Interplay between Hund's coupling and spin-orbit interaction on elementary excitations in Sr2IrO4

We study the elementary excitations in 5d transition metal oxideSr$_2$IrO$_4$ by calculating the particle-hole Green's function within the random phase approximation on an antiferromagnetic ground state in the two-dimensional multi-orbital Hubbard model. The obtained magnetic excitations of bound states show a characteristic dispersion in consistent with the experiments. In addition, two new types of excitations are found due to the interplay between spin-orbit interaction and Hund's coupling: a magnetic excitation as a bound state, which has energy gap at the $\Gamma$ point, and an exciton as a resonant mode in the continuum of electron-hole pair creation.