Insulator-metal transition in highly compressed NiO

Area of Science:

• Materials Science
• Condensed Matter Physics
• High-Pressure Physics

Background:

• Nickel monoxide (NiO) is a correlated insulator.
• Theoretical models, including Mott's theory, predicted an insulator-metal transition in NiO under specific conditions.
• Experimental verification of this transition has been challenging due to the required extreme pressures.

Purpose of the Study:

• To experimentally observe and characterize the insulator-metal transition in nickel monoxide (NiO).
• To provide the first experimental evidence for the long-theorized high-pressure phase transition in NiO.
• To correlate the observed transition with theoretical predictions regarding electronic band structure and electron correlation.

Main Methods:

• High-pressure generation using diamond anvil cells or similar techniques to reach pressures of approximately 240 GPa.
• Electrical resistance measurements as a function of applied pressure.
• Analysis of resistance changes to identify phase transitions and determine the nature of the transition (semiconducting to metallic).

Main Results:

• Electrical resistance of NiO became measurable around 130 GPa.
• A substantial decrease in resistance was observed with increasing pressure up to ~240 GPa.
• A sharp drop in resistance (approximately 3 orders of magnitude) occurred at ~240 GPa, indicating a transition from semiconducting to metallic behavior.

Conclusions:

• The first experimental observation of an insulator-metal transition in NiO at ~240 GPa was achieved.
• The results support theoretical predictions, particularly Mott's theory, regarding the electronic behavior of NiO under extreme pressure.
• The metallic phase is understood to form when the effective Hubbard energy (Ueff) approaches the bandwidth (2W), highlighting the role of electron correlation in this transition.