Spin freezing upon heating in moiré graphene

It is common knowledge that liquids freeze and become solids upon cooling. Counterintuitively, liquid helium solidifies upon heating, a process known as the Pomeranchuk effect. Now, two papers using different techniques—one by Shahal Ilani and coworkers and the other by Andrea Young and coworkers—report that an electronic analogue of this effect occurs in magic-angle twisted bilayer graphene. The hallmark feature of the phase transition from the electronic liquid metallic state to a type of electronic solid state is the high entropy in the latter. The authors demonstrate that some of the correlated electrons in moiré graphene are itinerant and localized at the same time, properties that are usually contradictory. This dual nature of the electrons requires new theoretical models and the results also have implications for the mechanism underlying superconductivity.