Charge-Transfer Excitations in the Model Superconductor HgBa$_2$CuO$_{\bf 4+\delta}$

TL;DR

This study uses resonant inelastic x-ray scattering to investigate charge-transfer excitations in HgBa$_2$CuO$_{4+\delta}$, revealing a 2 eV excitation indicative of a charge-transfer pseudogap within the superconducting phase.

Contribution

It demonstrates the importance of incident photon energy dependence in RIXS to resolve weakly-dispersive charge-transfer excitations in a high-temperature superconductor.

Findings

Identification of a 2 eV charge-transfer excitation

Observation of weakly-dispersive electron-hole excitations

Evidence of a charge-transfer pseudogap in the superconducting phase

Abstract

We report a Cu $K$-edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa$_2$CuO$_{4+\delta}$ at optimal doping ($T_{\rm c} = 96.5 $ K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly-dispersive ($ < 0.5$ eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a charge-transfer pseudogap deep in the superconducting phase. Quite generally, our data demonstrate the importance of exploring the incident photon energy dependence of the RIXS cross section.