Pressure-induced semiconductor-metal-semiconductor transitions in FeS

The semiconductor-metal-semiconductor transitions in stoichiometric FeS compounds were observed under nearly hydrostatic pressure condition up to 8 GPa and a temperature down to 4 K. In the low-pressure semiconductor state, the energy gap caused by the electron correlation decreases with a volume reduction. On the other hand, FeS in a high-pressure semiconductor state is a band insulator and the energy gap increases with pressure when the gap is formed between the occupied nonbonding and unoccupied antibonding bands. In the metallic phase at intermediate pressure range FeS can be regarded as a strongly correlated metal because of the large quadratic temperature dependence of resistivity at low temperature.