We measured, by angle-resolved photoemission spectroscopy, the electronic structure of ${\mathrm{LiCu}}_2{\mathrm{O}}_2$, 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.

• Received 16 September 2016
• Corrected 3 May 2017

DOI:https://doi.org/10.1103/PhysRevLett.118.176404