Researchers have uncovered a remarkable phenomenon in which the direction of a spin-polarized current can be confined to a single direction within a single-atom layer of thallium-lead alloys when exposed to light at room temperature. This finding challenges existing assumptions, as single-atom layers were previously believed to be nearly transparent, meaning they minimally absorb or interact with light. The observed unidirectional current flow opens up the potential for advancements beyond traditional diodes, which may lead to more sustainable data storage solutions and sophisticated two-dimensional spintronic devices in the future. The research has been documented in the journal ACS Nano.
Researchers Ibuki Taniuchi, Ryota Akiyama, Rei Hobara, and Shuji Hasegawa from the University of Tokyo have unveiled that the flow of spin-polarized current can be limited to just one direction in a single-atom layer of thallium-lead alloys when irradiated at room temperature. This groundbreaking discovery goes against established beliefs, as single-atom layers were thought to be almost completely transparent, minimally absorbing or interacting with light. The directional flow of the current observed in this research allows for functionality that exceeds that of traditional diodes, setting the stage for greener data storage and cutting-edge two-dimensional spintronic devices in the future. The findings are published in the journal ACS Nano.
Diodes are essential components in contemporary electronics, as they direct the flow of current in one specific direction. However, as devices become thinner, designing and manufacturing these critical components becomes increasingly complex. Therefore, demonstrating phenomena that could facilitate the development of such devices is of utmost importance. Spintronics is a field focused on manipulating the intrinsic angular momentum, or spin, of electrons, often by using light.
“Traditionally, spintronics has dealt with thicker materials,” comments Akiyama. “Yet, our interest lies in very thin systems due to their inherently fascinating properties. Therefore, we aimed to merge these two areas and explore the conversion of light into spin-polarized current within a two-dimensional framework.”
The process of converting light into spin-polarized current is referred to as the circular photogalvanic effect (CPGE). In this type of current, the electron spins align in one direction, thus restricting the electrical current’s flow based on the light’s polarization. This phenomenon mirrors that of traditional diodes, where the electrical current can only flow in one direction, depending on the voltage’s polarity. The researchers employed thallium-lead alloys to investigate whether they could observe this effect even in single-atom-thick layers (two-dimensional systems). They carried out experiments in an ultra-high vacuum environment to prevent adsorption and oxidation, allowing them to reveal the material’s “true colors.” By irradiating the alloys with circularly polarized light, they were able to track changes in both the direction and strength of the resulting electrical current.
In an unexpected twist, Akiyama states, “We discovered that it was a spin-polarized current: the direction of the electron spin aligned with the current’s direction due to the unique properties of these thin alloys.”
These lightweight alloys, previously developed by the research team, exhibited distinctive electronic characteristics, providing the researchers with an unexpected lead for their current investigation. With this newfound understanding, Akiyama envisions the future.
“These findings highlight the importance of fundamental research for applications and advancements. In this study, we aimed to explore an optimized system. Our next step is to look for innovative two-dimensional thin alloys with exceptional electronic properties, as well as to employ lower energy (terahertz) lasers to refine the excitation pathways that lead to CPGE. This approach could enhance the efficiency of converting light into spin-polarized current.”
