Charge Excitations In The Stripe-Ordered La5/3sr1/3nio4 And La2-X(Ba,Sr)(X)Cuo4 Superconducting Compounds

Charge excitations in stripe-ordered 214 compounds La5/3Sr1/3NiO4 and 1/8-doped La2-x(Ba or Sr)(x)CuO4 are studied using resonant inelastic x-ray scattering in the hard x-ray regime. We observe similar or equal to 1 eV excitation with a momentum transfer corresponding to the charge stripe spatial period both for the diagonal (nickelate) and parallel (cuprates) stripes. They are interpreted as collective stripe excitations or anomalous softening of the charge excitonic modes of the in-gap states.