共振非弹性X射线散射在量子材料领域的应用

Resonant inelastic X-ray scattering applications in quantum materials

共振非弹性X射线散射(resonant inelastic X-ray scattering,RIXS)是一种先进的基于同步辐射和自由电子激光光源的光进光出的谱学探测手段.在过去的十几年,RIXS的能量分辨不断被提高,其对凝聚态物质的研究也从最初的晶体场分裂和电荷转移激发,发展到产生于包括电荷、自旋、轨道、晶格4个量子自由度的集体激发行为及相关的序参量.本文总结了近几年高分辨软X射线RIXS在量子材料领域,如铜基和镍基高温超导材料等离子激发及磁激发的研究,量子材料中的集体轨道激发、激子激发和高阶磁激发,以及对磁性金属和拓扑磁性材料中磁激发的探测.

The essence of quantum materials lies in the intricate coupling among charge,spin,orbital and lattice degrees of freedom.Although X-ray photoemission spectroscopy and inelastic neutron scattering have advantages in detecting fermionic single-particle spectral function and bosonic spin excitations in quantum materials,respectively,probing other bosonic collective excitations especially their coupling is not possible until the establishment of the advanced resonant inelastic X-ray scattering(RIXS).In the past decades,RIXS has flourished with continuously improved energy resolution which made a paradigm shift from measuring crystalfield splitting and the charge-transfer excitation,to probing collective excitations and the order parameters of all degrees of freedom.This review paper summarises the latest research progress of quantum materials studied by the soft X-ray RIXS.For instance,three-dimensional collective charge excitations,plasmons,were discovered experiment ally by RIXS in both electron and hole doped cuprate superconductors.The collective orbital excitations and excitons were found in copper and nickel based quantum materials.For the newly discovered nickelate superconductors,RIXS has made substantial contributions to characterising their electronic and magnetic excitations and the related ordering phenomena critical for an in-depth understanding of the underlying superconducting mechanicsm.The RIXS is a unique tool in probing the higher-order spin excitations in quantum materials due to the strong spin-orbit coupling and the core-valence exchange interaction.The RIXS is also found to be superior in probing the Stoner magnetic excitations in magnetic metals and topological magnetic materials.Finally,the development of RIXS technology in Chinese large-scale research facilities is briefly prospected.