A team of researchers has managed for the first time to observe a fundamental characteristic in a supersolid, providing direct proof of the existence of this extreme state of matter which combines rigidity and fluidity.
In our daily lives, matter can exist in four classic states: solid, liquid, gas and plasma.
But scientists have long been interested in so-called “exotic” states of matter, which form at temperatures close to absolute zero (-273.15 degrees Celsius), at very high energy levels or at levels of gravity and extreme density as in black holes.
Under these conditions, matter presents physical properties or behaviors very different from those observed in classical states.
Thus, normal fluids (liquids and gases) have a greater or lesser resistance to flow, called viscosity – for example, oil is more viscous than water.
Superfluids have no viscosity: they flow without loss of energy, which allows them to circulate indefinitely in a container without slowing down.
More than 50 years ago, physicists predicted the existence of a supersolid state. In it, the matter exhibits both the properties of a classical solid, with a crystalline structure, and of a superfluid, where a fraction of the atoms flows without viscosity through the solid network.
– Neutron stars –
The crystal structure of these supersolids had already been imaged, but until now their superfluidity had only been inferred from various observations.
“Our work was still missing a direct observation of one of the characteristic and fundamental properties of superfluidity: flow without rotation,” underlines Francesca Ferlaino, who led the research published Wednesday in Nature.
“Imagine that you have a cup of coffee and you give it a little spin with a spoon. You will see the coffee rotating around the center, a classic example of a vortex in an ordinary fluid,” the physicist from the University of Innsbruck (Austria).
If we replace the coffee with a superfluid, it does not spin with the spoon, it remains perfectly still as if nothing had disturbed it.
“However, if you spin the spoon faster, instead of forming a large swirl in the center, a series of smaller swirls (called quantized vortices) begin to appear. These are like small holes in the fluid, each spinning at a speed specific, which are organized into beautiful regular patterns on the surface of the superfluid, almost like the holes in a piece of Swiss cheese,” continues Ms. Ferlaino.
His team managed to create and observe these quantified vortices in the laboratory. A particularly difficult feat to achieve.
In 2021, the Innsbruck team had already succeeded in creating a long-lived supersolid, by cooling certain atoms and molecules to very low temperatures.
We then had to find a way to agitate this supersolid without destroying its fragile state. The researchers used magnetic fields to carefully spin it. Which resulted in the formation of quantized vortices.
This work provides “strong and direct proof of the dual nature of a supersolid state,” underlines Ms. Ferlaino.
They will also make it possible to observe physical phenomena in the laboratory that only occur in nature under extreme conditions.
Like recreating what happens in the heart of neutron stars, these extremely dense and compact stars born from the collapse of massive stars.
“It is assumed that rotational speed variations in neutron stars – called glitches – are caused by superfluids trapped inside. Our platform offers the opportunity to simulate such phenomena here on Earth,” explains the researcher. Thomas Bland, who participated in the study, in a press release.
