Researchers have discovered that light 鈥斅爄n the form of a laser 鈥斅燾an trigger a form of magnetism in a normally nonmagnetic material. This magnetism centers on the behavior of electrons. These subatomic particles have an electronic property called 鈥渟pin,鈥� which has a potential application in quantum computing. The researchers found that electrons within the material became oriented in the same direction when illuminated by photons from a laser.
The experiment, led by scientists at the 天美影院, the University of Hong Kong and the Pacific Northwest National Laboratory, was April 20 in 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 , a Boeing Distinguished Professor at the 天美影院in the Department of Physics and the Department of Materials Science and Engineering, and scientist at the Pacific Northwest National Laboratory.
鈥淚n this system, we can use photons essentially to control the 鈥榞round state鈥� properties 鈥斅爏uch as magnetism 鈥斅爋f charges trapped within the semiconductor material,鈥� said Xu, who is also a faculty researcher with the UW鈥檚聽, the , and the . 鈥淭his is a necessary level of control for developing certain types of 鈥斅爋r 鈥榪uantum bits鈥� 鈥斅爁or and other applications.鈥�
Xu, whose research team spearheaded the experiments, led the study with co-senior author Wang Yao, professor of physics at the University of Hong Kong, whose team worked on the theory underpinning the results. Other 天美影院faculty members involved in this study are co-authors , a 天美影院professor of physics and of materials science and engineering who also holds a joint appointment at the Pacific Northwest National Laboratory, and , a 天美影院professor of chemistry, director of the , and faculty member in the Clean Energy Institute and the Molecular Engineering & Sciences Institute.
The team worked with ultrathin sheets 鈥斅爀ach just three layers of atoms thick 鈥斅爋f tungsten diselenide and tungsten disulfide. Both are semiconductor materials, so named because electrons move through them at a rate between that of a fully conducting metal and an insulator, with potential uses in photonics and solar cells. Researchers stacked the two sheets to form a 鈥渕oir茅 superlattice,鈥� 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 bound pairs of 鈥渆xcited鈥� 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 nonmagnetic material. Photons provided by the laser 鈥渆xcited鈥� excitons within the laser beam鈥檚 path, and these excitons induced a type of long-range correlation among other electrons, with their spins all orienting in the same direction.
鈥淚t鈥檚 as if the excitons within the superlattice had started to 鈥榯alk鈥� to spatially separated electrons,鈥� said Xu. 鈥淭hen, via excitons, the electrons established exchange interactions, forming what鈥檚 known as an 鈥榦rdered state鈥� with aligned spins.鈥�
The spin alignment that the researchers witnessed within the superlattice is a characteristic of ferromagnetism, the form of magnetism intrinsic to materials like iron. It is normally absent from tungsten diselenide and tungsten disulfide. Each repeating unit within the moir茅 superlattice is essentially acting like a to 鈥渢rap鈥� an electron spin, said Xu. Trapped electron spins that can 鈥渢alk鈥� to each other, as these can, have been suggested as the basis for a type of qubit, the basic unit for quantum computers that could harness the unique properties of quantum mechanics for computation.
In a separate published Nov. 25 in Science, Xu and his collaborators found new magnetic properties in moir茅 superlattices formed by ultrathin sheets of chromium triiodide. Unlike the tungsten diselenide and tungsten disulfide, chromium triiodide harbors intrinsic magnetic properties, even as a single atomic sheet. Stacked chromium triiodide layers formed alternating magnetic domains: one that is ferromagnetic 鈥斅爓ith spins all aligned in the same direction 鈥斅燼nd another that is 鈥渁ntiferromagnetic,鈥� where spins point in opposite directions between adjacent layers of the superlattice and essentially 鈥渃ancel each other out,鈥� according to Xu. That discovery also illuminates relationships between a material鈥檚 structure and its magnetism that could propel future advances in computing, data storage and other fields.
鈥淚t shows you the magnetic 鈥榮urprises鈥� that can be hiding within moir茅 superlattices formed by 2D quantum materials,鈥� said Xu. 鈥淵ou can never be sure what you鈥檒l find unless you look.鈥�
First author of the Nature paper is Xi Wang, a 天美影院postdoctoral researcher in physics and chemistry. Other co-authors are Chengxin Xiao at the University of Hong Kong; 天美影院physics doctoral students Heonjoon Park and Jiayi Zhu; Chong Wang, a 天美影院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.
For more information, contact Xu at xuxd@uw.edu.