Researchers from Lancaster University recently made an unexpected discovery that a foreign object travelling through a superfluid medium could exceed its critical speed limit without breaking the fragile superfluid. This contradicts our understanding of superfluidity.
Now, physicists have figured out how this happens by recreating and studying the phenomenon. When the foreign object moves in the superfluid, particles in the superfluid stick to the object, shielding the object from interacting with the rest of the superfluid, thus preventing the superfluid’s breakdown. Their research is published in Nature Communications.
Superfluids have zero viscosity and zero friction, and therefore flows without losing kinetic energy. They can be made relatively easy by cooling bosons of helium-4 isotope to temperatures just above absolute zero. At these temperatures, the bosons slow down enough to overlap and form a high-density atom cluster that acts as one ‘super-atom.’
These ‘super-atoms’ form just one type of superfluid. Another is based on the fermion. Fermions are particles similar to bosons but with a half integer-spin. Some fermions are elementary particles such as electrons, and some composites like protons.
When fermions are cooled below a certain temperature, they become bound together into Cooper pairs. Each pair comprises two fermions forming a composite boson, and they behave exactly like bosons. Thus a superfluid can be formed with Cooper pairs.
The research team created a fermionic superfluid out of helium-3. Helium-3 is a light, stable, and rare isotope of helium with two protons and one neutron. Helium-3 forms Cooper pairs when cooled to one ten-thousandth of a degree above absolute zero (0.0001 Kelvin, or -273.15 degrees Celsius/-459.67 degrees Fahrenheit).
In a 2016 paper, researchers from Lancaster University found that a wire rod moving through this fragile helium-3 superfluid could exceed the landau velocity without breaking the Cooper pairs. Landau velocity is the critical velocity above which an object moving through a superfluid would break the Cooper pairs.
According to physicist Samuli Autti of Lancaster University in the UK, “Superfluid helium-3 feels like a 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.”
An extremely small force was required to move the wire rod through the superfluid, but once it starts moving, the force needed to keep the motion was zero. The team concluded that the initial resistance comes from the movement of Cooper pairs to accommodate the wire’s motion. After that, the wire moves freely camouflaged in a coat of Cooper pairs.
According to physicist Ash Jennings of Lancaster University, by making the rod change its direction of motion, they concluded that the rod would be hidden from the superfluid by the bound particles covering it, even when its speed is very high.
This new finding could have some interesting applications in the field of quantum computing. Superconductors are critical components of quantum computers; fermionic superfluids can be used to create one.
Knowledge about the superfluid’s behaviour will bring us one step closer to that goal.
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