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Different external environments, such as temperature and pressure, can change physical properties, such as transport, magnetism, and light transparency. One phenomenon is the Pressure-induced Insulator-to-Metal Transition (PIMT), in which a transparent, nonconducting material (insulator) becomes opaque and begins to conduct electricity (metallization) when pressure is applied. Some materials exhibit the PIMT at pressures below 10,000 atm (approximately 1 GPa). One substance that exhibits PIMT at low pressure is samarium monosulfide (SmS), an insulator at ambient pressure (1 atm) with high electrical resistance and transmits infrared and terahertz regions. When SmS is damaged by a needle tip or applying pressure above approximately 7,000 atm (~0.7 GPa), it undergoes PIMT and transforms into a metal. This property change, as well as the color change of SmS from black to golden-yellow, has attracted the attention of many researchers and has been the subject of numerous studies. However, the origin of PIMT is yet to be identified, even more than 50 years after its discovery.

More recently, the behavior of SmS in external environments that differs from that of pressure has attracted attention. The first is the negative differential resistance (dV/dI) when a current higher than the normal measurement condition is applied to SmS at low temperatures. Because ordinary materials obey Ohm's law, dV/dI is constant regardless of the current magnitude. However, SmS exhibits negative dV/dI at low temperatures, where the voltage decreases when the current is increased.

Here, the origin of dV/dI was investigated using optical reflectivity spectra in the region from the terahertz to vacuum-ultraviolet and angle-integrated photoelectron spectra under the application of current. The obtained spectra suggest that the material exhibited a Current-induced Insulator-to-Metal Transition (CIMT). Detailed analysis of the electronic state change revealed that the CIMT was caused by the hybridization of the Sm 4f orbital and Sm 5d conduction band owing to the pushing out of the 4f electrons localized in the atom by the applied electric current. The metallic state of the CIMT has a lower conduction electron density than the metallic phase of the PIMT of SmS, suggesting that the same phase transition in the PIMT cannot occur by increasing only the 4f-5d hybridization strength. This result suggests that effects other than hybridization are required for PIMT.

(Written by S. Kimura on behalf of all the authors)