Abstract
Resonant inelastic x-ray scattering (RIXS) is a powerful probe of elementary excitations in solids. It is now widely applied to study magnetic excitations. However, its complex cross section means that RIXS has been more difficult to interpret than inelastic neutron scattering (INS). Here we report $\ensuremath{\sim}37$ meV resolution RIXS measurements of the magnetic excitations in ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$, the antiferromagnetic parent of one system of high-temperature superconductors. At high energies ($\ensuremath{\sim}2$ eV), the RIXS spectra show angular-dependent $dd$ orbital excitations in agreement with previous RIXS studies but show new structure. They are interpreted with single-site multiplet calculations. At low energies ($\ensuremath{\lesssim}0.3$ eV), we model the wave-vector-dependent single magnon RIXS intensity as the product of the calculated single-ion spin-flip RIXS cross section and the dynamical structure factor $S(\mathbf{Q},\ensuremath{\omega})$ of the spin-wave excitations. When $S(\mathbf{Q},\ensuremath{\omega})$ is extracted from our data, the wave-vector-dependence of the single-magnon pole intensity shows a similar variation to that observed by INS. Our results confirm that suitably corrected RIXS data can yield the genuine wave-vector and energy dependence of $S(\mathbf{Q},\ensuremath{\omega})$ for a cuprate antiferromagnet. In addition to spin waves, our data show structured multimagnon excitations with dispersing peaks in the intensity at energies higher than the single-magnon excitations.
Highlights
Unconventional superconductors constitute an important group of strongly correlated materials that include heavy fermions, cuprates, ruthenates, and iron-based superconductors [1,2,3,4]
We aim to improve the understanding of the magnetic RIXS cross section by examining the validity of the assumption made for LCO in the previous study [38], that is, whether the dynamical structure factor S(Q, ω) probed by RIXS is consistent to the spin-wave theory and inelastic neutron scattering (INS) results
The dd orbital excitations are clearly resolved owing to the high-energy resolution
Summary
Unconventional superconductors constitute an important group of strongly correlated materials that include heavy fermions, cuprates, ruthenates, and iron-based superconductors [1,2,3,4]. Braicovich et al [36] studied magnetic excitations in a number of undoped cuprates and found good agreement between measured and calculated the momentum and polarization dependence of single-magnon RIXS spectral weight This was demonstrated by the linear dichroic ratio of the RIXS spectral weight between the horizontal and vertical polarizations assuming that the dynamical structure factor S(Q, ω) could be divided out as a technique independent component. We aim to improve the understanding of the magnetic RIXS cross section by examining the validity of the assumption made for LCO in the previous study [38], that is, whether the dynamical structure factor S(Q, ω) probed by RIXS is consistent to the spin-wave theory and INS results. The extracted wave-vector-dependent spin susceptibilities χ (Q) show a remarkable match with those seen by INS
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