High resolution resonant inelastic X-ray scattering from solids in the soft range

In resonant inelastic X-ray scattering (RIXS), the incident photon having energy in the region of a core threshold promotes an electron into the empty states creating a strongly excited state; this is the intermediate state in scattering since it decays with a lower energy photon leaving behind the sample in an excited state. As a consequence, RIXS can give very detailed information on the elementary excitations of the sample in the absence of core holes but with site and chemical sensitivity typical of core level spectroscopy. The implementation of RIXS is an impressive extension of the possibilities offered by nonresonant X-ray scattering and by neutron spectroscopy of magnetic excitations and of lattice dynamics. RIXS does not match inelastic neutron scattering with respect to energy resolution, but the sensitivity to charge, the atomic and site sensitivity, the better momentum resolution, and the easy measurement of high-energy excitations make this option very appealing. A great advantage is that RIXS in the soft X-ray range (roughly below 1,800 eV) is possible with very tiny samples down to atomic monolayer while neutrons require very massive samples. This tremendously expanded the applications to high Tc magnetic excitations and charge ordering. Without any doubt, the cross- fertilization between RIXS and neutrons will be a crucial issue in the coming years. The success of RIXS has been possible because of the tremendous progress made at modern synchrotron radiation facilities and ongoing improvements of instrumentation including the implementation of polarization analysis. Moreover free-electron lasers will enable time-resolved RIXS in the near future. In this contribution, we limit ourselves to soft RIXS, and we chose some examples relevant also to the interplay with neutrons. We begin with a qualitative description of the basic concepts. The introduction concludes with the results on MnO as an example of traditional RIXS. In the subsequent text, we discuss the power of modern high- resolution RIXS by presenting state-of-the-art results on cuprates both undoped parent compounds and doped superconductors. We also show some peculiar aspects of one-dimensional systems directly visible with RIXS and give a short review of the new trends emerging at present. Finally, we present a series of topics on the design and operation of a high-resolution spectrometer.