  The isospin-phonon coupling in Sr2IrO4 is investigated by temperature
dependent phonon Raman scattering. Anomalous behavior in the frequency of all
studied optical phonons is observed below the magnetic transition temperature
TN ~240 K. The strongest effect is detected for the A1g mode at 272 cm-1
associated with the modulation of the Ir-O-Ir bond angle. Additionally, the A1g
mode at 560 cm-1 shows a Fano asymmetric lineshape sensitive to TN, supporting
the existence of low-energy (~70 meV) electronic excitations that are
renormalized by the magnetic order. These results reveal new aspects of the
interaction between the crystal lattice and electronic degrees of freedom in
this spin-orbit entangled Mott insulator.
