Supersolid explained: Matter that’s both solid and fluid

In a groundbreaking experiment, scientists have made a pivotal discovery about a strange matter called a “supersolid,” which combines the properties of both solids and superfluids.

This marks the first time the dual nature of supersolids has been directly observed—something that has puzzled physicists for decades.

In everyday life, matter exists in four primary states: solid, liquid, gas, and the rarer plasma. However, under extreme conditions—such as at incredibly high energy levels or temperatures approaching absolute zero (-273.15°C or -459.67°F)—matter can behave in unusual ways.

Physicists observe that fluids, such as liquids or gases, experience varying levels of resistance to flow, known as viscosity. For example, honey is more viscous than water.

Superfluids, a type of exotic matter created at ultra-low temperatures, have zero viscosity—there is no resistance, so they flow freely. When stirred, a superfluid continues to move indefinitely, never slowing down.

More than 50 years ago, physicists predicted the existence of an even stranger state of matter: the supersolid. A supersolid behaves as both a solid and a superfluid—its atoms are arranged in a rigid crystal lattice, yet a fraction of them can flow frictionlessly through the structure.

While researchers had previously observed the crystal formations inside supersolids, direct observation of their unusual flow behavior remained elusive—until now.

In a study led by Francesca Ferlaino at Austria’s University of Innsbruck, published in the journal Nature on Wednesday, the team made a breakthrough by directly observing this behavior.

In the experiment, the team stirred a supersolid and observed the formation of tiny whirlpools, called “quantized vortices,” which are the “smoking gun of superfluidity”, Ferlaino told AFP News.

“Imagine you have a cup of coffee, and you give it a little swirl with a spoon. You’ll see the coffee spinning around the center, and if you look closely, there might be a whirlpool in the middle where the liquid is swirling the fastest. This is a classic example of a vortex in a regular fluid,” she explained.

In a superfluid, however, the response is different.

“If you swirl the spoon slowly, you’ll be surprised to see that the superfluid doesn’t rotate along with the spoon at all—it remains perfectly still, as if nothing disturbed it,” Ferlaino said.

“However, if you swirl the spoon faster, instead of forming one large whirlpool in the center, something remarkable happens. A series of tiny whirlpools, or quantized vortices, begin to appear. These are like small holes in the fluid, each rotating at a specific speed,” she explained.

“(T)hey arrange themselves in beautiful, regular patterns across the surface of the superfluid, almost like the holes in a piece of Gruyere cheese, but perfectly organized,” the physicist added.

In 2021, Ferlaino’s team created a stable, two-dimensional supersolid by cooling atoms and molecules to near absolute zero.

“The next step—developing a way to stir the supersolid without destroying its fragile state—required even greater precision,” lead study author Eva Casotti said.

Using magnetic fields, the team carefully rotated the supersolid, stirring it to generate the distinctive quantized vortices.

This discovery provides strong, direct evidence of the dual nature of supersolids—both solid and superfluid.

Ferlaino and her team believe that this breakthrough could allow scientists to simulate extreme phenomena in the laboratory, such as those occurring in neutron stars—the incredibly dense remnants of massive stars that have exploded in supernovae.