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Synchrotron X-ray scattering


Fig. 1. Schematic of synchrotron X-ray scattering


Fig. 2. Left: correction of the atomic scattering factor near the absorption edge. Top is the X-ray absorption spectra (real part of the correction) and bottom is the imaginary part obtained from Kramer-Kronig transformation of the real part. Right: resonant profile across the Bi L edge for Bi2Te3 films. The sharp resonance contrast reveals the electron density from Bismuth.


Synchrotron X-ray scattering utilizes X-rays generated from synchrotron facilities to study the crystal structure and electronic orderings in materials. Compared to lab-based X-ray sources, synchrotron X-ray sources have many advantages, including a very high intensity (more than 6 orders of magnitude larger than lab sources), high coherence/collimation, large tunability of photon nergy and polarization, natural time structure, etc. These characteristics allow for quantitative analysis of atomic positions to high precision, and charge/orbital orderings in quantum materials, which cannot be easily probed by other experimental methods. The recent discovery of charge ordering in cuprates has attracted considerable research interest and such electronic ordering has become an important topic in many correlated electron systems. Since charge/orbital order involves only a small number of valence electrons/orbitals near the Fermi level, it is particularly difficult to detect via conventional scattering techniques. Resonant elastic X-ray scattering combines conventional X-ray scattering with X-ray absorption spectroscopy (see Fig. 2), which allows one to probe element/orbital specific ordering due to the strong variation of atomic scattering factor with incoming photon energy near the absorption edge. While X-ray absorption measurements directly reveal the imaginary part of the atomic scattering factor, the real part of the correction can be obtained directly by Kramer-Kronig transformation. Once the corrections to the atomic scattering factor is known, the scattering intensity can be directly linked to the orbital-specific ordering. The resonance contrast, i.e., the change of the scattering intensity due to the variation of the incoming photon energy/polarization, reveals the ordering pattern, strength, spatial periodicity/correlations, etc.  Careful analysis of such information and comparison with theoretical calculations allows us to obtain the detailed charge/orbital ordering information. In addition, for strongly penetrating hard X-rays, it can be also used to measure a variety of materials under extreme conditions, including strong magnetic field, high pressure, etc. This makes X-rays an important tool for understanding the electronic structure in these systems under extreme physical conditions.