Science researchers observe unprecedented Hall effect

Monday, May 17, 2021

Researchers studying two-dimensional crystalline materials have observed an electromagnetic effect, called the nonlinear anomalous Hall effect, of unprecedented size. Their finding opens the door to exploring other quantum materials using their techniques and hints at promising applications in spintronic devices.

“The nonlinear anomalous Hall effect could be found in other materials with low crystalline symmetry, but we cannot predict just how large of an effect we could expect,” said Wei Tsen, a professor at the University of Waterloo’s Department of Chemistry and the corresponding author on the paper. “So, this work opens the door to exploring other materials in a similar way.”

Wei Tsen in his lab.Wei Tsen in the Quantum Materials and Devices Lab.

Understanding the different Hall effects

In 1879, Edwin Hall found that introducing a magnetic field perpendicular to electric current flowing through a conductor would cause an unexpected voltage drop along the third perpendicular, which we now understand as due to the electrons being deflected sideways by the magnetic field. This phenomenon came to be known as the Hall effect.

Later, he found that using a magnetic conductor produced an even larger Hall effect, which can persist even after removing the introduced magnetic field. This variation is now called the anomalous Hall effect (AHE).

“It’s tempting to think that this is just due to the internal magnetization of the sample,” said Tsen, also a member of Waterloo’s Institute for Quantum Computing (IQC), “but it turns out that it actually has some deep quantum mechanical origins that people have only recently figured out.”

More recently, researchers have shown that materials with reduced crystal symmetries can generate a Hall effect with no sample magnetization or external magnetic field. Here, the electric current itself effectively magnetizes the material, and then that magnetization gives rise to the Hall effect. This unique property means that the size of the Hall effect increases quadratically with the size of the current, instead of linearly, and so is termed the nonlinear AHE.

Materials that maximize

The AHE has inspired researchers to see how large of an effect they can get with different materials when no magnetic field is applied. The larger the effect per a given input of current, the higher the Hall ratio. Materials with a high Hall ratio may prove useful for developing spintronic devices that use electric current to control spin.

Conventional magnetic conductors typically have a Hall ratio of about 0.01. Materials that exhibit a so-called giant AHE have a ratio of around 0.1. By taking advantage of the strong nonlinearity in molybdenum ditelluride and tungsten ditelluride, the researchers observed a Hall ratio of 2.47, more than an order of magnitude larger than previous records.

“It was exciting to see the manifestation of symmetry in the nonlinear AHE,” said Archana Tiwari, a PhD student with the Department of Physics and Astronomy and IQC, and first author on the paper.

“A lot of things came together to give rise to this large Hall ratio,” said Tsen. “The orientation in which we measured, the quality of the material, and the nonlinear mechanism itself all contributed.”

Many researchers also had to come together to make this experiment happen. The nonlinear AHE in the orientation and materials measured by Tiwari and Tsen was predicted by Binghai Yan of the Weizmann Institute of Science in Israel, also an author on the paper. The materials were grown by Tsen’s postdoctoral fellow Fangchu Chen and other collaborators while he was a PhD student at the Chinese Academy of Sciences. And researchers at the University of Michigan, Ann Arbor performed optical characterization of the samples.

Harnessing the Hall effect

Now that they have demonstrated the extremely large nonlinear AHE experimentally, Tsen hopes to continue exploring the effect’s ties to other properties like electron spin. There are already hints that this connection exists, which means it could one day be useful for developing spintronic devices like magnetic random-access memory.

But we’ll never know for sure unless we keep exploring. And that is just what Tsen and his team plan to do.

Giant c-axis nonlinear anomalous Hall effect in Td-MoTe2 and WTe2 was published in Nature Communications on April 6, 2021. This research was supported by the Canada First Research Excellence Fund through Transformative Quantum Technologies and by the Army Research Office.

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