At low exciton density, a superfluid suddenly stops flowing

Physicists at Columbia University in the US say they may have found evidence for a phenomenon in which a superfluid suddenly stops flowing inside a solid-state material. If confirmed, the finding – made in experiments using two atom-thin layers of graphene – could be the first superfluid-to-insulator phase transition ever observed in a naturally occurring material.

“For the first time, we’ve seen a superfluid undergo a phase transition to become what appears to be a supersolid,” says Cory Dean, who led the new study. “It’s like water freezing to ice, but at the quantum level.”

Supersolids are a hypothetical state of matter that can be both liquid- and solid-like at the same time – that is, they have a crystal structure and superfluid properties. In this description, first put forward by physicists in the 1970s, the crystal lattice and superfluidity are all part of the same phase coherent ground state and are not two separate systems, explains Dean.

In the new work, the researchers studied graphene, which is a sheet of carbon just one atom thick. When two of sheets of graphene are placed atop each other, they can be manipulated so that one layer contains extra electrons and the other extra holes.

The electrons and holes can combine to form quasiparticles known as excitons, which can then travel through the graphene bilayer as a superfluid when a strong magnetic field is applied.

Graphene, sometimes called the “wonder material”, is ideal for such fundamental physics studies because its properties can be fine-tuned by adjusting parameters like temperature, the applied electromagnetic fields and even the distance between the layers.

Controlling the density of excitons

In their experiments, Dean and co-workers were able to move the excitons in their bilayer samples by applying oppositely charged electric fields to the two layers. This, explains Dean, causes the positive and negative parts of each exciton to be pulled in the same direction, allowing them to indirectly drive and detect exciton flow. This ability to control layer imbalance allowed the team to tune the exciton density. Normally, such a process is difficult to achieve because excitons are electrically neutral and do not respond directly to ordinary electrical measurements, which makes tracking their motion difficult.

Thanks to their technique, which they detail in Nature, the researchers found that at high densities, the excitons behaved like a superfluid. At lower densities, however, these excitons “froze” and the superfluid became insulating. Even more striking, says Dean, is that warming the system restored the superfluid flow. “This result suggests that a supersolid-like phase emerges spontaneously, driven solely by particle interactions.”

The Dean lab has been studying the superfluid exciton phase for many years, though most of their work to date focused almost exclusively on the “layer balanced” condition that occurs when there is an equal density of electrons and holes in the two graphene layers. More recently, they began to study the layer imbalanced regime, which has been much less explored in experiments.

“To our surprise we found that under very large imbalance, the exciton transitions to an insulating state beyond some critical imbalance,” says Dean. “This observation alone could have many trivial explanations, but the real shock came when we found that upon heating the system, the superfluid is recovered.”

This behaviour, which has been discussed in some theoretical literature, has no precedence in any existing experiments of superfluidity, he explains, so it is something we should try to better understand.

“To view the situation in the opposite sense: when cooling a fluid and it transitions to a superfluid, the superfluid is already in a thermodynamic ground state. So why upon further cooling, should it undergo a transition to any other phase?” asks team member Jia Li. “We eventually realized that in our experiment, the role of layer imbalance is really a tuning of the exciton density, and the insulating phase onsets when the exciton density crosses a critical value,” he tells Physics World. “Once we had adopted this view, understanding the observed phase transition, and how it fits in with existing theoretical predictions, fell into place.”

A true supersolid or not?

While the researchers say they have firmly established the existence of an insulating state within the superfluid phase diagram, whether this state is truly a supersolid or some other as-yet unknown quantum ground state remains less clear. The challenge with understanding an insulating material is that it becomes more difficult to probe its behaviour, says Dean. “This is made even more difficult by the experimental requirements to stabilize the insulating phase: we need ultraclean samples, low temperatures and high magnetic fields.”

And the difficulties do not end there: “having to work with strong magnetic fields also limits what experimental probes we can use,” he adds. “To progress further, we need to develop new tools to probe the insulating state – for example, we are developing a scan probe technique that we hope can directly image and spatially map the exciton condensate.”

“We have also been working on realizing this condensate in material systems with strong interactions that do not require magnetic fields,” he reveals.

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