Possible Observation of a 'Zhang-RiceTriplet': RIXS Study of Li2CuO2

The electronic structure of cuprates has been of considerable interest since the discovery of high T$_{c}$ superconductivity in such materials. Li$_{2}$CuO$_{2}$ consists of edge-sharing Cu-O chains, with a quasi-one dimensional crystal structure. The electronic structure, however, is regarded as quasi-zero dimensional, since the resistivity is similar along all three crystal axes, and the insulating gap is estimated at 3.2eV. We have measured the O K edge and Cu L edge XAS, SXE, and RIXS spectra of Li$_{2}$CuO$_{2}$. The results indicate the presence of Cu d-d excitations, as well as a higher energy charge transfer excitation that is tentatively identified as a two hole final state ``Zhang-Rice Triplet.'' This identification is supported by energetic and magnetic arguments, as well as more general theoretical considerations. Supported in part by the U.S. DOE under DE-FG02-98ER45680.