Physicists Reveal the Reason for Absence of Speed Limit in Superfluid

At Lancaster University, physicists have determined why objects traveling through superfluid helium-3 do not have a speed limit as part of a previous Lancaster study.

Researchers found the reason for the absence of the speed limit: exotic particles that stick to all surfaces in the superfluid. Image Credit: Lancaster University.

Helium-3 is known to be a rare isotope of helium, which lacks one neutron. At very low temperatures, helium-3 changes into superfluid allowing extraordinary properties like the absence of friction for moving objects.

It was believed that the speed of objects traveling through the superfluid helium-3 was basically restricted to the crucial Landau velocity and that surpassing this speed limit would damage the superfluid.

However, previous experiments performed in Lancaster University have identified that it is not a stringent rule and that objects can travel at relatively higher speeds without damaging the delicate superfluid state.

Exotic Particles and the Lack of Speed Limits

At present, Lancaster University researchers have identified the reason for the lack of the speed limit—exotic particles that adhere to all surfaces in the superfluid.

The finding might guide applications in quantum technology, including quantum computing, in which numerous research teams have already aimed to use these exotic particles.

To bring the bound particles into view, the team cooled down the superfluid helium-3 to less than one ten-thousandth of a degree from absolute zero (that is, 0.0001K or −273.15 °C).

They subsequently moved a wire via the superfluid at an extreme speed and quantified the amount of force that was required to shift the wire. Aside from a very small force associated with shifting the bound particles around when the wire begins to move, the quantified force was zero.

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.

Dr Samuli Autti, Study Lead Author, Lancaster University

According to Ash Jennings, a PhD student, “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.

Dr Dmitry Zmeev, Project Supervisor, Lancaster University

The Lancaster team 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 Zmeev.

The results of the study were published in the Nature Communications journal.

Journal Reference

Autti, S., et al. (2020) Fundamental dissipation due to bound fermions in the zero-temperature limit. Nature Communications.

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