In a Cornell University laboratory, scientists stumbled by chance upon a material that behaves as if two opposite worlds of electrons coexist within it at the same time. The material in question is rhombohedral six-layer graphene — six sheets of carbon stacked with a shift in a repeating pattern. Instead of studying twisted layers, the researchers obtained a flat stack, and it was precisely this that revealed unexpected quantum phases.
Graphene has long been known for its strength and conductivity, but when its layers are stacked in a particular way, electrons begin to interact with one another far more strongly. In the six-layer version, at a certain charge-carrier density and under an external electric field, a regime arises in which electrons and holes coexist and are strongly correlated. It resembles a crowd of people in which each person influences a neighbour's movement rather than simply going about their own business.
Measurements showed the disappearance of electrical resistance — a hallmark of superconductivity — and an anomalous Hall effect indicating orbital magnetism. Two superconducting states arose precisely where electrons and holes intersect. Their interaction apparently plays a key role in the formation of these phases, something that was not observed in five-layer graphene.
The device was placed between layers of boron nitride and controlled by gate fields. By varying the temperature, the magnetic field and the carrier concentration, the team constructed a detailed phase diagram. It clearly shows interaction-induced insulators, regions of superconductivity and two types of orbital magnetism. The results were published in Nature Physics.
The discovery matters not only for fundamental physics. Understanding how the interaction of electrons and holes gives rise to superconductivity without traditional mechanisms could bring closer the creation of materials that work at higher temperatures. Such materials will be useful in quantum computers, energy-efficient transmission lines and new sensors.
Researchers are now delving deeper into the nature of orbital magnetism. They are testing the hypothesis that some of the carriers form a crystal-like structure while the rest remain in a liquid-like state. These questions go beyond the scope of the current work, but it is already clear that six-layer graphene has become a new arena for studying quantum correlations.
The accidental discovery once again reminds us that in the world of two-dimensional materials it is enough to change the number of layers or their relative arrangement — and familiar properties turn into something entirely new.
