Shedding New Light on Spin Excitations: Magnon-Magnon Scattering in a High-T$_{c}$ Parent Compound Using Soft X-rays

The struggle to understand high-temperature superconductivity (HTSC) has taken place in the field of condensed matter physics for a relatively long time. The difficulty in discovering a HTSC mechanism seems to originate from the confluence of two very complicated problems; electron correlation and quantum magnetism. Novel techniques that specifically probe the electronic or magnetic behavior of these materials are highly sought. We report the direct observation of spin-flip (SF) excitations using resonant inelastic soft x-ray scattering (RIXS). The antiferromganetic (AFM) charge transfer insulator, CaCuO$_{2}$, was irradiated by soft x-rays tuned to the Cu M-edge ($\sim $ 75 eV). Magnon-magnon (2 spin-flips) excitations were revealed as low-energy loss (0.39 eV) features in photon-in/photon-out experiments. The process is analogous to 2-magnon laser Raman scattering. Collected RIXS spectra provide a measurement of the 2 spin-flip excitation energy and the extraction of the AFM exchange of CaCuO$_{2.}$ These results mark the first report of two-spin-flip magneto-optical excitations revealed through Raman scattering using x-rays. This work reveals the utility of RIXS as a new probe of the spin structure in strongly correlated electron materials.