European physicists have created the first 2D supersolid – a seemingly contradictory type of matter that simultaneously has the properties of a solid and a superfluid.
You’re likely intimately familiar with the states of matter surrounding you every day – solid, liquid, gas and plasma – but when you start looking into the quantum world, everything gets a bit wild.
A superfluid, for example, is a peculiar state of matter that can flow without friction, so if you stir a cup full of it, it will continue to swirl around forever. (This allows for some wacky research.)
Back in the 1950s, physicists wondered if there was an equivalent for solids: a state of matter with the rigid structure of atoms yet able to move without resistance.
“To picture a supersolid, consider an ice cube immersed in liquid water, with frictionless flow of the water through the cube,” explains Bruno Laburthe-Tolra from Paris North University, in a News & Views article accompanying the new research into 2D supersolids.
Two major candidates were in the running for the first supersolidity, since they were already known to display superfluidity: helium atoms, because they’re so ultralight they can act like waves; and ultracold atomic gases, produced at a cool 100 nanokelvins.
The latter won out. After years of searching, two independent research teams finally observed this elusive state of matter for the first time in 2017, starting with superfluidic atoms and using lasers to arrange the atoms into regular formations.
Then in 2019, the team responsible for this current study tried a different approach. They produced supersolid states in ultracold quantum gases of magnetic atoms. The magnetism allowed these systems to self-organise into regular, 1D patterns, like a string of ‘droplets’.
“We have now extended this phenomenon to two dimensions, giving rise to systems with two or more rows of droplets,” explains Matthew Norcia, lead author of the new paper published in Nature.
Laburthe-Tolra writes that “this demonstration is a key advance because one direct way to prove that a system exhibits superfluidity is to study its properties under rotation, and this analysis cannot be achieved if the system has only one dimension”.
Norcia says this expands the research horizon of this bizarre state of matter – literally.
“For example, in a two-dimensional supersolid system, one can study how vortices form in the hole between several adjacent droplets,” he says. “These vortices described in theory have not yet been demonstrated, but they represent an important consequence of superfluidity.”
The research was led by the University of Innsbruck, Austria, and the Institute for Quantum Optics and Quantum Information at the Austrian Academy of Sciences.