation, which conserves isospin, and presence of Jahn-Teller coupling can explain some of the peculiar features of measured resonant inelastic x-ray scattering spectra of Sr$_2$IrO$_4$.