Researchers have shown that laser light can cause a sort of magnetism in a substance that is ordinarily non-magnetic. The behavior of electrons is the focal point of this magnetism. The “spin” of these subatomic particles is an electronic characteristic that may be used in quantum computing.
When irradiated by light from a laser, the researchers discovered that electrons within the material started to align themselves in the same direction. The study, conducted by researchers at the Universities of Washington and Hong Kong, was released in Nature on April 20.
According to co-senior author Xiaodong Xu, a Boeing Distinguished Professor at the UW in the Departments of Physics and Materials Science and Engineering, this platform could be used in the area of quantum simulation by precisely manipulating and aligning electron spins.
“In this system, we can use photons essentially to control the ‘ground state’ properties such as magnetism of charges trapped within the semiconductor material,” said Xu, who is also a faculty researcher with the UW’s Clean Energy Institute and the Molecular Engineering & Sciences Institute. “This is a necessary level of control for developing certain types of qubits or ‘quantum bits for quantum computing and other applications.”
Together with co-senior author Wang Yao, a professor of physics at the University of Hong Kong, Xu, whose research team led the tests, oversaw the study. Wang Yao’s team developed the theory that underpinned the findings.
Di Xiao, a UW professor of physics and materials science and engineering with a joint appointment at the Pacific Northwest National Laboratory, and Daniel Gamelin, a UW professor of chemistry and the director of the Molecular Engineering Materials Center, are co-authors of the study and UW faculty members as well.
The researchers used tungsten diselenide and tungsten disulfide sheets that were each only three layers (atoms) thick. Both are semiconductor materials, which can be used in photonics and solar cells because electrons travel through them at a rate that is halfway between that of a completely conducting metal and an insulator.
In this system, we can use photons essentially to control the ‘ground state’ properties such as magnetism of charges trapped within the semiconductor material. This is a necessary level of control for developing certain types of qubits or ‘quantum bits for quantum computing and other applications.
Xiaodong Xu
The two sheets were stacked to create a repeating unit structure called a “moiré superlattice.” The superlattice structure of these stacked sheets makes them effective study platforms for quantum physics and materials science.
Excitons are bonded pairs of “stimulated” electrons and the positive charges that go along with them, and researchers may see how their characteristics and behavior vary depending on the structure of the superlattice.
When the researchers made the startling discovery that light activates a crucial magnetic feature within the typically non-magnetic material, they were investigating the exciton properties of the substance.
Within the laser beam’s path, photons from the laser “activated” excitons, and these excitons caused a sort of long-range correlation among other electrons, causing their spins to all point in the same direction.
“It’s as if the excitons within the superlattice had started to ‘talk’ to spatially separated electrons,” said Xu. “Then, via excitons, the electrons established exchange interactions, forming what’s known as an ‘ordered state’ with aligned spins.”
The researchers’ observation of spin alignment within the superlattice is a feature of ferromagnetism, a type of magnetism that is inherent to materials like iron. Normal tungsten diselenide and tungsten disulfide compositions do not contain it.
According to Xu, every repeating unit in the moiré superlattice functions effectively like a quantum dot to “capture” an electron spin. The foundation for a certain form of qubit, the fundamental building block for quantum computers that might exploit the special capabilities of quantum mechanics for computation, has been proposed as trapped electron spins that can “speak” to each other, as these can.
Separately, Xu and his colleagues discovered novel magnetic characteristics in moiré superlattices made of ultrathin sheets of chromium triiodide in an article that appeared in Science on November 25. Chromium triiodide, even as a single atomic sheet, possesses intrinsic magnetic characteristics, in contrast to tungsten diselenide and tungsten disulfide.
Stacked chromium triiodide layers formed alternating magnetic domains: “one that is ferromagnetic with spins all aligned in the same direction and another that is “antiferromagnetic,” where spins point in opposite directions between adjacent layers of the superlattice and essentially “cancel each other out,” according to Xu.
That finding also reveals connections between a material’s structure and magnetism, which may lead to future developments in computing, data storage, and other areas.
“It shows you the magnetic ‘surprises’ that can be hiding within moiré superlattices formed by 2D quantum materials,” said Xu. “You can never be sure what you’ll find unless you look.”
First author of the Nature paper is Xi Wang, a UW postdoctoral researcher in physics and chemistry. Other co-authors are Chengxin Xiao at the University of Hong Kong; UW physics doctoral students Heonjoon Park and Jiayi Zhu; Chong Wang, a UW researcher in materials science and engineering; Takashi Taniguchi and Kenji Watanabe at the National Institute for Materials Science in Japan; and Jiaqiang Yan at the Oak Ridge National Laboratory.
The research was funded by the U.S. Department of Energy; the U.S. Army Research Office; the U.S. National Science Foundation; the Croucher Foundation; the University Grant Committee/Research Grants Council of Hong Kong Special Administrative Region; the Japanese Ministry of Education, Culture, Sports, Science, and Technology; the Japan Society for the Promotion of Science; the Japan Science and Technology Agency; the state of Washington; and the UW.
