Now, a research team from Brown University has found a surprising new phenomenon that can arise in magic-angle graphene. In research published in the journal Science, the team showed that by inducing a phenomenon known as spin-orbit coupling, magic-angle graphene becomes a powerful ferromagnet.
"Magnetism and superconductivity are usually at opposite ends of the spectrum in condensed matter physics, and it's rare for them to appear in the same material platform," said Jia Li, an assistant professor of physics at Brown and senior author of the research. "Yet we've shown that we can create magnetism in a system that originally hosts superconductivity. This gives us a new way to study the interplay between superconductivity and magnetism, and provides exciting new possibilities for quantum science research."
Magic-angle graphene has caused quite a stir in physics in recent years. Graphene is a two-dimensional material made of carbon atoms arranged in a honeycomb-like pattern. Single sheets of graphene are interesting on their own -- displaying remarkable material strength and extremely efficient electrical conductance. But things get even more interesting when graphene sheets are stacked. Electrons begin to interact not only with other electrons within a graphene sheet, but also with those in the adjacent sheet. Changing the angle of the sheets with respect to each other changes those interactions, giving rise to interesting quantum phenomena like superconductivity.
This new research adds a new wrinkle -- spin-orbit coupling -- to this already interesting system. Spin-orbit coupling is a state of electron behavior in certain materials in which each electron's spin -- its tiny magnetic moment that points either up or down -- becomes linked to its orbit around the atomic nucleus.
"We know that spin-orbit coupling gives rise to a wide range of interesting quantum phenomena, but it's not normally present in magic-angle graphene," said Jiang-Xiazi Lin, a postdoctoral researcher at Brown and the study's lead author. "We wanted to introduce spin-orbit coupling, and then see what effect it had on the system."
Spin-orbit–driven ferromagnetism at half moiré filling in magic-angle twisted bilayer graphene, Science (DOI: 10.1126/science.abh2889)
Also at phys.org