Charge response function probed by resonant inelastic x-ray scattering: Signature of electronic gaps of YBa2⁢Cu3⁢O7-\delta

In strongly correlated systems, the complete determination of the dynamical susceptibility χ(q,ω) is of special relevance because of the entwinement of the spin and charge components. Although resonant inelastic x-ray scattering (RIXS) spectra are directly related to both the charge [χc′′(q,ω)] and the spin [χs′′(q,ω)] contributions, only the latter has been extensively studied with RIXS so far. Here we show how to extract from RIXS spectra of high-Tc superconducting cuprates relevant properties of χc′′, such as the presence of the superconducting gap and of the pseudogap. In particular, we exploit the temperature dependence of the Cu L3 edge RIXS spectra of underdoped YBa2Cu3O7-δ at specific wave vectors q. The signature of the two gaps is given by the departure of the low-energy excitation continuum from the Bosonic thermal evolution. This approach can be immediately used to investigate systematically the nature of the pseudogap in cuprates, thereby taking advantage of the RIXS technique that does not suffer the limitations of surface-sensitive electron spectroscopies. Its extension to other interesting materials is foreseen.