Scientists use light to trigger magnetism in non-magnetic material

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An illustrative representation of the light-induced ferromagnetism that researchers observed in ultrathin sheets of tungsten diselenide and tungsten disulfide. Laser light, shown in yellow, excites an exciton – a bonded pair of an electron (blue) and its associated positive charge, also known as a hole (red). This activity induces long-range exchange interactions between other holes trapped in the moiré superlattice, orienting their spins in the same direction. Credit: Xi Wang/University of Washington

Lasers trigger magnetism in atomically thin quantum materials

Researchers have found that light – in the form of a laser – can trigger a form of magnetism in normally non-magnetic material. This magnetism is centered on the behavior of electrons. These subatomic particles have an electronic property called “spin”, which has potential application in quantum computing. The researchers found that the electrons inside the material point in the same direction when illuminated by photons from a laser.

The experiment, conducted by scientists from the University of Washington and the University of Hong Kong, was published on April 20, 2022 in the journal Nature.

By controlling and aligning electron spins at this level of detail and accuracythis platform could have applications in the field of quantum simulation, according to co-lead author Xiaodong Xu, Boeing Professor Emeritus at UW in the Department of Physics and the Department of Materials Science and Engineering.

“In this system, we can use photons essentially to control the ‘ground state’ properties – such as magnetism – of the charges trapped in the semiconductor material,” said Xu, who is also a researcher at the UW’s Clean Energy Institute and the Molecular Institute of Engineering and Science. “This is a level of control needed to develop certain types of qubits – or ‘quantum bits’ – for quantum computing and other applications.”

Stacked layers of tungsten diselenide and tungsten disulfide

A top view image, taken by piezoelectric response force microscopy, of stacked layers of tungsten diselenide and tungsten disulfide, forming what is known as a heterostructure. The triangles indicate the repeating “units” of the moiré superlattice. Credit: Xi Wang/University of Washington

Xu, whose research team led the experiments, led the study with co-lead author Wang Yao, a professor of physics at the University of Hong Kong, whose team worked on the underlying theory. tends the results. Other UW faculty members involved in this study are co-authors Di Xiao, a UW professor of physics and materials science and engineering who also holds a cross-appointment at Pacific Northwest National Laboratory, and Daniel Gamelin, professor of chemistry and director of the UW. of the Center for Molecular Engineering Materials.

The team worked with ultrathin sheets – each just three atom layers thick – of tungsten diselenide and tungsten disulfide. Both are semiconductor materials, so named because electrons move through them at a speed between that of a fully conductive metal and that of an insulator, with potential uses in photonics and solar cells. . The researchers stacked the two sheets to form a “moiré superlattice”, a stacked structure made up of repeating units.

Stacked sheets like these are powerful platforms for quantum physics and materials research because the superlattice structure can hold excitons in place. Excitons are bonded pairs of “excited” electrons and their associated positive charges, and scientists can measure how their properties and behavior change in different superlattice configurations.

The researchers were studying the properties of the exciton in the material when they made the startling discovery that light triggers a key magnetic property in the normally non-magnetic material. The photons provided by the excitons “excited” by the laser in the path of the laser beam, and these excitons induced a type of long-range correlation between the other electrons, their spins all pointing in the same direction.

“It’s as if the superlattice excitons have started ‘talking’ to spatially separated electrons,” Xu said. “Then, via excitons, the electrons established exchange interactions, forming what is called an ‘ordered state’ with aligned spins.”

The spin alignment observed by researchers in the superlattice is a feature of ferromagnetism, the form of magnetism intrinsic to materials like iron. It is normally absent from tungsten diselenide and tungsten disulfide. Each repeating unit in the moiré superlattice essentially acts as a quantum dot to “trap” an electron spin, Xu said. Spins of trapped electrons that can “talk” to each other, however they can, have been suggested as the basis for a type of qubit, the basic unit of quantum computers that could exploit the unique properties of quantum mechanics. for the calculation.

In a separate article published on November 25, 2021 in the journal Science, Xu and co-workers discovered new magnetic properties in moire superlattices formed by ultrathin sheets of chromium triiodide. Unlike tungsten diselenide and tungsten disulfide, chromium triiodide has intrinsic magnetic properties, even as a single atomic sheet. Stacked layers of chromium triiodide formed alternating magnetic domains: one that is ferromagnetic – with spins all aligned in the same direction – and another that is “antiferromagnetic”, where the spins point in opposite directions between adjacent layers of the superlattice and essentially “cancel each other”. “, according to Xu. This discovery also sheds light on the relationships between a material’s structure and its magnetism that could propel future advances in computing, data storage and other fields.

“It shows you the magnetic ‘surprises’ that can lurk in moiré superlattices formed by 2D quantum materials,” Xu said. “You can never be sure what you’ll find unless you look.”

Reference: “Light-induced ferromagnetism in moire superlattices” by Xi Wang, Chengxin Xiao, Heonjoon Park, Jiayi Zhu, Chong Wang, Takashi Taniguchi, Kenji Watanabe, Jiaqiang Yan, Di Xiao, Daniel R. Gamelin, Wang Yao and Xiaodong Xu, April 20, 2022, Nature.
DOI: 10.1038/s41586-022-04472-z

First author of Nature paper is Xi Wang, a UW postdoctoral fellow in physics and chemistry. Other co-authors are Chengxin Xiao from the University of Hong Kong; Heonjoon Park and Jiayi Zhu, PhD students in physics from UW; Chong Wang, a UW researcher in materials science and engineering; Takashi Taniguchi and Kenji Watanabe of the National Institute of Materials Science in Japan; and Jiaqiang Yan at Oak Ridge National Laboratory. The research was funded by the US Department of Energy; the US Army Research Office; the US National Science Foundation; the Croucher Foundation; the University Scholarship Committee/Research Scholarship Council of the Hong Kong Special Administrative Region; the Japanese Ministry of Education, Culture, Sports, Science and Technology; Japan Society for the Promotion of Science; Japan Science and Technology Agency; Washington State; and the UW.

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