Tuesday, March 25, 2014

Superfluid Helium: Hunt for an 'unidentified electron object'

This image shows vortex rings as the result of vortex multiplication in a quantum fluid. Some electrons are free, some got trapped by one or more vortices. 

Credit: Gleb and Sofia Berloff, ISM

New research sheds light on the nature of 'unidentified electron objects'—a mysterious class of objects that exists in superfluid helium at low temperature.

Researchers have developed a new mathematical framework capable of describing motions in superfluids – low temperature fluids that exhibit classical as well as quantum behaviour.

The framework was used to lift the veil of mystery surrounding strange objects in superfluid helium (detected ten years ago at Brown University).

The study, conducted by an international collaboration of researchers from the UK, Russia and France is published today in the journal Proceedings of National Academy of Sciences (PNAS).

The quantum nature of superfluids manifests itself in the form of quantized vortices, tiny twisters, with the core sizes of the order of an Angstrom (0.1nm – approximately the diameter of an atom) that move through fluid severing and coalescing, forming bundles and tangles.

To make these processes even more intricate and distinct from motions in usual classical fluids, these tiny twisters live on the background consisting of a mixture of viscous and inviscid fluid components that constitute superfluid.

The mathematical modelling of such complex systems that involve a range of scales is a notoriously difficult problem.

Natalia Berloff
The international team of researchers; Natalia Berloff of the University of Cambridge and the Russian Skolkovo Institute of Science and Technology, Marc Brachet of Universit├ę Pierre-et-Marie-Curie and Nick Proukakis of Joint Quantum Centre Durham came up with a novel framework for achieving this task.

The team applied their method to elucidate an intriguing phenomenon in liquid helium research.

Electrons immersed in superfluid helium are useful experimental probes. As they move through superfluid they form soft bubbles of about 2 nm in diameter that get trapped by quantized vortices quite similar to how houses and cars become trapped and transported by a tornado.

A research team from Brown University led by Professor Humphrey Maris has studied the effect of oscillating pressures on electron bubbles.

As pressure decreases below the criticality, the bubble expands and explodes, reaching micron sizes, with the bubble trapped by a vortex exploding at a pressure larger than that for the free bubble.

Maris' team also discovered another class of object that existed at very low temperatures only and exploded at even larger pressures. They termed these "unidentified electron objects".

More information: The paper 'Modeling quantum fluid dynamics at nonzero temperatures' was published today, 24 March 2014, in the journal Proceedings of the National Academy of Sciences (PNAS). www.pnas.org/cgi/doi/10.1073/pnas.1312549111

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