While ordinary matter transitions from gas to liquid to solid upon cooling, quantum matter exhibits distinct behaviors. Superfluids, for instance, flow without energy loss and possess unique quantum properties, such as the ability to climb out of containers. The transition of superfluids upon further cooling has been a long-standing question in physics.
Breakthrough in Quantum Matter Observation
A team led by physicists Cory Dean from Columbia University and Jia Li from the University of Texas at Austin has reported observing a superfluid cease motion, transitioning into what appears to be a supersolid state. This observation was published in Nature.
Supersolids are theorized as a quantum counterpart to classical solids, expected to be both crystalline and capable of frictionless flow.
Previously, the definitive observation of a spontaneous superfluid-to-supersolid transition in naturally occurring matter had not occurred. While researchers had simulated supersolids using periodic traps formed by lasers and optical elements, a spontaneous formation remained unconfirmed.
Graphene: The Quantum Playground
The team utilized graphene, a single-atom-thick carbon material. Graphene can host excitons, which are quasiparticles formed when two-atom-thin graphene layers are manipulated to have electron and hole imbalances. These excitons can form a superfluid when subjected to a strong magnetic field.
Two-dimensional materials like graphene offer platforms to investigate phenomena such as superfluidity due to adjustable parameters like temperature and electromagnetic fields.
Unveiling the Transition
Researchers adjusted these controls for excitons in their samples and observed an unexpected correlation between quasiparticle density and temperature. At high densities, excitons exhibited superfluid characteristics. However, as their density decreased, they became immobile insulators. When the temperature was subsequently raised, superfluidity returned.
Confirmation and Future Steps
Jia Li noted the significance of these findings:
Observing an insulating phase melt into a superfluid is an unusual occurrence, indicating that the low-temperature phase is likely an exciton solid.
While the characteristics align with a supersolid, further direct measurement tools are being developed by the team to confirm the state, particularly since insulators do not conduct current.
The research team plans to investigate the boundaries of this insulating state and develop new measurement techniques. They are also exploring other layered materials. The excitonic superfluid and potential supersolid in bilayer graphene currently require a strong magnetic field. Future research aims to stabilize these quasiparticles at higher temperatures and without magnetic fields, potentially offering a more accessible way to study these quantum phases. Excitons, being significantly lighter than helium, could allow for the formation of quantum states like superfluids and supersolids at higher temperatures, indicating 2D materials' role in advancing the understanding of this quantum phase.