European and Australian physicists recently published a collaborative study in Physical Review Letters suggesting that layered 2D semiconductors can harbor a unique quantum phase of matter known as the supersolid. This counterintuitive quantum phase maintains the rigid structure of a crystal while simultaneously allowing particles to flow freely like a liquid, defying our conventional understanding of matter.
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What is a Supersolid?
A supersolid phase is characterized by a repeating pattern of particles, making it a solid, while the particles can also flow freely, making it a superfluid.
The concept of supersolid phases has been of great interest for a long time because it seems counterintuitive to imagine particles flowing freely without resistance while forming a rigid lattice.
This exotic state is characterized by the coexistence of spatially periodic solid and off-diagonal long-range order, resulting in the spontaneous breaking of continuous translational and particle conservation invariance.
The 50-Year Pursuit of the Supersolid and Its Challenges
In 1970, Geoffrey Chester proposed a theory suggesting that solid helium-4 at low temperatures would exhibit a supersolid ground state under pressure. He proposed that each helium atom would arrange in a regular lattice structure to exhibit crystalline solid order and simultaneously undergo Bose-Einstein condensation of the atoms, causing them to flow without resistance.
This idea sparked a significant experimental search for supersolidity in solid helium-4, but no positive results were obtained.
The concept of supersolidity raised a fundamental question: can a system exhibit periodicity in space without being localized? Achieving this would require long-range atom exchange, but will it be feasible in perfect crystals, or will it require the presence of defects or vacancies?
Various researchers attempted to create a state similar to supersolidity, with some success in producing supersolid-like phases in cold-atom systems using optical lattices. They are either condensate clusters or condensates with varied densities categorized by trapping geometries.
However, it is important to distinguish these supersolid-like clusters from the original Chester supersolid, where each particle is localized in its specific position in the crystal lattice solely due to the forces between particles.
The issue of supersolidity in free space for solids with one or two atoms in the unit cell is not easily solvable through any existing analytical method due to the intense competition between the two orders - crystalline and superfluid - at the interatomic distance. However, the crystalline lattice can continuously adjust its period to remove any vacancies or interstitials that may have been introduced out of equilibrium.
Engineering a Supersolid State Using Interlayer Excitons in Semiconductor Structures
A recent study published in Physical Review Letters suggests that a supersolid can be synthesized in a 2D semiconductor structure consisting of two conducting layers separated by an insulator. The lower layer is infused with positively charged holes, while the upper layer is infused with negatively charged electrons.
This causes interlayer exciton formation, a bound state consisting of a hole and an electron attracted to each other by strong coulomb forces. The average separation between the excitons can be adjusted by applying voltages to the top and bottom metal gates.
The team anticipates that the interlayer excitons in this semiconductor structure will develop a supersolid across a wide range of average separations between the excitons and layer separations. In addition, the excitons' electrical repulsion will confine them to a hard-crystalline lattice.
Significance of the Study
Researchers have been trying to create supersolid states in semiconductors by bringing the layers close together, which makes the exciton binding energy strong but the exciton-exciton interaction weak. However, this new study suggests that a supersolid can be created at larger layer separations, which are easier to achieve experimentally.
Ironically, the layer separations are relatively large and are easier to fabricate than the extremely small layer separations in such systems that have been the focus of recent experiments aimed at maximizing the interlayer exciton binding energies.
Prof. Alex Hamilton, Co-Corresponding Author of the Study
To detect the presence of a supersolid, the researchers propose looking for an anomaly in the rotational moment of inertia, similar to how a superfluid is identified by its ability to host a quantum vortex.
The phase diagram of the system was determined at low temperatures, showing a triple point at the intersection of normal-solid melting, supersolid to normal-solid transition, and supersolid melting. This triple point spreads over an area of phase space due to the ever-present disorder in experiments, resulting in a co-existing supersolid, normal liquid, and normally solid domains with exotic interfaces.
The existence of a triple point is also particularly intriguing. At this point, the boundaries of supersolid and normal-solid melting, and the supersolid to normal-solid transition, all cross. There should be exciting physics coming from the exotic interfaces separating these domains, for example, Josephson tunneling between supersolid puddles embedded in a normal background.
Dr. Sara Conti, Lead Author of the Study
The proposed supersolid is of the Chester type, as each supersolid site is occupied by precisely one exciton and fundamentally differs from other supersolids with periodic density or periodic clustering modulation resembling density waves.
This discovery of a supersolid in excitons extends our understanding of the behavior of matter and could have implications for future developments in quantum computing and other technologies.
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References and Further Reading
Conti, S., Perali, A., Hamilton, A. R., Milošević, M. V., Peeters, F. M., & Neilson, D. (2023). Chester supersolid of spatially indirect excitons in double-layer semiconductor heterostructures. Physical Review Letters, 130(5), 057001. https://doi.org/10.1103/PhysRevLett.130.057001
FLEET. (2023). Engineering a Novel Supersolid State Using Layered 2D Materials. Available from: https://www.fleet.org.au/blog/engineering-a-novel-supersolid-state-using-layered-2d-materials/ (Accessed on April 02, 2023)
Kuklov, A. B., Prokof’ev, N. V., & Svistunov, B. V. (2011). How Solid is Supersolid? Physics, 4, 109. Retrieved from https://physics.aps.org/articles/v4/109 (Accessed on April 02, 2023)
Norcia, M. A., Politi, C., Klaus, L., Poli, E., Sohmen, M., Mark, M. J., ... & Ferlaino, F. (2021). Two-Dimensional Supersolidity in a Dipolar Quantum Gas. Nature. https://doi.org/10.1038/s41586-021-03725-7