In their experiments, the researchers stacked monolayer tungsten ditelluride and CGT. Image: Shi lab, UC Riverside.
In their experiments, the researchers stacked monolayer tungsten ditelluride and CGT. Image: Shi lab, UC Riverside.

A research team led by a physicist at the University of California, Riverside (UCR) has demonstrated a new magnetized state in a monolayer of tungsten ditelluride (WTe2), a new quantum material. Termed a magnetized or ferromagnetic quantum spin Hall insulator, this one-atom-thick material has an insulating interior but a conducting edge, which has important implications for controlling electron flow in nanodevices.

In a typical conductor, electrical current flows evenly everywhere. Insulators, on the other hand, do not readily conduct electricity. Ordinarily, monolayer WTe2 is a special insulator with a conducting edge, but magnetization can bestow even more unusual properties upon it.

“We stacked monolayer WTe2 with an insulating ferromagnet of several atomic layer thickness – of Cr2Ge2Te6, or simply CGT – and found that the WTe2 had developed ferromagnetism with a conducting edge,” said Jing Shi, a distinguished professor of physics and astronomy at UCR, who led the study. “The edge flow of the electrons is unidirectional and can be made to switch directions with the use of an external magnetic field.” The researchers report their work in a paper in Nature Communications.

Shi explained that when a material only conducts electricity at its edge, the size of its interior is inconsequential. This means electronic devices that utilize such materials can be made smaller — indeed, nearly as small as the conducting edge. Because devices using this material would consume less power and dissipate less energy, they could be made more energy efficient. Batteries using this technology, for example, would last longer.

Currently, the technology only works at very low temperatures – CGT is ferromagnetic at around 60K (-350°F). The goal of future research would be to make the technology work at higher temperatures, allowing for nanoelectronic applications such as the non-volatile memory chips used in computers and cell phones.

According to Shi, the conducting edge in ideal quantum spin Hall insulators comprises two narrow channels running alongside each other, akin to a two-lane highway with cars driving in opposite directions. Electrons flowing in one channel cannot cross over to the other channel unless impurities are introduced. The conducting edge in monolayer WTe2 was first visualized in an earlier study by co-author Yongtao Cui, an associate professor of physics and astronomy at UCR and Shi’s colleague.

“It is two channels per edge,” Shi said. “If you eliminate one channel, you end up with a current flowing only in one direction, leaving you with what is called a quantum anomalous Hall insulator, yet another special quantum material. Such an insulator has only one highway lane, to use the highway analogy. This insulator transports electrons in a fully spin-polarized manner.”

On the other hand, the magnetized WTe2 that Shi and his colleagues studied is called a ferromagnetic quantum spin Hall insulator, and has a conducting edge with partially spin-polarized electrons.

“In the two channels of ferromagnetic quantum spin Hall insulators, we have an unequal number of electrons flowing in opposite directions resulting in a net current, which we can control with an external magnet,” Shi said.

According to Shi, quantum materials such as WTe2 are the future of nanoelectronics. “The CHIPS Act will encourage researchers to come up with new materials whose properties are superior to those of current silicon materials,” he said.

This story is adapted from material from the University of California, Riverside, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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