Atomic Diffusion in the Surface State of Mott Insulator NiS2

TL;DR

This study investigates how heat treatment induces sulfur vacancies in NiS2, affecting surface conduction, with atomic diffusion playing a key role in the surface state modifications of this Mott insulator.

Contribution

It provides the first direct evidence linking atomic diffusion and surface state changes in NiS2 after heat treatment, highlighting sulfur vacancies as a key factor.

Findings

Resistivity increases after heat treatment and relaxes over days.

Diffusion constant estimated at approximately 10^{-10} m^2/s.

Sulfur vacancies likely cause surface state localization.

Abstract

We present resistivity measurements of Mott insulator NiS2 single crystals after heat treatment. We find a strong increase of the low temperature resistivity that relaxes back towards the pristine behaviour over several days with a time constant of 45 h at room temperature. The low temperature resistivity has previously been shown to be dominated by surface conduction (T. Thio and J. Bennett, PRB 50 10574 1994). Consequently, the changes induced by heat treatment are attributed to changes to surface states. Our results suggest the creation of vacancies in the surface that re refilled from the bulk via atomic diffusion. We estimate a diffusion constant of $D \approx 10^{-10}$ m$^2$/s at room temperature. We identify sulphur vacancies as the most likely to form via oxidisation of sulphur forming volatile SO2 during heat treatment. Our results point towards these sulphur vacancies to be the source of surface state localisation in NiS2.