How can electrons split into fractions of themselves? An anomalous effect!

Amazing stuff!

"MIT physicists have taken a key step toward solving the puzzle of what leads electrons to split into fractions of themselves. Their solution sheds light on the conditions that give rise to exotic electronic states in graphene and other two-dimensional systems.

The new work is an effort to make sense of a discovery that was reported earlier this year by a different group of physicists ... found that electrons appear to exhibit “fractional charge” in pentalayer graphene — a configuration of five graphene layers that are stacked atop a similarly structured sheet of boron nitride. ...

The phenomenon was coined the “fractional quantum anomalous Hall effect,” and theorists have been keen to find an explanation for how fractional charge can emerge from pentalayer graphene.

The new study ... provides a crucial piece of the answer. Through calculations of quantum mechanical interactions ... show that the electrons form a sort of crystal structure, the properties of which are ideal for fractions of electrons to emerge.

“This is a completely new mechanism, meaning in the decades-long history, people have never had a system go toward these kinds of fractional electron phenomena,” ... “It’s really exciting because it makes possible all kinds of new experiments that previously one could only dream about.” ...

They found that the moiré arrangement of pentalayer graphene, in which each lattice-like layer of carbon atoms is arranged atop the other and on top of the boron-nitride, induces a weak electrical potential. When electrons pass through this potential, they form a sort of crystal, or a periodic formation, that confines the electrons and forces them to interact through their quantum correlations. This electron tug-of-war creates a sort of cloud of possible physical states for each electron, which interacts with every other electron cloud in the crystal, in a wavefunction, or a pattern of quantum correlations, that gives the winding that should set the stage for electrons to split into fractions of themselves. ..."

From the abstract:

"Remarkable recent experiments on the moiré structure formed by pentalayer rhombohedral graphene aligned with a hexagonal boron nitride substrate report the discovery of a zero field fractional quantum Hall effect. These “(fractional) quantum anomalous Hall” [(F)QAH] phases occur for one sign of a perpendicular displacement field, and correspond, experimentally, to full or partial filling of a valley polarized Chern-1 band. Such a band is absent in the noninteracting band structure. Here we show that electron-electron interactions play a crucial role, and present microscopic theoretical calculations demonstrating the emergence of a nearly flat, isolated, Chern-1 band and FQAH phases in this system. We also study the four- and six-layer analogs and identify parameters where a nearly flat isolated Chern-1 band emerges which may be suitable to host FQAH physics."

How can electrons split into fractions of themselves? | MIT News | Massachusetts Institute of Technology "Physicists surprised to discover electrons in pentalayer graphene can exhibit fractional charge. New study suggests how this could work."

Theory of Quantum Anomalous Hall Phases in Pentalayer Rhombohedral Graphene Moiré Structures (no public access, but article above contains a link to request the PDF file)