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The spin-orbit exciton (SOE) in iridates is a propagating quasiparticle whose dispersion in a Mott insulator directly reflects the underlying antiferromagnetic (AF) order. The Ruddlesden-Popper iridates Srₙ₊₁IrₙO₃ₙ₊₁ evolve from the Jeff = ½ AF insulators Sr₂IrO₄ and Sr₃Ir₂O₇ to the semimetallic perovskite SrIrO₃, with an insulator-metal transition (IMT) governed by strong spin-orbit coupling, dimensionality, and electronic correlations. Despite extensive RIXS studies of the insulating members, the fate of the magnetic and spin-orbital excitations on the metallic side of the phase diagram has remained unresolved.
Here I present a study of SrIrO₃, a “strange (semi-)metal” with high-mobility carriers and no magnetic order, but on the verge of a Mott insulating state. Using Ir L₃-edge RIXS on a series of heterostructures spanning the metallic side of the IMT, we find a magnetic mode with an AF-like dispersion, but with a large gap and large linewidth, revealing remnants of AF Mott physics. In contrast, the SOE shows an inverted dispersion and doubled periodicity, reflecting its decoupling from the magnetic sector and the dominant role of charge fluctuations. Localized and itinerant physics therefore coexist in this material, and the SOE dispersion emerges as a sensitive fingerprint of the balance between spin correlations and charge fluctuations.
[1] E. Paris et al., Phys. Rev. Lett. 135, 186506 (2025).
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