Sutter, D.Fatuzzo, C. G.Moser, S.Kim, M.Fittipaldi, R.Vecchione, A.Granata, V.Sassa, Y.Cossalter, F.Gatti, G.Grioni, M.Rønnow, Henrik M.Plumb, N. C.Matt, C. E.Shi, M.Hoesch, M.Kim, T. K.Chang, T. -R.Jeng, H. -T.Jozwiak, C.Bostwick, A.Rotenberg, E.Georges, A.Neupert, T.Chang, J.2017-05-302017-05-302017-05-30201710.1038/ncomms15176https://infoscience.epfl.ch/handle/20.500.14299/137882WOS:000400851100001A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realized in Ca2RuO4. Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide-using angle-resolved photoemission electron spectroscopy-the band structure of the paramagnetic insulating phase of Ca2RuO4 and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund's coupling J = 0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the d(xy) orbital due to c-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund's coupling that together generate the Mott insulating state of Ca2RuO4.MPBHtext::journal::journal article::research article