Physicists push quantum boundaries by turning a superfluid into a supersolid — and back — for the first time

Scientists just watched a bizarre phase of matter turn into an even stranger one. For the first time, they saw a superfluid turn into a supersolid — a transition they weren't sure was even possible.

In a Jan. 28 study in the journal Nature, researchers observed a group of excitons — quasiparticles that combine an electron and an electron hole — transforming from a superfluid into a supersolid and back again. It is the first time excitons have been seen condensing into a supersolid, undergoing a reversible phase transition the way water can transform from a liquid to ice and back.

Secret phases of matter

There are many more phases of matter than the typical three we encounter every day (gases, liquids and solids), although most of these other matter states exist only under extreme conditions. Superfluids are one type that occurs only when some particles, like helium isotopes and excitons, are cooled to just above absolute zero — the complete absence of heat. They're not quite liquids — they flow without resistance from friction — and when stirred, they form tiny eternal tornadoes called quantum vortices.

Supersolids, on the other hand, are a state of matter theorized to exist when superfluids are cooled even more. They keep superfluidity's zero viscosity, but instead of particles moving around in a liquid-like blob, they form an orderly structure, like a crystal lattice, while maintaining their ability to flow and form quantum vortices.

Supersolids have been made in labs before, including in 2021, when researchers created 2D supersolid dysprosium and in 2024 when they saw quantum vortices in a supersolid. However, they achieved this only by using extra equipment and energy to force particles into an orderly lattice. The new study, by contrast, demonstrates a natural phase transition.

"For the first time, we've seen a superfluid undergo a phase transition to become what appears to be a supersolid," Cory Dean, a physicist at Columbia University and co-author of the study, said in a statement.

Exploring new boundaries

To do it, researchers put two pieces of graphene — which is like a very thin sheet of paper made entirely of carbon atoms — very close together. Then, they added a strong magnetic field and cooled the system to form an exciton "soup."

When cooled to between 2.7 and 7.2 degrees Fahrenheit (1.5 to 4 degrees Celsius) above absolute zero, the excitons formed a superfluid. When cooled more than that, the excitons changed into an electrically insulative mysterious new phase that the team suspects is the theorized supersolid state.

"Superfluidity is generally regarded as the low-temperature ground state," Jia Li, a physicist at the University of Texas at Austin and co-author of the study, said in the statement. "Observing an insulating phase that melts into a superfluid is unprecedented. This strongly suggests that the low-temperature phase is a highly unusual exciton solid."

The team is looking at other materials to test, as well as finding new ways to measure and study the exciton supersolid state.

"For now, we're exploring the boundaries around this insulating state, while building new tools to measure it directly," Dean said. Further study will help scientists understand how supersolids and superfluids behave, deepen our understanding of particle physics and work toward applications of higher-temperature supersolids.