Charge response function probed by resonant inelastic x-ray scattering: the signature of electronic gaps of YBa$_2$Cu$_3$O$_{7-\delta}$

TL;DR

This paper demonstrates how to extract information about electronic gaps in high-temperature superconducting cuprates from RIXS spectra, revealing the presence of superconducting and pseudogaps through temperature-dependent analysis.

Contribution

It introduces a method to analyze RIXS spectra to identify electronic gaps in cuprates, focusing on the charge susceptibility component often overlooked in previous studies.

Findings

Identification of superconducting gap signatures in RIXS spectra.

Detection of pseudogap features through temperature dependence.

Method applicable to other correlated materials.

Abstract

In strongly correlated systems the complete determination of the dynamical susceptibility $\chi(\textbf{q}, \omega)$ 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 ($\chi''_c(\textbf{q}, \omega)$) and the spin ($\chi''_s(\textbf{q}, \omega)$) contributions, only the latter has been extensively studied with RIXS so far. Here we show how to extract from RIXS spectra of high-$T_c$ superconducting cuprates relevant properties of $\chi''_c$, such as the presence of the superconducting gap and of the pseudogap. In particular, we exploit the temperature dependence of the Cu L$_3$ edge RIXS spectra of underdoped YBa$_2$Cu$_3$O$_{7-\delta}$ at specific wave-vectors q. The signature of the two gaps is in the departure of the low energy Bosonic excitation continuum from the statistical temperature-dependence. 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.