Raman and fluorescence characteristics of resonant inelastic X-ray scattering from doped superconducting cuprates

TL;DR

This study demonstrates that resonant inelastic X-ray scattering (RIXS) can effectively probe collective spin excitations, such as magnons and paramagnons, in doped high-temperature cuprate superconductors, clarifying previous debates.

Contribution

The paper provides conclusive experimental and theoretical evidence that Raman-like RIXS signals correspond to collective spin excitations in doped cuprates, confirming RIXS as a tool for studying spin dynamics.

Findings

Raman-like RIXS excitations are collective spin excitations (magnons and paramagnons).

Fluorescence-like shifts are mainly due to particle-hole charge excitations.

RIXS can be used under proper conditions to probe paramagnons in doped cuprates.

Abstract

Measurements of spin excitations are essential for an understanding of spin-mediated pairing for superconductivity; and resonant inelastic X-ray scattering (RIXS) provides a considerable opportunity to probe high-energy spin excitations. However, whether RIXS correctly measures the collective spin excitations of doped superconducting cuprates remains under debate. Here we demonstrate distinct Raman- and fluorescence-like RIXS excitations of Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_{2}$O$_{8+{\delta}}$ in the mid-infrared energy region. Combining photon-energy and momentum dependent RIXS measurements with theoretical calculations using exact diagonalization provides conclusive evidence that the Raman-like RIXS excitations correspond to collective spin excitations, which are magnons in the undoped Mott insulators and evolve into paramagnons in doped superconducting compounds. In contrast, the fluorescence-like shifts are due primarily to the continuum of particle-hole excitations in the charge channel. Our results show that under the proper experimental conditions RIXS indeed can be used to probe paramagnons in doped high-$T_c$ cuprate superconductors.