Researchers have witnessed a superfluid in graphene halt its motion, transitioning into a supersolid—a quantum phase blending solid-like order with frictionless flow. This breakthrough, achieved in bilayer graphene under specific conditions, challenges long-held assumptions about quantum matter. The findings, published in Nature, mark the first natural observation of such a phase without artificial constraints.
Quantum matter often defies classical expectations. Over a century ago, scientists found that helium at ultra-low temperatures becomes a superfluid, flowing without resistance and exhibiting odd traits like climbing container walls. For decades, researchers pondered what happens if such a fluid cools further, potentially forming a supersolid: a state with crystalline structure yet liquid-like properties.
A team led by Cory Dean of Columbia University and Jia Li of the University of Texas at Austin addressed this in experiments with bilayer graphene. By stacking two atom-thin carbon sheets and tuning one with extra electrons and the other with holes, they created excitons—quasiparticles that, under strong magnetic fields, act collectively as a superfluid.
As they adjusted exciton density and temperature, an unexpected shift occurred. At high densities, excitons flowed freely. Lowering density stopped the flow, turning the system into an insulator—a solid-like state. Increasing temperature then revived the superfluid behavior, inverting typical phase transitions.
"For the first time, we've seen a superfluid undergo a phase transition to become what appears to be a supersolid," Dean said. Li added, "Superfluidity is generally regarded as the low-temperature ground state. 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, including Yihang Zeng (now at Purdue University), used transport measurements to detect these changes. Dean noted limitations: "We are left to speculate some, as our ability to interrogate insulators stops a little." Future work explores other 2D materials, where lighter excitons might enable supersolids at higher temperatures, without magnetic fields.
This discovery highlights graphene's role in probing quantum phases, potentially advancing understanding of exotic matter states.