Metal-insulator crossover behavior at the surface ofNiS2

We have performed a detailed high-resolution electron spectroscopic investigation of ${\mathrm{NiS}}_2$ and related Se-substituted compounds, ${\mathrm{NiS}}_{2-x}{\mathrm{Se}}_x,$ which are known to be gapped insulators in the bulk at all temperatures. A large spectral weight at the Fermi energy of the room temperature spectrum, in conjunction with the extreme surface sensitivity of the experimental probe, however, suggests that the surface layer is metallic at about 300 K. Interestingly, the evolution of the spectral function with decreasing temperature is characterized by a continuous depletion of the single-particle spectral weight at the Fermi energy and the development of a gap-like structure below a characteristic temperature, providing evidence for a metal-insulator crossover behavior at the surfaces of ${\mathrm{NiS}}_2$ and of related compounds. These results provide a consistent description of the unusual transport properties observed in these systems.