Superfluids are supposed to flow indefinitely. Physicists just watched one stop moving

Physicists observed a superfluid transition into what appears to be a supersolid for the first time using bilayer graphene. The team created excitons (quasiparticles formed from electrons and holes) in graphene sheets under a magnetic field, which behaved as a superfluid at high density but stopped moving and became an insulator at lower density. This insulating phase melted back into a superfluid at higher temperatures, suggesting the formation of an exotic exciton solid. The discovery provides the first solid evidence of a spontaneous superfluid-to-supersolid transition, resolving a decades-old question in condensed matter physics.