Metal-insulator Crossover Behavior at the Surface of NiS_2

TL;DR

This study reveals a temperature-dependent metal-insulator crossover at the surface of NiS$_2$ and related compounds, showing surface metallicity at high temperatures and a gap formation at lower temperatures, explaining their unusual transport properties.

Contribution

It provides the first detailed spectroscopic evidence of surface metallicity and a crossover to an insulating state in NiS$_2$ compounds, highlighting surface-specific electronic behavior.

Findings

Surface metallicity at 300 K confirmed by spectral weight at Fermi energy.

Depletion of spectral weight and gap formation observed at lower temperatures.

Results explain the unusual transport phenomena in NiS$_2$ systems.

Abstract

We have performed a detailed high-resolution electron spectroscopic investigation of NiS$_2$ and related Se-substituted compounds NiS$_{2-x}$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 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 NiS$_2$ and of related compounds. These results provide a consistent description of the unusual transport properties observed in these systems.