In medieval times the seemingly impossible effect of fluids flowing uphill against gravity was considered a miracle, even one of the Seven Wonders of Fore Abbey. In modern times, quantum physics has shown us that certain systems can form superfluids: fluids that have zero viscosity and are able to flow without losing kinetic energy. Superfluidity has been discovered in quantum liquids formed by 4He and 3He (a much rarer isotope of helium) at extremely low temperatures, and is also theorised to describe matter in neutron stars. This property has been exploited by physicists for many years, allowing for a wide range of quantum phenomena to be studied. Recently, ISIS researchers along with colleagues from the University of Lancaster and Oak Ridge National Lab have used neutrons to study this extremely pure quantum system at temperatures close to absolute zero and understand how the addition of a drop of another quantum fluid can disrupt the superfluid state.
The group were interested in understanding how superfluidity manifests and behaves in 4He upon the addition of 3He at very low temperatures – the millikelvin range. X-ray reflectivity has previously been one of the techniques used to investigate thin films of helium, however the use of neutron reflectivity instead allows the creation of much colder temperatures due to the neutrons’ weak interaction with matter. This weak interaction with the sample minimises the energy deposited into it. Neutrons also allowed the researchers to distinguish between 3He and 4He, due to their extremely different absorption cross sections.
For this experiment, the team utilised the PolRef instrument at ISIS, a neutron reflectometry instrument using the Time-of-Flight technique, as PolRef is especially suited to studying surfaces and interfaces. Using a dilution refrigerator to reach and maintain very low temperatures, and a specially designed experimental cell which allowed the helium film to be formed on a silicon substrate, the team were able to create a nano-film with a thickness of only 160Å and probe the delicate interplay between 3He and 4He which occurs within this quantum system.
The question that had originally motivated the researchers was: can the unstoppable uphill flow of superfluid helium be interrupted? Analysing the experimental data produced by PolRef, they were able to resolve structural features and phase transitions as a function of temperature. The researchers reported a phase-separated mixture film at a temperature of 170 millikelvin, and the experiment offered some tantalising hints about some possible as-yet unstudied phase transitions at 300 millikelvin. The behaviour at 300 millikelvin appeared to destroy the superfluid state entirely, the uphill flowing liquid had been tamed.
This study gives researchers a unique window into the quantum world. Not only does it demonstrate the well-known behaviour of a bosonic 4He superfluid, but also highlights how the addition of other quantum matter (in this case the fermionic 3He) can disrupt the delicate states present in superfluids at ultra-low temperatures. It could also contribute to dilution refrigerator development, e.g. of the powerful machines needed for cooling quantum computers whose performance is currently limited by helium film effects. A better understanding of the latter could lead to enhanced cooling power.
The researchers hope this experiment can serve as an example of the power of neutron techniques in the study of quantum systems, whilst also hinting at the unexplored phenomena present in thin helium mixtures films.
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