In the pursuit of faster and smaller electronics, researchers are seeking new techniques and materials to revolutionize current technological approaches. A collaborative effort between Associate Professor Yang Shengyuan and his team from the Singapore University of Technology and Design (SUTD) and Research Assistant Professor Cong Xiao from the University of Hong Kong has uncovered a previously unknown phenomenon in electron transport. Their findings, published as “Intrinsic nonlinear planar Hall effect” in Physical Review Letters, shed light on the intrinsic mechanisms behind electron transport and open up possibilities for novel materials and applications.
According to Associate Professor Yang, understanding the transport properties of materials is crucial for advancing our knowledge and exploring potential applications. Electron transport involves both extrinsic and intrinsic contributions. Extrinsic mechanisms are influenced by structural and geometrical factors, such as defects and impurities, while intrinsic mechanisms arise from the inherent characteristics of the material itself. The intrinsic contribution can be likened to the unique identity of each material.
In their study, the research team developed an extended theoretical framework and applied it to different materials under various conditions to identify those with distinctive behavior. They discovered that certain crystal structures possessed the necessary symmetries to exhibit a new phenomenon called the intrinsic nonlinear planar Hall effect (NPHE).
In conventional electronic materials, the flow of electrons between two points increases linearly with applied voltage, enabling precise control for calculations and data storage. However, when a magnetic field is introduced, electron transport can exhibit unusual behavior. These magnetic field-induced effects are collectively known as Hall effects. They result in a current that may not align with the direction of the applied voltage, leading to the development of highly sensitive magnetic field sensors.
Previous research focused on the planar Hall effect (PHE), where the magnetic field, applied voltage, and induced current lie on the same plane. However, most of these phenomena were extrinsic and did not fully leverage the material’s inherent properties. Moreover, these effects typically exhibited linear behavior, where the induced current scaled proportionally with the applied voltage. To challenge conventional electronics, researchers are particularly interested in nonlinear complex behaviors exhibited by materials.
The intrinsic NPHE discovered by Associate Professor Yang’s team allows for a broader range of crystals to demonstrate nonlinear complex behaviors, unlike other PHEs, which are limited to specific crystal structures. Additionally, the size of the intrinsic NPHE can be altered by changing the directions of the applied magnetic field and electric voltage. This additional controllability offers potential applications in nonlinear rectifiers or terahertz detectors for long-range communications. By simply rotating the material, the effect can be manipulated.
To validate their proposed mechanism, the research team investigated well-known materials that possess the required symmetries. Two-dimensional (2D) materials, which are highly sought after for compact and efficient electronics, served as an excellent starting point. These materials consist of crystalline monolayers of atoms arranged in a sandwich-like structure and often exhibit desirable electronic properties for use in components like transistors.
The team focused on a 2D material called monolayer MoSSe, which was first synthesized in 2017 and has been actively researched since then. This material was found to possess the necessary crystal structure to demonstrate the proposed mechanism. Through comprehensive calculations starting from fundamental principles, the team determined that significant intrinsic NPHE responses could be observed in the material under specific conditions.
With the growing interest in investigating this material within the scientific community, the research team anticipates that their theory will soon be supported by experimental evidence. Meanwhile, Associate Professor Yang is already exploring other novel transport phenomena. His team’s primary objective is to deepen our understanding of the fundamental physics of materials and identify new effects that can occur in nature. By predicting and theorizing new effects, they hope to propose potential applications for these discoveries.