Physicists have observed a superfluid—a substance that normally flows indefinitely without friction—suddenly stop moving and freeze into a solid-like state. This groundbreaking observation, made by researchers at Columbia University and the University of Texas at Austin, marks the first time a superfluid has been seen undergoing a phase transition into a “supersolid.” The discovery challenges long-held assumptions about quantum matter and provides new insights into one of the most elusive states in physics.
The findings, published in the journal Nature, describe how the team used ultra-thin graphene to witness this rare event. While ordinary matter transitions from gas to liquid to solid as it cools, quantum matter follows different rules. For decades, scientists believed that superfluids represented a low-temperature ground state that would persist indefinitely. However, this new research demonstrates that under specific conditions, a superfluid can “freeze” into a quantum state that possesses the crystalline order of a solid while retaining quantum properties.
A Quantum Phase Transition
The research team, led by physicists Cory Dean of Columbia University and Jia Li of the University of Texas at Austin, achieved this result by manipulating bilayer graphene. In their experiments, they observed a superfluid come to a complete standstill, effectively defying the conventional behavior of quantum fluids.
“For the first time, we’ve seen a superfluid undergo a phase transition to become what appears to be a supersolid,” said Dean. He compared the phenomenon to a familiar real-world process, explaining, “It’s like water freezing to ice, but at the quantum level.”
This transition is significant because it suggests that superfluids are not always the final state of matter at extremely low temperatures. Instead, they can evolve into a supersolid, a phase that combines the rigid structure of a solid with the unique physics of a quantum wave.
Understanding the Supersolid
A supersolid is a theoretical state of matter that has puzzled scientists for roughly 50 years. By definition, a classical solid is characterized by a fixed arrangement of atoms in a repeating crystal lattice. A supersolid, however, is a counterintuitive blend of two opposing states: it maintains the crystalline structure of a solid but is predicted to exhibit the frictionless flow of a superfluid.
In this experiment, the “freezing” observed by the researchers represents the formation of this ordered state. The team found that the superfluid entered an insulating phase, effectively locking into place.
“Superfluidity is generally regarded as the low-temperature ground state,” said Li. He noted the rarity of the event, adding, “Observing an insulating phase that melts into a superfluid is unprecedented.”
The Role of Graphene
The experiment relied on the unique properties of graphene, a material consisting of a single layer of carbon atoms. By using a bilayer graphene setup, the researchers could create the necessary conditions for excitons—bound pairs of electrons and holes—to spontaneously self-assemble into a solid phase.
Previous attempts to study supersolids often involved simulating the state using lasers and optical traps to force fluids into patterns. This new observation in graphene offers a more direct look at the transition in a naturally occurring material system. The ability to switch between superfluid and supersolid states in such a controlled environment could open new pathways for understanding quantum materials and their potential applications.
