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There is no speed limit in a superfluid universe. Now we know why



Scientists recently made an unexpected discovery in the cold, dense medium of a helium-3 superfluid. A foreign object moving through the medium can exceed a critical speed limit without breaking the fragile superfluid itself.

Since this contradicts our understanding of superfluidity, it posed quite a puzzle – but now, by recreating and studying the phenomenon, physicists have figured out how it happens. Particles in the superfluid adhere to the object and protect it from interactions with the bulk superfluid, which prevents the superfluid from degrading.

“Superfluid Helium-3 feels like a vacuum to a rod moving through it, even though it is a relatively dense liquid. There is no resistance, none at all,”

; said physicist Samuli Autti of Lancaster University in the UK. “I find that very fascinating.”

Superfluids are a type of liquid that has no viscosity or 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 become so slow that they overlap and form a high-density cluster of atoms that act as a “super atom”.

However, these “superatoms” form only one type of superfluid. Another is based on the boson’s sibling, the fermion. Fermions are particles that contain atomic building blocks such as electrons and quarks.

Standard model of the particle physics map

When cooling below a certain temperature, fermions are bound together in so-called Cooper pairs, each of which consists of two fermions that together form a composite boson. These Cooper pairs behave exactly like bosons and can thus form a superfluid.

The team created their fermionic superfluid from helium-3, a rare helium isotope that lacks a 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 quite fragile, and the Cooper pairs can break apart if an object moves through them above a certain speed known as the Landau critical speed.

However, in a 2016 paper, Lancaster University researchers found that a wire moving through a helium-3 superfluid can exceed that speed without breaking the pairs.

In their follow-up experiments, they measured the force required to move the wire through the superfluid. You measured an extremely small force when the wire started moving, but once it moved the force it took to keep going was zero – just give it a nudge and there you go.

The team concluded that the initial force comes from the Cooper pairs who move a little to accommodate the movement and apply that little starting force to the wire. After that, the wire is free to move, essentially camouflaged in a layer of Cooper pairs.

“By getting the rod to change its direction of movement, we were able to conclude that the rod is hidden from the superfluid by the bound particles that cover it, even if its speed is very high,” said the physicist Ash Jennings from Lancaster University.

This new finding could have some interesting implications.

Fermionic superfluids can be used to create superconductors, which in turn are being investigated as a critical component of quantum computers. Knowing more about how and why superfluids behave the way they do will likely only bring us closer to that goal.

The research was published in Nature communication.


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