Optical study of the metal-nonmetal transition in NiS

TL;DR

This study investigates the electronic properties of NiS across its metal-nonmetal transition using optical reflectivity, revealing a charge-transfer semiconductor behavior and how the energy gap varies with temperature and doping.

Contribution

It provides detailed optical spectra analysis of NiS during the metal-nonmetal transition, highlighting the charge-transfer nature and the evolution of the energy gap with temperature and doping.

Findings

Reflectivity spectra show a pronounced dip at transition.

Optical conductivity indicates a charge-transfer semiconductor behavior.

Energy gap increases with decreasing temperature and decreases with doping.

Abstract

Optical reflectivity study has been made on the hexagonal (NiAs-type) Ni_{1-d}S in order to probe its electronic properties, in particular those associated with the metal-nonmetal transition in this compound. Samples with d=0.005 and 0.02 are studied, which have transition temperatures Tt=246 K and 161 K, respectively. A pronounced dip appears in the reflectivity spectra upon the transition, and the optical conductivity spectra show that the electronic structure below Tt is similar to that of a carrier-doped semiconductor with an energy gap of ~ 0.15 eV. The optical spectra indicate that the gap becomes larger with decreasing temperature, and it becomes smaller as d increases. It is also found that the overall spectrum including the violet region can be described based on a charge-transfer-type semiconductor, consistent with recent photoemission results.