Unoccupied electronic states in ${\mathrm{NiS}}_{2-x}{\mathrm{Se}}_x$ across the metal-insulator transition

We investigate the evolution of the electronic structure across the insulator-metal transition in ${\mathrm{NiS}}_{2-x}{\mathrm{Se}}_x$ with changing composition, but in the absence of any structural or magnetic changes. A comparison of the inverse photoemission spectra with band-structure calculations establishes the importance of correlation effects in these systems. Systematic changes in the spectral distribution establish the persistence of the upper Hubbard band well into the metallic regime, with the insulator-to-metal transition being driven by a transfer of spectral weight from the Hubbard band to states close to the Fermi energy.