Collective excitations such as plasmons and paramagnons are fingerprints of atomic-scale Coulomb and exchange interactions between conduction electrons in metals. The strength and range of these interactions, which are encoded in the excitations’ dispersion relations, are of primary interest in research on the origin of collective instabilities such as superconductivity and magnetism in quantum materials. Here we report resonant inelastic x-ray scattering experiments on the correlated 4d-electron metal Sr₂RhO₄, which reveal a spin-orbit entangled collective excitation. The dispersion relation of this mode is opposite to those of antiferromagnetic insulators such as Sr₂IrO₄, where the spin-orbit excitons are dressed by magnons. The presence of propagating spin-orbit excitons implies that the spin-orbit coupling in Sr₂RhO₄ is unquenched, and that collective instabilities in 4d-electron metals and superconductors must be described in terms of spin-orbit entangled electronic states.

等离激元和顺磁振子等集体激发是原子尺度库仑和金属中传导电子之间交换相互作用的指纹。这些相互作用的强度和范围被编码在激发色散关系中，是研究集体不稳定性（例如量子材料中的超导性和磁性）起源的主要兴趣。在这里，我们报告了相关 4 d电子金属 Sr ₂ RhO ₄的共振非弹性 X 射线散射实验，揭示了自旋轨道纠缠集体激发。该模式的色散关系与反铁磁绝缘体如Sr ₂ IrO _{4的色散关系相反}，其中自旋轨道激子由磁振子修饰。_{传播自旋轨道激子的存在意味着Sr 2} RhO ₄中的自旋轨道耦合是未淬灭的，并且4 d电子金属和超导体中的集体不稳定性必须用自旋轨道纠缠电子态来描述。