This work is supported by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC02-76SF00515. L.C. acknowledges support from the Department of Energy, SLAC Laboratory Directed Research and Development funder contract under DE-AC02-76SF00515. RIXS data were taken at beamline ID32 of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) using the ERIXS spectrometer designed jointly by the ESRF and the Politecnico di Milano. G.G. and Y.Y.P. were supported by the by ERC-P-ReXS project (2016-0790) of the Fondazione CARIPLO and Regione Lombardia, in Italy. R.L.G. and T.S. acknowledge support from NSF award DMR-1708334. Computational work was performed on the Sherlock cluster at Stanford University and on resources of the National Energy Research Scientific Computing Center, supported by the US DOE under contract number DE-AC02-05CH11231.

Reviewer information

Nature thanks D. M. Casa, D. van der Marel and the other anonymous reviewer(s) for their contribution to the peer review of this work.