Model compounds of the high temperature superconductors (HTCS) are considered of big interest for the understanding of superconductivity but also for their fascinating, tunable magnetic and electronic properties. Thanks to the low dimensionality, these model systems allow for a thorough theoretical analysis enabling to achieve a good understanding of the interactions at work. CuGeO_3 is a well known realization of one-dimensional anti-ferromagnetic S=1/2 spin chain, made of edge-sharing CuO_4 – plaquettes, and undergoing a Spin-Peierls transition at T ≈14 K. Recently, thanks to the improved energy resolution, soft x-ray resonant inelastic scattering (RIXS) has demonstrated to be a suitable tool for the study of the HTCS. We performed RIXS experiments on CuGeO_3 at the ADRESS beamline of the Swiss Light Source. Intra-site d-d excitations can be clearly observed at the Cu-L edge, while inter-site Cu-O charge transfer processes are enhanced at the O-K edge. Temperature depend!
ent studies unambiguously identify the presence of two Zhang-Rice (ZR) singlet excitons in CuGeO_3. The strong variation of their spectral weight with temperature can be directly linked to the local magnetic correlation at the ground state. Complementary data measured with high resolution electron energy loss spectroscopy (EELS) at the IFW Dresden support the RIXS results. Finally RIXS calculations are in qualitative good agreement with our measurements by presenting two temperature dependent features in the same energy range as the experiment: therefore the theory allows for a direct interpretation of the different ZR singlet excitons, respectively formed between nearest neighbor and next nearest neighbor plaquettes.
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