Physicists give weird new phase of matter an extra dimension

An artist's impression of the supersolid, which is like a solid and a liquid at the same time.
An artist's impression of the supersolid, which is like a solid and a liquid at the same time. (Image credit: IQOQI Innsbruck/Harald Ritsch)

Physicists have created the first ever two-dimensional supersolid — a bizarre phase of matter that behaves like both a solid and a frictionless liquid at the same time.

Supersolids are materials whose atoms are arranged into a regular, repeating, crystal structure, yet are also able to flow forever without ever losing any kinetic energy. Despite their freakish properties, which appear to violate many of the known laws of physics, physicists have long predicted them theoretically — they first appeared as a suggestion in the work of the physicist Eugene Gross as early as 1957.

Now, using lasers and super-chilled gases, physicists have finally coaxed a supersolid into a 2D structure, an advancement that could enable scientists to crack the deeper physics behind the mysterious properties of the weird matter phase.

Related: 12 stunning quantum physics experiments 

Of particular interest to the researchers is how their 2D supersolids will behave when they're spun in a circle, alongside as the tiny little whirlpools, or vortices, that will pop up inside them.

"We expect that there will be much to learn from studying rotational oscillations, for example, as well as vortices that can exist within a 2D system much more readily than in 1D," lead author Matthew Norcia, a physicist at Innsbruck University's Institute for Quantum Optics and Quantum Information (IQOQI) in Austria, told Live Science in an email. 

To create their supersolid, the team suspended a cloud of dysprosium-164 atoms inside optical tweezers before cooling the atoms down to just above zero Kelvin (minus 459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius) using a technique called laser-cooling. 

Firing a laser at a gas typically heats it up, but if the photons (light particles) in the laser beam are traveling in the opposite direction of the moving gas particles, they can actually cause slow and cool the gas particles. After cooling the dysprosium atoms as far as they could with the laser, the researchers loosened the "grip" of their optical tweezers, creating just enough space for the most energetic atoms to escape. 

Since "warmer" particles jiggle faster than cooler ones, this technique, called evaporative cooling, left the researchers with just their super-cooled atoms; and these atoms had been transformed into a new phase of matter — a Bose-Einstein condensate: a collection of atoms that have been super-cooled to within a hair's breadth of absolute zero

When a gas is cooled to near zero temperature, all its atoms lose their energy, entering into the same energy states. As we can only distinguish between the otherwise identical atoms in a gas cloud by looking at their energy levels, this equalizing has a profound effect: The once disparate cloud of vibrating, jiggling, colliding atoms that make up a warmer gas then become, from a quantum mechanical point of view, perfectly identical. 

This opens the door to some truly weird quantum effects. One key rule of quantum behavior, Heisenberg's uncertainty principle, says you cannot know both a particle's position and its momentum with absolute accuracy. Yet, now that the Bose-Einstein condensate atoms are no longer moving, all of their momentum is known. This leads to the atoms' positions becoming so uncertain that the places they could possibly occupy grow to be larger in area than the spaces between the atoms themselves. 

Instead of discrete atoms, then, the overlapping atoms in the fuzzy Bose-Einstein condensate ball act as if they are just one giant particle. This gives some Bose-Einstein condensates the property of superfluidity — allowing their particles to flow without any friction. In fact, if you were to stir a mug of a superfluid Bose-Einstein condensate, it would never stop swirling. 

The researchers used dysprosium-164 (an isotope of dysprosium) because it (alongside its neighbor on the periodic table Holmium) is the most magnetic of any discovered element. This means that when the dysprosium-164 atoms were supercooled, in addition to becoming a superfluid, they also clumped together into droplets, sticking to each other like little bar magnets. 

By "carefully tuning the balance between long-range magnetic interactions and short-range contact interactions between atoms," Norcia said, the team was able to make a long, one dimensional tube of droplets that also contained free-flowing atoms — a 1D supersolid. That was their previous work.

To make the leap from a 1D to a 2D supersolid, the team used a larger trap and dropped the intensity of their optical tweezer beams across two directions. This, alongside keeping enough atoms in the trap to maintain a high enough density, finally allowed them to create a zig-zag structure of droplets, similar to two offset 1D tubes sitting next to each other, a 2D supersolid.

With the task of its creation behind them, the physicists now want to use their 2D supersolid to study all of the properties that emerge from having this extra dimension. For instance, they plan to study vortices that emerge and are trapped between the droplets of the array, especially as these eddies of swirling atoms, at least in theory, can spiral forever.

This also brings researchers one step closer to the bulk, 3D, supersolids envisioned by early proposals like Gross', and the even more alien properties they may have.

The researchers published their findings Aug. 18 in the journal Nature.

Originally published on Live Science.

Ben Turner
Staff Writer

Ben Turner is a U.K. based staff writer at Live Science. He covers physics and astronomy, among other topics like tech and climate change. He graduated from University College London with a degree in particle physics before training as a journalist. When he's not writing, Ben enjoys reading literature, playing the guitar and embarrassing himself with chess.