NSLS-II Friday Lunchtime Seminar

Resonant Inelastic X-ray Scattering (RIXS) has become a versatile tool for probing quantum materials, allowing simultaneous access to charge, lattice, spin and orbital degrees of freedom. Much of its success in recent years has been driven by huge improvements in instrumentation, and with the recent advent of X-ray free electron lasers such as the LCLS, it has become possible to translate this technique into the time-domain. Given its ability to address multiple degrees of freedom at once, time-resolved RIXS (trRIXS) therefore has huge potential for investigating materials with cooperative dynamics. Nevertheless, it remains technically very challenging to perform such experiments. We performed trRIXS at the oxygen K-edge in order to probe short-range spin correlations in CuGeO3. This quasi-1D material formed of chains of edge-sharing CuO4 plaquettes, displays pronounced anti-ferromagnetic (AFM) nearest-neighbor spin correlations and transitions into a spin-Peierls phase below 14 K, highlighting a close relation between magnetic and lattice sub-systems. As a result of these AFM correlations a Zhang-Rice singlet (ZRS) exciton can develop during the RIXS process, resulting in a distinct energy loss peak within the charge transfer gap. The amplitude of this ZRS exciton directly reflects the nearest-neighbor AFM correlations [1]. By photoexciting CuGeO3 across the charge gap (~4 eV) we induce a sudden reduction of the intensity of the ZRS exciton within 1 ps, which rapidly recovers before gradually decreasing again towards a stable value after ~10 ps. Comparison to equilibrium measurements and a thermal model reveal that the longer time scale dynamics are dominated by heating of the lattice. However, the initial drop and recovery of the ZRS at short time scales clearly imply a non-thermal behavior. In comparison, lower fluence data shows only a thermal component, suggesting a threshold effect. By comparison with model Hamiltonian calculations we reveal