Quasiparticle camera images superfluid vortices in helium-3

Diagram showing Andreev reflectionsOn reflection Diagram showing how some particles are blocked by superfluid vortices by the process of Andreev reflection. (Courtesy: MT Noble et al/Phys. Rev. B)”>
On reflection Diagram showing how some particles are blocked by superfluid vortices by the process of Andreev reflection. (Courtesy: MT Noble et al/Phys. Rev. B)

Physicists in the UK have created a camera that can image the complex tangles of vortices that form inside a helium-3 superfluid. Developed by Theo Noble and colleagues at Lancaster University, the approach could help researchers to better understand the behaviour of quantum fluids.

When cooled to temperatures just above absolute zero, liquid helium-3 becomes a superfluid, which below a certain critical velocity, can flow without any loss of kinetic energy. The effect arises because at very low temperatures atoms of helium-3 – which are fermions – can form Cooper pairs. These pairs are bosons, which means that helium-3 can become a superfluid.

Physicists are fascinated by the dynamics of superfluid helium-3 at high flow velocities. Here, thermal fluctuations break Cooper pairs to create quasiparticles that propagate through the superfluid. These structures cannot exist within a certain energy range, which can prevent them from entering certain regions of a superfluid. As quasiparticles approach these regions, they will trap a partner to form a Cooper pair, leaving behind a quasiparticle called a hole, which propagates in the opposite direction – a process called “Andreev reflection”.

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Tangled vortices

This process can be triggered by the quantized vortices that form around obstacles to the flow of a superfluid. In liquid helium-3, these vortices can exist as a disorderly tangle of strings just tens of nanometres thick and can shift the forbidden range of quasiparticles in the fluid by a certain amount – which varies with distance from the vortex.

A variety of techniques have been used to probe these structures: including measuring the magnetic fields surrounding helium-3 nuclei and passing sound waves through the fluid. Yet so far, physicists have struggled to image these tangles directly without the use of invasive techniques, such as artificial tracer particles.

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The Lancaster team used a partially closed box within their superfluid to create quasiparticles using a vibrating curved wire. Some of the quasiparticles could move into the rest of the superfluid via a small hole in the box – thus creating a beam of quasiparticles. Upon leaving the box, the beam encounters another vibrating wire that creates a “turbulent tangle” of vortices. Quasiparticles that pass through the tangle are then detected using a 5×5 array of quartz tuning fork resonators.

New discoveries

This allowed the team to produce a series of pixelated images revealing the shadows of vortices, where the quasiparticle beam had been blocked by Andreev reflection. Using this method, the team has already made new discoveries about the properties of superfluid helium-3. For example, they observed many more vortices appeared on the inner edge of the curved wire than its outer edge, despite flow velocities being roughly the same on each side.

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The team intends to study these effects in more detail through further improvements to the set-up: including larger pixel arrays, and higher operation speeds to enable video recordings. If achieved, these improvements could allow researchers to mimic a wide variety of complex, large-scale flow patterns in quantum fluids: including sudden accelerations in the rotations of neutron stars; and the break-up of Cooper pairs by incoming cosmic rays, or even by as-yet undiscovered dark matter particles.

The research is described in Physical Review B.

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