The First Signals of a Magnetic Supersolid

We live in a world full of color, noise, and causes that demand attention. In order to avoid being completely overwhelmed, most people quickly and instinctively sort things into neat categories that help them make sense of the world.

But what happens when something doesn’t fit into the organizational structure we depend on?

For scientists, that means it’s time to get excited.

Depending on the strength of the interaction between the dysprosium atoms, the system is either a standard Bose-Einstein condensate (top row), an array of quantum droplets (bottom row), or a novel type of systems with periodic structure and supersolid properties (middle row). Image Credit: Tanzi et al. 2019

This is the context for research recently published in Physical Review Letters by scientists from the National Institute of Optics in Italy (CNR-INO), the University of Florence (UniFl), and the University of Hanover in Germany. The team has detected some of the first signals of a strange type of matter called a supersolid.

A supersolid isn’t really a solid, liquid, gas, or plasma. The atoms in a supersolid form a periodic structure like those solid, but they can flow like the atoms in a liquid or gas. Actually, they can flow even more efficiently, because they do so without friction.

That might sound impossible, but it’s within the realm of possibility thanks to quantum mechanics. In fact, 50 years ago this year a duo of Russian scientists predicted that superfluid helium-4, which flows without friction, could become a supersolid under the right conditions. The science community has tried earnestly to realize this prediction, but so far without clear success.

The quest to create a supersolid has recently regained momentum. Labs around the world can produce systems of ultracold atoms known as Bose-Einstein condensates (BECs), explains lead researcher Giovanni Modugno (CNR-INO and UniFl). Known as the fifth state of matter, BECs are superfluids. Research suggests that under the right conditions, they can form “quantum droplets” that have a periodic arrangement and could facilitate a supersolid. But determining the right conditions has been tricky.

To increase their odds of creating the right conditions, Modugno and his team focused their attention on a highly magnetic element that you’ve probably never heard of—dysprosium, element number 66 on the periodic table. Why dysprosium? It can be manipulated due to its magnetic properties. Scientists have created quantum droplets before, but the interactions between the droplets weren’t right to create a supersolid. Modugno and his team hoped that by using highly magnetic atoms, they could alter the interactions in the right way. “[W]ith an external magnetic field we can align all the atoms like the needles of compasses, and this creates a special interaction between the particles,” he explains.

The team spent several years building a machine that traps dysprosium atoms: it cools them into a BEC, and lets the BEC freely arrange in the presence of a magnetic field. Once the dysprosium atoms were trapped, the team gradually changed the magnetic field. This affected the interactions between atoms, changing their arrangement. To take a photo of these arrangements at this microscopic size, the researchers had to let it expand freely as a gas.

Dipolar atoms in a Bose-Einstein condensate. Dark blue droplets will form when interacting with the trapping potential (the gray line here), the atoms’ dipolar and contact interactions, and quantum fluctuations. The correspondence here indicates the presence of a supersolid. Image credit: APS/Alan Stonebreaker 

The data they collected shows that under some conditions, the atoms formed just the pattern you’d expect from a supersolid—atoms gathered in groups, spaced equidistantly apart. The pattern lasted for about 30 milliseconds which is too short for the material to be thoroughly tested, but longer lifetimes should be within reach. Once observed in the laboratory, the existence of the supersolid was confirmed theoretically by Luis Santos and his group at the University of Hanover. Two other groups have since found similar results that you can read about in Physics Magazine.

The excitement is palpable. “I’m of course very happy for the exciting period ahead, in which we will have to explore how this new material behaves differently from the ordinary gases, liquids, and solids,” says Modugno.

“Our supersolid will not produce any application or device for the general public in the short run, since in the end, we are working with a tiny gas of atoms in a vacuum, at the center of a machine that occupies a whole room. But it will definitely help us to see the fundamental quantum properties of a potentially new material.”

Kendra Redmond

You may also read these articles