Establishing theoretically the capacity of resonant inelastic X-ray scattering to probe the phase and excitations of the superconducting order parameter

The capability to probe the dispersion of elementary spin, charge, orbital, and lattice excitations has positioned Resonant Inelastic X-ray Scattering (RIXS) at the forefront of photon science. Here we develop the scattering theory for RIXS on superconductors, calculating its momentum-dependent scattering amplitude. Considering unconventional superconductors with different pairing symmetries and, in particular, evaluating the theoretical RIXS spectra for cuprate and pnictide-like systems, we show that the low-energy scattering is strongly affected by the superconducting gap and coherence factors. This establishes RIXS as a tool to disentangle pairing symmetries and to probe the elementary excitations of unconventional superconductors.