When it comes to the fascinating world of physics, a groundbreaking observation has emerged that challenges our traditional understanding of matter. Typically, when we cool down common materials, they undergo a predictable transformation: gases condense into liquids, and further cooling leads to the formation of solids. However, the behavior of quantum matter can be quite different from these expectations. Over a century ago, researchers made a remarkable discovery regarding helium—it exhibits surprising characteristics at extremely low temperatures. Instead of transitioning into a solid state, helium transforms from a gaseous state into a superfluid, a unique form of matter that flows effortlessly without any resistance, often displaying bizarre behaviors such as climbing up the walls of its container.
This peculiar nature of superfluids has led physicists to ponder an intriguing question: what if we could cool a superfluid even more? For nearly half a century, this query remained unanswered, leaving scientists eager for insights.
A Superfluid Comes to a Standstill
Exciting new research recently published in the journal Nature by a team led by physicists Cory Dean from Columbia University and Jia Li from the University of Texas at Austin reveals a groundbreaking finding. They observed a superfluid—known for its perpetual motion—suddenly come to a complete halt. Dean stated, "For the first time, we've witnessed a superfluid undergoing a phase transition to what seems to be a supersolid." This phenomenon is akin to water freezing into ice, but it unfolds in the realm of quantum mechanics.
Defining a Supersolid
To understand this discovery, it's essential to grasp what a supersolid actually is. In classical terms, a solid is characterized by atoms arranged in a stable, repeating crystal lattice. In contrast, a supersolid is a quantum counterpart of this concept, predicted to maintain a solid-like arrangement while simultaneously exhibiting behaviors typically associated with liquids, such as frictionless flow. The unusual combination of these properties positions supersolids as one of the most intriguing states of matter theorized by physicists.
Until this recent study, no experiments had convincingly demonstrated a superfluid transitioning into a supersolid naturally—this held true for helium and other known materials. While some laboratory experiments have successfully replicated supersolid behavior through highly controlled conditions involving atomic, molecular, and optical (AMO) physics, these setups often rely on lasers and optical components to create periodic traps that force particles into structured patterns, comparable to how Jello forms in an ice cube tray.
Exploring Graphene for Insights
The quest for a naturally occurring supersolid—one that forms without external constraints—has been a topic of intense debate in condensed matter physics. Dean's team adopted a novel approach by investigating graphene, a remarkable material composed of a single layer of carbon atoms. The collaborative effort included Li, who conducted his research during his postdoctoral fellowship at Columbia, and Yihang Zeng, now an assistant professor at Purdue University, who was a former PhD student in the group.
Graphene possesses the ability to support quasiparticles known as excitons. These excitons arise when two atom-thin graphene sheets are stacked together and adjusted so that one layer contains excess electrons while the other holds extra holes (the vacancies left behind when electrons depart the layer due to light). Because electrons carry a negative charge and holes behave like positive charges, they can combine to form excitons. Under a strong magnetic field, these excitons can collectively exhibit superfluid behavior.
A Surprising Phase Transition in Two-Dimensional Materials
Two-dimensional materials like graphene serve as powerful platforms for examining quantum phenomena due to their adjustable properties. Researchers can manipulate variables such as temperature, electromagnetic fields, and even the distance between layers. As Dean's team fine-tuned these parameters, they uncovered an unexpected relationship between exciton density and temperature.
When excitons were densely concentrated, they flowed freely as a superfluid. However, as the density decreased, the flow came to a standstill, and the system transitioned into an insulating state. Remarkably, increasing the temperature restored the superfluidity. This sequence of events contradicts long-held beliefs about the nature of superfluidity.
Li commented, "Superfluidity is generally considered to be the state that exists at low temperatures. Observing an insulating phase that melts into a superfluid is unprecedented. This strongly implies that the low-temperature phase is a highly unusual exciton solid."
Is This Phenomenon Truly a Supersolid?
The classification of this newly observed state as a supersolid is still a matter of debate. Dean stated, "We find ourselves in a position where we have to speculate a bit, as our ability to study insulators has limitations; after all, insulators do not conduct electricity. For the time being, we are probing the edges of this insulating state while developing new tools to measure it directly."
What Lies Ahead for Supersolid Research
The research team is actively exploring other layered materials that may host similar quantum phases. In bilayer graphene, for example, the excitonic superfluid and potential supersolid only manifest under strong magnetic fields. While other materials might prove more challenging to produce in the necessary configurations, they could enable excitons to remain stable at higher temperatures without requiring a magnetic field.
The ability to manage superfluids within two-dimensional materials could have significant implications for future technologies. Unlike helium, excitons are thousands of times lighter, making it feasible for them to form exotic quantum states at much higher temperatures. Although the mysteries surrounding supersolids are far from fully resolved, these compelling findings underscore the pivotal role that 2D materials will likely play in deepening our understanding of this enigmatic quantum phase.
