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arXiv · 1610.02854

Hallmarks of Hund's 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. Ronnow·N. C. Plumb·C. E. Matt·M. Shi·M. Hoesch·T. K. Kim·T. R. Chang·H. T. Jeng·C. Jozwiak·A. Bostwick·E. Rotenberg·A. Georges·T. Neupert·J. Chang
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Abstract

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.

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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. Ronnow, N. C. Plumb, C. E. Matt, M. Shi, M. Hoesch, T. K. Kim, T. R. Chang, H. T. Jeng, C. Jozwiak, A. Bostwick, E. Rotenberg, A. Georges, T. Neupert, J. Chang. 2016-10-10. Hallmarks of Hund's coupling in the Mott insulator Ca$_2$RuO$_4$. https://doi.org/10.1038/ncomms15176

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