Reexamination of the electronic phase diagram of doped Ni⁢S₂: Electronic, magnetic, and structural inhomogeneity across the Mott insulator-metal transition

Pyrite-structure NiS2 is, in principle, a model antiferromagnetic Mott insulator that can be electron doped, hole doped, and bandwidth controlled. Despite decades of study, however, the electronic and magnetic behavior of NiS2 have proven challenging to understand. Here, we build on recent advances establishing surface conduction in NiS2 to completely reexamine the electronic phase behavior of electron- and hole-doped single-crystal Ni1-xCuxS2 and Ni1-xCoxS2. Magnetometry, heat capacity, neutron diffraction, and electronic transport measurements suggest that prior studies missed vital details of the magnetic ordering in this system. While electron and hole doping rapidly increase the antiferromagnetic ordering temperature (by as much as fourfold by x≈0.1), signatures remain of antiferromagnetic and weak ferromagnetic ordering at the same temperatures as in undoped NiS2. As these undoped ordering temperatures remain constant, the associated magnetic moments are diminished by doping, strongly implicating electronic/magnetic phase coexistence across the Mott insulator-metal transition. Substantial structural changes and inhomogeneity accompany these evolutions, highlighting the importance of structural-chemical-electronic-magnetic coupling in NiS2. The insulator-metal transition is also strongly electron/hole asymmetric, which we interpret with the aid of complementary dynamical mean-field theory results. These findings significantly revise and advance our understanding of the electronic phase behavior of this prototypical Mott insulator, highlighting the essential role of electronic, magnetic, structural, and chemical inhomogeneity across the Mott transition.