Observation of a 500 meV Collective Mode inLa2−xSrxCuO4andNd2CuO4Using Resonant Inelastic X-Ray Scattering

Utilizing resonant inelastic x-ray scattering, we report a previously unobserved mode in the excitation spectrum of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ at 500 meV. The mode is peaked around the ($\pi$, 0) point in reciprocal space and is observed to soften, and broaden, away from this point. Samples with $x=0$, 0.01, 0.05, and 0.17 were studied. The new mode is found to be rapidly suppressed with increasing Sr content and is absent at $x=0.17$, where it is replaced by a continuum of excitations. This mode is only observed when the incident x-ray polarization is normal to the CuO planes.