Closing of the Mott gap near step edges in NiS2

TL;DR

This study uses scanning tunneling microscopy to reveal that in NiS2, the Mott gap closes near step edges, indicating potential one-dimensional conduction channels, challenging prior assumptions of metallic surface states.

Contribution

The paper provides the first direct spectroscopic evidence that the Mott gap closes at step edges in NiS2, highlighting a new surface phenomenon in this charge-transfer insulator.

Findings

Mott gap closes near step edges in NiS2

Surface is not metallic despite expectations

Possible one-dimensional conduction channels at edges

Abstract

A prototypical charge-transfer type Mott insulator NiS2 pyrite exhibits a metal-insulator transition with bandwidth control. Recent discoveries on surface-specific electronic states on other 3d transition-metal disulfide pyrites motivate us to further investigate the surface of NiS2, where metallic surface conduction is discussed. Here, the spectroscopic-imaging scanning-tunneling-microscopy observations revealed that the surface is not metallic, contrary to the expectation. Instead, the Mott gap is closed near step edges, suggesting possible electrical conduction from one-dimensional channels. The edge anomaly was observed irrespective of its magnetic order and is limited to the insulator phases.