  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.
