Hund correlation near the Mott metal-insulator transition: NiS₂ under pressure

Strong correlation effects due to Hund coupling J in multi-band systems have been intensively studied in so-called Hund’s metal. The Hund correlation effects are usually contrasted with the Mott correlation effects, as Hund’s metal exhibits a correlated metallic behavior while being far from the Mott insulating limit. Recently, however, the Hund correlation effects were revealed to be essential also in an archetypal Mott material, NiS_2-xSe_x, near the Mott metal-insulator transition. A kink structure inside the quasiparticle bands is observed from the angle-resolved photoemission spectroscopy experiment, and the role of Hund’s coupling in producing the kink was theoretically demonstrated from the density functional theory plus the dynamical mean-field theory study.

To unveil the underlying physics of the Hund correlation near the Mott metal-insulator transition of a half-filled multi-band system, we studied the pressure-induced metallic state of NiS₂ using first principles DFT+DMFT methods. Our main results consist of 1) definitizing the Hund versus Mott physics by investigating the local impurity properties, 2) finding a new interplay of correlations produced by Hund physics together with Mott physics, which is exhibited as the kink scaled by J times Z, and 3) the prediction of observable Hund trace in optical conductivity as the non-Drude-like tail and the non-monotonic temperature evolution of optical spectral weight.