Resonant Elastic X-ray Scattering (REXS)
In the study of complex materials, resonant elastic x-ray scattering provides a sensitive probe for spatial modulations of spins, charges, and orbitals. This unique sensitivity is achieved by merging diffraction and x-ray absorption spectroscopy (XAS) into a single experiment, where the scattering provides information about spatial modulations and the XAS provides sensitivity to the electronic structure. More precisely, resonant x-ray scattering close to an absorption edge involves virtual transitions from core levels into unoccupied states close to the Fermi level and these virtual transitions depend strongly on the spin, charge and orbital configuration of the resonant scattering centres. Furthermore, the resonance process strongly enhances the scattering cross-section and allows the opportunity to study selected atomic species in a crystal .
With accesses to the state-of-the-art facilities in SLAC and LBNL, our group enjoys a distinct advantage in utilizing soft x-ray scattering techniques to investigate a variety of intriguing material systems such as high temperature superconductors and rare-earth tritelluride charge density wave (CDW) compounds . We have also been able to combine the pump-probe approach with this technique to realize time-resolved femtosecond resonant x-ray diffraction, which aids us in probing the interesting physics in nonequilibrium states of materials such as the stripe-ordered nickelate where spin and charge orders are found to be strongly coupled even during the nonequilibrium process despite distinct spin and charge energy scales .
Resonant Inelastic X-ray Scattering (RIXS)
We also perform resonant inelastic x-ray scattering (RIXS) experiments, in which one scatters x-ray photons inelastically off matter. The changes in energy, momentum, and polarization of the scattered photon are transferred to intrinsic excitations of the material under study, so RIXS is able to probe many elementary excitations such as plasmons, charge-transfer excitations, crystal-field and orbital excitations, magnons, and phonons. Besides, similar to REXS, the resonance in RIXS greatly enhances the cross-section and adds selectivity to atomic species. In this aspect, RIXS offers a wide landscape in the study of strongly correlated electron systems. .
 J. Fink et al. Resonant elastic soft x-ray scattering. Rep. Prog. Phys. 76, 056502 (2013)
 Luuk J.P. Ament et al. Resonant inelastic x-ray scattering studies of elementary excitations. Rev. Mod. Phys. 83, 705 (2011)
 M. Hashimoto et al. Direct observation of bulk charge modulations in optimally doped Bi1.5Pb0.6Sr1.54CaCu2O8+δ. Phys. Rev. B 89, 220511(R) (2014)
 Y. D. Chuang et al. Real-time manifestation of strongly coupled spin and charge order parameters in stripe-ordered La1.75Sr0.25NiO4 nickelate crystals using time-resolved resonant x-ray diffraction. Phys. Rev. Lett. 110, 127404 (2013)
 W. S. Lee et al. Phase fluctuations and the absence of topological defects in a photo-excited charge-ordered nickelate. Nature Comm. 3, 838 (2012)
 W. S. Lee et al. Resonant enhancement of charge density wave diffraction in the rare-earth tritellurides. Phys. Rev. B 85, 155142 (2012)
 J. J. Lee et al. Charge-orbital-lattice coupling effects in the dd excitation profile of one-dimensional cuprates. Phys. Rev. B 89, 041104(R) (2014)
 W. S. Lee et al. Role of lattice coupling in establishing electronic and magnetic properties in quasi-one-dimensional cuprates. Phys. Rev. Lett. 110, 265502 (2013)