Anisotropic electron damping and energy gap in B2Sr2CaCu2O8-d

The many-body electron-electron interaction in cuprates causes broadening of the electronic bands in k space, leading to a deviation from the standard Fermi liquid. While a k-dependent anisotropic electronic scattering (k-DAES) has been assessed by photoemission, its fingerprint in Q space has been scarcely considered. Here, we explore the Q-dependent electron dynamics in optimally doped Bi2Sr2CaCu2O8+δ through the evolution of low-energy charge excitations as measured by resonant inelastic x-ray scattering (RIXS). In the normal state, the RIXS spectra display a continuum of excitations down to 0 meV, while the superconducting state features a spectral weight suppression below 80 meV without any enhancement at higher energies. To interpret the energy and Q evolution of our data, we introduce a phenomenological expression of the charge susceptibility by including the k-DAES. We show that only the charge susceptibility with k-DAES captures the RIXS data, highlighting the importance of k-DAES when describing the Q dependence of charge excitations from 0 to a few eV scale. Furthermore, we also find that the inclusion of k-DAES is essential when quantitative parameters such as the electronic energy gap are extracted from RIXS data.