The SUTD researchers' discovery of the intrinsic nonlinear planar Hall effect suggests a new method for characterizing new materials and their complex nonlinear behavior that could translate into useful applications in electronics.

figure: (a) Schematic diagram of measurement setup for NPHE. (b),(c) For materials with point group Cnv or Dn (n>2), the internal NPHE response exhibits a simple cosine or sine dependence on the angle (φ−θ) between E and B fields, when B field is rotated in the plane.
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Credit: SUTD

Like the blood that flows through our veins, the electrons at the heart of our microchips breathe life into our computers. In the race to speed up and shrink our electronics, there is a growing demand for new methods and materials to redefine our current technology practices.

Associate Professor Yang Shengyuan and his team from the Singapore University of Technology and Design (SUTD) collaborated with Research Assistant Professor Cong Xiao from the University of Hong Kong to investigate a never-before-seen phenomenon of electron transport. The results of their research were recently published as “Intrinsic nonlinear planar Hall effect” in Physical audit letter.

“The transport properties of materials are fundamental to our understanding of materials and their potential applications,” said Associate Professor Yang, who explained that the electron transfer process can be attributed to external and internal contributions. While extrinsic systems result from structural and geometrical properties such as defects and impurities, intrinsic systems result solely from the properties of the material itself. “Intrinsic contributions are like the information on each subject’s identity card,” he elaborated.

In the study, the team developed an extended theoretical framework and applied it to various situations in different materials to search for materials with novel behaviors. They found that a selection of specific crystal structures had the necessary symmetry needed to produce a new phenomenon – the intrinsic nonlinear planar Hall effect (NPHE).

In conventional materials used in electronics, a voltage between two points causes electrons to flow between them. Flux increases linearly with voltage, allowing for the precise control needed in computing and data storage. However, in the presence of a magnetic field, the transport of electrons across the material can behave in unusual ways. The effect of magnetic fields on electron transport is collectively known as the Hall effect. They produce a current that does not necessarily go in the direction of the applied voltage, which has led to the development of hypersensitive magnetic field sensors.

Previous studies investigated the planar Hall effect (PHE), where the magnetic field, applied voltage and induced current all lie in the same plane. However, most of these phenomena are external and do not utilize the inherent properties of matter itself. Additionally, this effect typically has a linear behavior, with the induced current scaling proportional to the applied voltage. In order to overcome conventional electronics, nonlinear complex behavior in materials is more desirable.

The intrinsic NPHE discovered by Associate Professor Yang’s team allows for a much wider range of possible crystals that would exhibit nonlinear complex behavior, unlike other PHEs, which develop only under a very dense portion of crystal structures.

As an added bonus, the team found that the size of the internal NPHE changes depending on the direction of the applied magnetic field and voltage. This creates an extra control knob in possible applications with a simple twist. Associate Professor Yang was optimistic about how the effect could be used in today’s devices and suggested that the process could lead to new designs for nonlinear rectifiers or terahertz sensors for long-range communications.

To demonstrate the plausibility of their proposed system, the team looked to well-known materials that exhibit the necessary symmetry required. The growing interest in two-dimensional (2D) materials for compact and efficient electronics was an excellent starting point for their search. Consisting of crystalline monolayers of atoms stacked in a sandwich-like structure, these materials often have electronic properties that make them desirable for use in components such as transistors.

First synthesized in 2017 and under active research since then, 2D monolayer MoSSe was found to have the necessary crystal structure to demonstrate the proposed system of Associate Professor Yang’s team. By starting their calculations from basic fundamentals, the team discovered that a significant intrinsic NPHE response could be generated in the material under the right conditions.

Given the community’s interest in researching this topic, the team is hopeful that their theory will soon be supported by experimental data. Meanwhile, Associate Professor Yang is already looking for other new transport phenomena.

“Our main goal is to understand the fundamental physics of matter and to know what effects can occur in nature,” he said. “We will predict new effects, develop theories for them and propose possible applications.”

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