Researchers have discovered that light – in the form of a laser – can induce a form of magnetism in a normally non-magnetic material. This magnetism focuses on the behavior of electrons. These subatomic particles have an electronic property called “spin” that has potential application in quantum computing. The researchers found that electrons within the material were aligned in the same direction when illuminated by photons from a laser.
The experiment, led by scientists from the University of Washington and the University of Hong Kong, was carried out on April 20 Nature.
By controlling and aligning electron spins at this level of detail and accuracy, this platform could have applications in the field of quantum simulation, according to co-senior author Xiaodong Xu, Boeing Distinguished Professor at UW in the Faculty of Physics and the Faculty of Materials Science and Engineering .
“In this system, we can essentially use photons to control the ‘ground-state’ properties – like magnetism – of charges trapped in the semiconductor material,” said Xu, who is also a faculty researcher at Clean Energy Institute and UW’s Molecular Engineering & Sciences Institute. “This is a necessary level of control for the development of certain types of qubits – or ‘quantum bits’ – for quantum computing and other applications.”
Xu, whose research team led the experiments, led the study along with co-senior author Wang Yao, a professor of physics at the University of Hong Kong, whose team worked on the theory that underpinned the findings. 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 joint appointment at the Pacific Northwest National Laboratory, and Daniel Gamelin, a UW professor of chemistry and director of the Molecular Engineering Materials Center.
The team worked with ultra-thin layers – each only three atomic layers thick – made of tungsten diselenide and tungsten disulfide. Both are semiconductor materials, so named because electrons move through them at speeds between that of a fully conductive metal and that of an insulator, with potential applications in photonics and solar cells. The researchers stacked the two sheets to form a “moire superlattice,” a stacked structure of repeating units.
Stacked layers like this are powerful platforms for quantum physics and materials research because the superlattice structure can hold excitons in place. Excitons are bound 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 exciton properties within the material when they made the surprising discovery that light triggers a key magnetic property within the normally non-magnetic material. The photons provided by the laser “excited” excitons in the path of the laser beam, and these excitons induced a kind of long-distance correlation between other electrons, with their spins all aligning in the same direction.
“It’s as if the excitons within the superlattice have started to ‘talk’ to spatially separated electrons,” Xu said. “Then the electrons established exchange interactions via excitons and formed a so-called ‘ordered state’ with aligned spins.”
The spin alignment the researchers observed within the superlattice is a feature of ferromagnetism, the form of magnetism inherent in materials like iron. It is usually absent in tungsten diselenide and tungsten disulfide. Each repeating unit within the moiré superlattice essentially acts like a quantum dot to “capture” an electron spin, Xu said. Trapped electron spins, which like these can ‘talk’ to each other, have been proposed as the basis for a kind of qubit, the basic unit for quantum computers that could exploit the unique properties of quantum mechanics for calculations.
In a separate paper published Nov. 25 in Science, Xu and his collaborators found new magnetic properties in Moiré superlattices composed of ultrathin layers of chromium triiodide. Unlike tungsten diselenide and tungsten disulfide, chromium triiodide exhibits intrinsic magnetic properties even as a single atomic layer. Stacked layers of chromium triiodide formed alternating magnetic domains: one that is ferromagnetic – with spins all oriented in the same direction – and another that is “antiferromagnetic” with spins pointing in opposite directions between adjacent layers of the superlattice and centered in the Essentially “extinguish”. others out,” Xu said. This discovery also sheds light on relationships between a material’s structure and its magnetism, which could fuel future advances in computing, data storage, and other fields.
“It shows you the magnetic ‘surprises’ that can hide in moiré superlattices formed by 2D quantum materials,” Xu said. “You can never be sure what you will find if you don’t look.”
First author of Nature Paper is Xi Wang, a UW postdoctoral researcher in physics and chemistry. Additional co-authors are Chengxin Xiao from the University of Hong Kong; UW Physics PhD students Heonjoon Park and Jiayi Zhu; Chong Wang, a UW researcher in materials science and engineering; Takashi Taniguchi and Kenji Watanabe from the National Institute for Materials Science in Japan; and Jiaqiang Yan at Oak Ridge National Laboratory. The research was funded by the US Department of Energy; the U.S. Army Research Bureau; the US National Science Foundation; the Croucher Foundation; the Hong Kong SAR University Grants Committee/Research Grants Council; the Japanese Ministry of Education, Culture, Sports, Science and Technology; the Japan Society for the Advancement of Science; the Japan Science and Technology Agency; the state of Washington; and the UW