Direct observation of kink evolution due to Hund's coupling on approach to metal-insulator transition in NiS2-xSex

Abstract

Understanding characteristic energy scales is a fundamentally important issue in the study of strongly correlated systems. In multiband systems, an energy scale is affected not only by the effective Coulomb interaction but also by the Hund's coupling. Direct observation of such energy scale has been elusive so far in spite of extensive studies. Here, we report the observation of a kink structure in the low energy dispersion of NiS2-xSex and its characteristic evolution with x, by using angle resolved photoemission spectroscopy. Dynamical mean field theory calculation combined with density functional theory confirms that this kink originates from Hund's coupling. We find that the abrupt deviation from the Fermi liquid behavior in the electron self-energy results in the kink feature at low energy scale and that the kink is directly related to the coherence-incoherence crossover temperature scale. Our results mark the direct observation of the evolution of the characteristic temperature scale via kink features in the spectral function, which is the hallmark of Hund's physics in the multiorbital system. A decisive spectroscopic evidence of the Hund's coupling energy scale in multi-orbital correlated systems has been lacking. Here, the authors identify a kink feature due to Hund ' s coupling in the spectral function of NiS2xSex as they track its evolution across the Mott-insulator transition.