The Brinkman-Rice picture, based on the single-band Hubbard model, has long served as the standard framework for understanding correlated systems, where the quasiparticle weight Z has been a central quantitative indicator of correlation strength. However, this traditional picture breaks down in realistic multiorbital correlated systems where Hund's coupling J plays a decisive role alongside the Coulomb interaction U, such as iron-based superconductors and ruthenates.

In the first part of this talk, I discuss the hallmark of Hund's physics, the kink feature at low energy scale, emerging in the half-filled two-orbital system NiS₂. By tuning correlation strength through both composition and hydrostatic pressure in NiS₂, we directly observe a kink in the low-energy quasiparticle dispersion and confirm that it originates from Hund's coupling via DFT+DMFT calculations. The kink energy scale is directly related to the characteristic temperature scale of the systems and the underlying Hund physics is fundamentally distinct from the charge-fluctuation suppression of Mott physics. A clear scaling relation E_kink ~ JZ is established, and the Hund effect is further manifested in optical conductivity as a non-Drude-like tail.

These findings ultimately reveal that quasiparticle weight Z alone is insufficient to fully characterize correlation strength in multiorbital systems. In the second part, I will introduce a complementary high-energy correlation factor Z_H, which quantifies charge fluctuation suppression. The discrepancy between the newly introduced Z_H and the conventional quasiparticle weight Z captures the distinct roles of spin and charge correlations, providing a universal framework for correlated materials validated across model systems and real materials.