Observation of a 500 meV collective mode in La(2-x)Sr(x)CuO(4) and Nd(2)CuO(4) using resonant inelastic x-ray scattering

Utilizing resonant inelastic x-ray scattering, we report a previously unobserved mode in the excitation spectrum of La(2-x)Sr(x)CuO(4) and Nd(2)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.