Universe

A scientist from Lancaster University accidentally discovered why objects moving through superfluid helium-3 in-universe could exceed a critical speed limit without breaking the fragile superfluid itself.

Helium-3 is a rare isotope of helium, in which one neutron is missing. It becomes superfluid at extremely low temperatures, enabling unusual properties such as a lack of friction for moving objects.

As this contradicts our understanding of superfluidity, it presented quite a puzzle – but now, by recreating and studying the phenomenon, physicists have figured out how it happens.

Particles in the universe superfluid stick to the object, shielding it from interacting with the bulk superfluid, thus preventing the super fluid’s breakdown.

Now scientists from Lancaster University have found the reason for the absence of the speed limit: exotic particles that stick to all surfaces in the superfluid.

The discovery may guide applications in quantum technology, even quantum computing, where multiple research groups already aim to make use of these unusual particles.

Superfluids are a type of fluid that has zero viscosity and zero friction, and therefore flows without losing kinetic energy.

They can be made relatively easily from bosons of the helium-4 isotope, which, when cooled to just above absolute zero, slow down enough to overlap and form a high-density cluster of atoms that act as one ‘super-atom’.

These ‘super-atoms’ form just one type of superfluid, though. Another is based on the boson’s sibling, the fermion. Fermions are particles that include atomic building blocks like electrons and quarks.

Lead author Dr Samuli Autti said: “Superfluid helium-3 feels like vacuum to a rod moving through it, although it is a relatively dense liquid. There is no resistance, none at all. I find this very intriguing.”

PhD student Ash Jennings added: “By making the rod change its direction of motion we were able to conclude that the rod will be hidden from the superfluid by the bound particles covering it, even when its speed is very high.”

“The bound particles initially need to move around to achieve this, and that exerts a tiny force on the rod, but once this is done, the force just completely disappears,” said Dr. Dmitry Zmeev, who supervised the project.

The team created their fermionic superfluid out of helium-3, a rare isotope of helium missing one neutron.

When cooled to one ten-thousandth of a degree above absolute zero (0.0001 Kelvin, or -273.15 degrees Celsius/-459.67 degrees Fahrenheit), helium-3 forms Cooper pairs.

These superfluids are fairly fragile, and the Cooper pairs can break apart if an object moves through it above a certain velocity, called the critical Landau velocity.

And yet, in a 2016 paper, researchers from Lancaster University found that a wire rod moving through a helium-3 superfluid could exceed this velocity without breaking the pairs.

In their follow-up experiments, they measured the force required to move the wire rod through the superfluid.

They measured an extremely small force when the wire started moving, but once it was moving, the force required to keep going was zero – just give it a nudge, and off it goes.

The team concluded that the initial force comes from the Cooper pairs moving around a little to accommodate the motion, exerting that small starting force on the wire rod.

But, after that, the wire can move freely, essentially camouflaged in a coat of Cooper pairs.

The Lancaster researchers included Samuli Autti, Sean Ahlstrom, Richard Haley, Ash Jennings, George Pickett, Malcolm Poole, Roch Schanen, Viktor Tsepelin, Jakub Vonka, Tom Wilcox, Andrew Woods, and Dmitry Zeev. The results are published in Nature Communications.

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