Hallmarks of Hunds coupling in the Mott insulator Ca 2 RuO 4

D. Sutter, C. G. Fatuzzo, S. Moser, M. Kim, R. Fittipaldi, A. Vecchione, V. Granata, Y. Sassa, F. Cossalter, G. Gatti, M. Grioni, H. M. Rønnow, N. C. Plumb, C. E. Matt, M. Shi, M. Hoesch, T. K. Kim, T. R. Chang, H. T. Jeng, C. JozwiakA. Bostwick, E. Rotenberg, A. Georges, T. Neupert, J. Chang

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A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realised in Ca$_2$RuO$_4$. 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 Ca$_2$RuO$_4$ 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 stabilisation of the d$_{xy}$ orbital due to $c$-axis contraction is shown to be important in explaining the nature of the insulating state. It is thus a combination of multiband physics, Coulomb interaction and Hund's coupling that generates the Mott insulating state of Ca$_2$RuO$_4$. These results underscore the importance of Hund's coupling in the ruthenates and related multiband materials.
Original languageEnglish
JournalNature Communications
Publication statusPublished - 2017 May 5

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    Sutter, D., Fatuzzo, C. G., Moser, S., Kim, M., Fittipaldi, R., Vecchione, A., Granata, V., Sassa, Y., Cossalter, F., Gatti, G., Grioni, M., Rønnow, H. M., Plumb, N. C., Matt, C. E., Shi, M., Hoesch, M., Kim, T. K., Chang, T. R., Jeng, H. T., ... Chang, J. (2017). Hallmarks of Hunds coupling in the Mott insulator Ca 2 RuO 4. Nature Communications, 8. https://doi.org/10.1038/ncomms15176