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Femtosecond dynamics of magnetic excitations from resonant inelastic x-ray scattering in CaCu2O3

Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we demonstrate that femtosecond dynamics of magnetic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them and determine their respective timescales. By comparing theory and experiment, we find that double spin flips need a finite amount of time to be generated, rendering them sensitive to the core-hole lifetime, whereas single spin flips are to a very good approximation independent of it. This shows that RIXS can grant access to time-domain dynamics of excitations and illustrates how RIXS experiments can distinguish between excitations in correlated electron systems based on their different time dependence.