Physicists have finally watched a fluid that should never freeze lock itself into a crystal pattern, then melt back again, all without losing its quantum strangeness. In a sheet of graphene, they drove a gas of ghostlike particles called excitons from a frictionless superfluid into a rigid supersolid and then reversed the process, revealing a long theorized state of matter in real time. The experiment does more than tick off a prediction from the 1970s, it opens a controlled window onto phases of matter that could underpin future quantum devices.
At the heart of the work is a simple but radical idea: a material can be both solid and fluid at once. By tuning density and temperature in an exquisitely engineered graphene structure, the team showed that a superfluid can freeze into a crystal while still allowing its constituents to flow, then thaw back into a pure quantum liquid. That reversible switch, achieved with excitons rather than ordinary atoms, gives researchers a powerful new handle on how quantum order emerges and collapses.
From frictionless flow to frozen quantum crystal
The central breakthrough came when physicists working with bilayer graphene watched a quantum liquid of excitons abruptly stop flowing as they dialed down its density. At high density, the excitons, which are bound pairs of electrons and holes, moved collectively without resistance, a hallmark of superfluidity. As the researchers reduced how tightly these quasiparticles were packed, the flow suddenly halted and the system behaved like an insulator, a transition that had been predicted but never directly seen in such a clean, tunable platform.
In detailed measurements, Researchers from Columbia University, Brown University, University of Texas, Austin and the National Institute for Materials Sci found that this superfluid to insulator jump was driven by interactions among the excitons themselves rather than by disorder. That interaction driven transition is exactly the regime where theory expects a supersolid to emerge, with excitons arranging into a crystal while still sharing a coherent quantum wave. The group could then nudge the system back toward the flowing phase, effectively toggling between a superfluid and a more ordered state in the same device.
Excitons, quantum droplets and the road to supersolids
To understand why this matters, it helps to look at how excitons became a serious playground for quantum phases. Earlier work showed that Physicists could make these electron hole pairs condense into a collective state that behaves like a single quantum object. In San Diego, San Diego researchers reported phenomena that they interpreted as the first confirmation of exciton superfluidity, where these quasiparticles slide without friction across a material. Those results established excitons as a platform for exploring exotic states that are hard to reach with ordinary atoms.
Even before supersolids entered the picture, quantum engineers were already coaxing new composite particles out of semiconductors. In work described in Feb in the journal Nature and highlighted on its cover, a JILA team identified a microscopic complex of electrons and holes in a new, unpaired arrangement, a so called quantum droplet. These droplets showed that many body interactions in semiconductors can stabilize unexpected bound states, a theme that now carries into exciton condensates and their potential to crystallize into supersolids under the right conditions.
Watching a superfluid freeze, then thaw
The graphene experiment that captured global attention went a step further by directly tying flow and structure together. Physicists have watched a quantum liquid in a graphene based device freeze into a phase that blurs the line between solid and fluid, resolving a puzzle that has lingered for decades. In this setup, excitons formed across two closely spaced graphene layers and, when densely packed, they flowed freely as a superfluid. As the density dropped, the flow stopped entirely and the system became an insulator, a behavior that points to the formation of an ordered exciton lattice.
The key was control. Researchers could tune temperature, electromagnetic fields and the spacing between layers, watching how the excitons responded. As Dean’s team adjusted these knobs, densely packed excitons flowed freely as a superfluid, but at lower density they locked into place. Scientists described the transformation as watching one bizarre phase of matter turn into an even stranger one, bringing a long theorized quantum world tantalizingly close to reality in a device that fits on a chip.
Defining a supersolid and proving it exists
For decades, theorists have argued about what exactly counts as a supersolid. Superfluids are supposed to flow indefinitely, yet in the graphene device the excitons stopped moving and formed what the team describes as a highly unusual exciton solid. In parallel, As the density dropped, the flow vanished and the system turned insulating, while raising the temperature restored the superfluid, a counterintuitive sequence that suggests the insulating phase is not a mundane frozen state but a delicate quantum crystal.
Independent signatures are starting to back that up. At elevated densities, Dean and colleagues saw their excitons exhibit superfluid behavior, then, as the density diminished, the same system became insulating, only to regain superfluidity when the temperature increased. In ultracold atomic setups, Physicists Spot Quantum in a Supersolid, with New observations of microscopic vortices confirming that a material can host both crystalline order and superfluid flow. Together, these results are converging on a consistent picture of what a supersolid looks like in the lab.
Why freezing a superfluid matters for quantum tech
On a conceptual level, the graphene work reframes how I think about low temperature phases. Superfluidity has long been regarded as the low temperature ground state, yet here an insulating phase that melts into a superfluid as the temperature rises appears instead. That inversion is like water freezing to ice at the quantum level, but with the twist that the “ice” still carries a coherent wave function. Theoretical work, including an analysis in solid more fluid, has emphasized that a supersolid marries solid and liquid properties in space, and the new experiments finally give that idea a concrete platform.
Recent theory on bilayer excitons goes further. Particularly interesting is the prediction that an interaction driven exciton supersolid may be stabilized within experimentally realizable parameters, exactly the regime now being probed. As one researcher put it, Superfluidity is generally regarded as the low temperature ground state, so observing an insulating phase that melts into a superfluid as temperature increases, in a repeating crystal lattice, forces a rethink of long held assumptions in condensed matter physics.
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*This article was researched with the help of AI, with human editors creating the final content.
