A Non-Perturbative, Variational Technique of Calculating RIXS Spectral Response Functions

The field of strongly correlated electrons, and in fact condensed matter physics in general, makes heavy use of spectroscopic experiments to pry into the intricate details of the physical properties of solids. In order to explain the spectra measured in such an experiment a theoretical calculation has to be provided, often relying on some effective model of the physical system under consideration. To explain phenomena such as high-T_c superconductivity in cuprates or exotic magnetic and orbital phases in manganites, a wide range of spectroscopic measurements are performed, each requiring a different setup and apparatus. Among them, RIXS is a prominent technique which allows one to probe different degrees of freedom of a system with precise control over electron transitions. Our aim is to develop a simple yet accurate standard theoretical tool for calculating RIXS response functions, relying on the non-perturbative, variational (momentum average) approximation to calculate the relevant propagators. This approach should bridge between the exact diagonalization of small clusters vs. perturbative expansion techniques, which until now have dominated the theory of RIXS.