REXS

REXS stands for resonant elastic X-ray scattering, a photon-based spectroscopic technique used to probe electronic order in solid-state materials. By tuning the incident X-ray energy to the absorption edge of a target element, REXS enhances the scattering amplitude at resonance, increasing sensitivity to charge, orbital, and magnetic order as well as lattice distortions.

In practice, a crystal is illuminated with monochromatic X-rays at a chosen energy; elastic scattering is measured as a function of momentum transfer. The resonance condition amplifies weak diffraction signals and makes the scattering cross-section sensitive to the valence state and symmetry of the resonant atom, enabling element-specific investigations of ordered phases.

Applications include studies of transition-metal oxides, cuprates, manganites, and layered materials exhibiting charge density waves, orbital ordering, or complex magnetism. REXS can map long-range order, short-range correlations, and temperature-dependent transitions, complementing neutron and non-resonant X-ray diffraction.

Instrumentation typically relies on synchrotron radiation, with a monochromator to select the resonant energy, a diffractometer to access reciprocal-space regions, and detectors to measure scattered photons. Polarization analysis and energy scans further enhance sensitivity to electronic anisotropy and symmetry.

REXS is distinct from resonant inelastic X-ray scattering (RIXS), which probes dynamical excitations, whereas REXS provides snapshots of static or quasi-static order. Together with other techniques, REXS helps characterize complex ordering phenomena in correlated electron systems.

In physics literature, REXS has been in use since the early 2000s and is routinely available at major synchrotrons. Developments continue in soft X-ray REXS to investigate light elements and surface phenomena.

Note that the acronym REXS can have other meanings in technology and industry, but in condensed-matter physics it most commonly refers to resonant elastic X-ray scattering.