We report resonant X-ray scattering measurements on the orbitally-degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below 365 K. We observe a strong resonant enhancement of the supercell reflections through the Ni K-edge. The empirically extracted K-edge shift between the crystallographically-distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of ~1 eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.
@article{arxiv.1009.2053,
title = {Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering},
author = {G. L. Pascut and R. Coldea and P. G. Radaelli and A. Bombardi and G. Beutier and I. I. Mazin and M. D. Johannes and M. Jansen},
journal= {arXiv preprint arXiv:1009.2053},
year = {2015}
}