Moser, S.Nomura, Y.Moreschini, L.Gatti, G.Berger, H.Bugnon, P.Magrez, A.Jozwiak, C.Bostwick, A.Rotenberg, E.Biermann, S.Grioni, M.2017-05-302017-05-302017-05-30201710.1103/PhysRevLett.118.176404https://infoscience.epfl.ch/handle/20.500.14299/137933WOS:000400241400006We measured, by angle-resolved photoemission spectroscopy, the electronic structure of LiCu2O2, a mixed-valence cuprate where planes of Cu(I) (3d(10)) ions are sandwiched between layers containing one-dimensional edge-sharing Cu(II) (3d(9)) chains. We find that the Cu(I)- and Cu(II)-derived electronic states form separate electronic subsystems, in spite of being coupled by bridging O ions. The valence band, of the Cu(I) character, disperses within the charge-transfer gap of the strongly correlated Cu(II) states, displaying an unprecedented 250% broadening of the bandwidth with respect to the predictions of density functional theory. Our observation is at odds with the widely accepted tenet of many-body theory that correlation effects generally yield narrower bands and larger electron masses and suggests that present-day electronic structure techniques provide an intrinsically inappropriate description of ligand-to-d hybridizations in late transition metal oxides.text::journal::journal article::research article