Resonant inelastic X-ray scattering (RIXS) is a powerful bulk-sensitive photon-in / photon-out spectroscopic probe of the electronic structure of condensed matter with atomic and orbital sensitivity. It is a unique tool for studying excitations from the electronic ground state in correlated transition-metal oxides, being directly sensitive to charge-, orbital- and spin-degrees of freedom. Dedicated instrumentation for soft X-ray RIXS with ultra-high resolution has become available for users at the ADvanced RESonant Spectroscopies (ADRESS) beamline  at the Swiss Light Source (SLS).
We report on construction and capabilities of the ADRESS beamline at the SLS. Its optical scheme is based on a Plane Grating Monochromator operated in collimated light. The beamline is calculated to deliver soft X-rays with variable polarization (circular and linear) between 0.4 and 1.8 keV at high resolving power up to 33000 near 1 keV. The flux of 1 keV photons on the sample ranges from 3x1011 to 1x1013 photons/s/0.01%BW for a resolving power of 33’000 and 10’000, respectively. The undulator for this beamline adopts an Apple-type scheme with 4 arrays of permanent magnets with a fixed magnetic gap. In this design horizontal movements of the magnetic arrays are used to adjust polarization (opposite arrays) and photon energy (two adjacent arrays). Ellipsoidal refocusing optics in front of the experimental station for the RIXS investigations reduces the vertical beam dimension on the sample to below 6 mm as required for high detection efficiency of the inelastically scattered X-rays.
The SAXES (Super Advanced X-ray Emission Spectrograph) RIXS spectrometer  of the ADRESS beamline is based on a variable line spacing spherical grating and has a resolving power of ca. 12000 for 1 keV. It allows varying the scattering geometry between incident and inelastically scattered X-rays in order to study low-energy excitations as a function of momentum transfer, thereby enabling characterization of localized vs. delocalized character of the respective excitations. We present RIXS investigations of magnetic and electronic excitations in the low dimensional spin systems Sr14Cu24O41  and Sr2CuO3, demonstrating the unique capabilities of our set-up for momentum transfer dispersion studies in correlated materials. For the metal-insulator transition in VO2, we show how RIXS can reveal important spectral information on low-energy excitations in moderately correlated materials with large degree of delocalization.
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