Electrons in Twisted Layers—Design, Surprise and a New Set of Eyes

When two atomically-thin layers of a material are stacked one atop each other, with a relative twist angle between them, properties can emerge that bear little resemblance to the behavior of the individual layers. Though much can be predicted and designed about such structures, I will share two vignettes about how my students aimed for a particular behavior but found something quite different. The first led to the discovery of the first experimentally-known "orbital magnet", a ferromagnet in which the tiny microscopic magnets that align with each other are not electron spins but tiny circulating current loops. The second surprise was observation of resistance that skyrocketed with the application of a magnetic field, along with other striking electronic properties -- this one took years to figure out, but we've recently explained it.

Each of these two surprises turned out to be caused by a structural feature of the layered stack which had not previously been considered important. I'll describe a refined approach to stacking and a newly-developed technique for mapping the structure of twisted layers, which together might help us get more repeatable control of structure and thus electronic properties in such twisted systems.