Symmetry-Resolved Two-Magnon Excitations in a Strong Spin-Orbit-Coupled Bilayer Antiferromagnet

We used a combination of polarized Raman spectroscopy and spin wave calculations to study magnetic excitations in the strong spin-orbit-coupled bilayer perovskite antiferromagnet Sr3Ir2O7. We observed two broad Raman features at similar to 800 and similar to 1400 cm(-1) arising from magnetic excitations. Unconventionally, the similar to 800 cm(-1) feature is fully symmetric (A(1g)) with respect to the underlying tetragonal (D-4h) crystal lattice which, together with its broad line shape, definitively rules out the possibility of a single magnon excitation as its origin. In contrast, the similar to 1400 cm(-1) feature shows up in both the A(1g) and B-2g channels. From spin wave and two-magnon scattering cross-section calculations of a tetragonal bilayer antiferromagnet, we identified the similar to 800 cm(-1) (1400 cm(-1)) feature as two-magnon excitations with pairs of magnons from the zone-center r point (zone-boundary van Hove singularity X point). We further found that this zone-center two-magnon scattering is unique to bilayer perovskite magnets which host an optical branch in addition to the acoustic branch, as compared to their single layer counterparts. This zone-center two-magnon mode is distinct in symmetry from the time-reversal symmetry broken "spin wave gap" and "phase mode" proposed to explain the similar to 92 meV (742 cm(-1)) gap in resonant inelastic x-ray spectroscopy magnetic excitation spectra of Sr3Ir2O7.