Superfluids are supposed to flow indefinitely. Physicists just watched one stop moving

Ordinary matter, when cooled, transitions from a gas into a liquid.  Cool it further still, and it freezes into a solid. Quantum matter, however, can behave very differently. In the early 20^th century, researchers discovered that when helium is cooled, it transitions from a seemingly ordinary gas into a so-called superfluid. Superfluids flow without losing any energy, among other quantum quirks, like an ability to climb out of containers. 

What happens when you cool a superfluid down even more? The answer to this question has eluded physicists since they first started asking it half a century ago. 

Writing today in Nature, a team led by physicists Cory Dean from Columbia University and Jia Li from the University of Texas at Austin has observed a superfluid, which normally remains in constant motion, come to a standstill. “For the first time, we’ve seen a superfluid undergo a phase transition to become what appears to be a supersolid,” said Dean. It’s like water freezing to ice, but at the quantum level.

 Supersolids are the predicted quantum version of a classical solid, which is defined as a fixed arrangement of atoms in a repeating crystal lattice. Supersolids, counterintuitively, can be both liquid-like and solid-like at the same time: crystalline, like classical solids, but predicted to exhibit the same frictionless flow as a superfluid.

Despite those predictions, no one has definitively observed the transition from superfluid to supersolid in helium, or any other naturally occurring matter, yet. Researchers in the atomic, molecular, and optical (AMO) sub-branch of physics have simulated versions of supersolids in recent years, but using lasers and optical elements to create what’s known as a periodic trap, which helps coax the fluid into a crystal-like pattern—a bit like Jello confined in an ice cube tray.

 A spontaneously forming supersolid remained enigmatic, leaving one of the great controversies in condensed matter physics unsolved. That is, until Dean’s team, which included Li while he was a postdoc at Columbia and a former PhD student, Yihang Zeng (now an assistant professor at Purdue University), turned to a naturally occurring crystal: graphene, a single-atom-thick sheet of carbon atoms.

Graphene can host what are known as excitons. These quasiparticles form when two-atom-thin sheets of graphene are layered together and manipulated such that one layer has extra electrons and the other, extra holes (which are left behind when electrons leave the layer in response to light). Negatively charged electrons and positively charged holes can combine into excitons. Add a strong magnetic field, and excitons can form a superfluid.

2D materials like graphene have emerged as promising platforms to explore and manipulate phenomena like superfluidity and superconductivity. That’s because there are a number of different “knobs” researchers can adjust, like temperature, electromagnetic fields, and even the distance between the layers, to fine-tune their properties. When Dean’s team began turning the knobs to control the excitons in their samples, they saw an unexpected relationship between the density of the quasiparticles and temperature. At high density, their excitons behaved like a superfluid, but as their density decreased, they stopped moving and became insulators. When the team increased the temperature, superfluidity returned. 

“Superfluidity is generally regarded as the low-temperature ground state,” said Li. “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.”

 So, is it a supersolid? “We are left to speculate some, as our ability to interrogate insulators stops a little,” explained Dean—their expertise is in transport measurements, and insulators don’t transport a current. “For now, we’re exploring the boundaries around this insulating state, while building new tools to measure it directly.”

They are also looking at other layered materials. The excitonic superfluid, and likely supersolid, that forms in bilayer graphene only does so with the help of a strong magnetic field. Alternatives are somewhat more challenging to fabricate into the necessary arrangements, but they could stabilize the quasiparticles at even higher temperatures and without the need for a magnet.

 Controlling a superfluid in a 2D material is an exciting prospect—compared to helium, for example, excitons are thousands of times lighter, so they could potentially form quantum states such as superfluids and supersolids at much higher temperatures. The future of supersolids remains to be realized, but there is now solid evidence that 2D materials will help researchers understand this enigmatic quantum phase.