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Electrons in a metal are like a crowd in a subway: each on their own, but at the slightest push, everyone moves at once. Physicists call this collective oscillation of the electron gas a plasmon. In superconductors, these oscillations are especially important because they reveal how electrons interact with each other.

In nickelates — recently discovered nickel-based superconductors — plasmons had not been observed until now. Now, scientists used resonant inelastic X-ray scattering (a method where X-rays transfer some energy to electrons, and the losses reveal collective motions) and for the first time detected plasmons in the metallic nickelate Pr4Ni3O8. It turns out that in this material, plasmons decay noticeably more strongly and their velocity is lower than in cuprates — another class of high-temperature superconductors based on copper.

Such behavior indicates that in nickelates, electrons hop between atoms more weakly, and electric charges are screened more strongly. This means that the analogies often drawn between nickelates and cuprates only partially hold. Understanding plasmons will help clarify how superconductivity works in nickelates and whether recipes developed for cuprates can be transferred to them.

Original abstract

as written by the authors arXiv:2601.12160

The discovery of nickelate superconductors has opened a new arena for studying the behavior of correlated electron liquids that give rise to unconventional superconductivity. While critical information about a material's charge dynamics is encoded in its plasmons, collective modes of the electron gas, these excitations have not yet been observed in nickelate materials. Here, we use resonant inelastic x-ray scattering (RIXS) to detect plasmons in the metallic, low-valence nickelate Pr4Ni3O8. Although qualitatively similar to those in cuprates, the nickelate plasmons are more heavily damped and have a lower velocity than those in a cuprate at comparable doping, which we attribute to reduced electronic hopping and enhanced screening of the long-range Coulomb interactions. Furthermore, the plasmons in Pr4Ni3O8 soften with increasing temperature, in contrast to the cuprate, where plasmons remain at nearly fixed energy but become more strongly damped. Taken together, these results reveal a distinct charge-screening landscape in nickelates and place quantitative constraints on analogies to cuprates.