Clean realization of the Hund physics near the Mott transition: NiS_2 under pressure

Strong correlation effects caused by Hund's coupling have been actively studied during the past decade. Hund's metal, strongly correlated while far from the Mott insulating limit, was studied as a representative example. However, recently, it was revealed that a typical Mott system also exhibits a sign of Hund physics by investigating the kink structure in the spectral function of NiS_2-xSe_x. Therefore, to understand the Hund physics in a half-filled multi-orbital system near the metal-insulator transition, we studied pressure-induced metallic states of NiS_2 by using density functional theory plus dynamical mean-field theory. Hund physics, responsible for suppressing local spin fluctuation, gives low-energy effective correlations, separated from Mott physics, which suppresses charge fluctuation at higher energy. This effect is prominent when J becomes comparable to the quasiparticle kinetic energy, showing apparent scaling behavior of the kink position E_kink∼ J · Z. We suggest that the Hund effect can also be observed in the optical conductivity as a non-Drude-like tail with 1/ω frequency dependence and non-monotonic temperature evolution of the integrated optical spectral weight at a fixed frequency. Our study demonstrates the important role of Hund's coupling for electronic correlations even in a half-filled system.