magnet*

The competition between spin-orbit coupling λ and electron-electron interaction U leads to a plethora of novel states of matter, extensively studied in materials such as ruthenates and iridates. Recent theoretical studies predicted that excitonic magnets -- the antiferromagnetic state due to the condensation of excitons (bounded electron-hole pairs) -- can be found in the ground-state of spin-orbit-coupled (t_2g)⁴  Hubbard models [1,2]. We present a detailed study of the magnetic excitations in that excitonic magnet, employing one-dimensional chains (via density matrix renormalization group) and small two-dimensional clusters (via Lanczos).  First we show that the low-energy spectrum is dominated by a dispersive (acoustic) magnonic mode, with extra features arising from the λ=0 state in the phase diagram. Second, and more importantly, we found a novel magnetic excitation forming a high-energy optical mode with the highest intensity at wavevector q→0 .  These unique fingerprints of the excitonic magnet are important in the analysis of neutron and RIXS experiments.

[1] N. Kaushal et al., Phys. Rev. B 101, 245147 (2020), Phys. Rev. B 96, 155111(2017),  Phys. Rev. B 99, 155115 (2019).

[2] G. Khalliulin, Phys. Rev. Lett. 111, 197201 (2013).

[3] N. Kaushal et al., arXiv:2110.11828 (2021) .