Beyond spin wave theory on a 5d transtion-metal oxide

5d transition metal oxides (TMOs) offer a new playground for quantum magnetism with some unique features. For instance, a Heisenberg antiferromagnet in square lattice iridate Sr2IrO4 reveals similar low-energy effective physics with high-Tc superconductor parent antiferromagnet, La2CuO4, with its spin-orbit coupling driven isospins. However, there are some different features in detailed physics between them, one of which is small deviation of low-energy magnetic dispersion from what spin wave theory gives. In this regard, we used resonant inelastic x-ray scattering to show that the spin wave theory cannot perfectly explain the low-energy magnetic behavior of a 5d transition-metal oxide, Sr2IrO4. The high momentum-resolution low-energy dispersion and its temperature dependent behaviors cannot be fully understood only by spin wave theory, but higher-order exchange order terms should be included to describe these behaviors.