In the study of complex materials, Resonant Elastic X-ray Scattering (REXS) has emerged as a powerful tool for probing spatial modulation of spin, charge, and orbital configuration. This technique combines the advantages of diffraction and x-ray absorption spectroscopy (XAS), providing unique insights into the electronic structure and magnetic properties of materials.

Theory

Resonant Elastic X-ray Scattering occurs when the energy of the incident x-ray photon is tuned close to some absorption edge of an element in the target sample under study. This resonance enhances the scattering cross-section, allowing for the selective probing of specific atomic species in a crystal. The process involves virtual transitions from core levels into unoccupied states close to the Fermi level, which depend strongly on the spin, charge, and orbital configuration of resonant scattering centers.

Applications

With its unique sensitivity, REXS has been used to study a variety of intriguing material systems, including high-temperature superconductors and rare-earth tritelluride charge density wave (CDW) compounds. The technique has also been combined with the pump-probe approach to realize time-resolved femtosecond resonant x-ray diffraction, enabling the probing of nonequilibrium states in materials.

For example, in a study on stripe-ordered nickelates, researchers found that spin and charge orders are strongly coupled during a nonequilibrium process despite distinct spin and charge energy scales. This information provides valuable insights into the complex physics of these systems.

Resonant Inelastic X-ray Scattering (RIXS)

In addition to REXS, our research group also performs Resonant Inelastic X-ray Scattering (RIXS) experiments. RIXS is a powerful tool for probing elementary excitations in materials, such as plasmons, charge-transfer excitations, crystal-field and orbital excitations, magnons, and phonons.

Selected Publications

Our research group has published several papers on the application of REXS and RIXS to various material systems. Some selected publications include:

• 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, 1204 (2012))

These studies demonstrate the power of REXS and RIXS in uncovering the complex physics of materials and provide valuable insights into their electronic structure and magnetic properties.

****, Resonant Elastic X-ray Scattering is a powerful tool for studying complex materials, providing unique insights into their electronic structure and magnetic properties. By combining the advantages of diffraction and x-ray absorption spectroscopy, REXS offers a powerful probe for spatial order and electronic structure in materials.