Flash Physics: Helium half-quantum vortices, Tokamak Energy gets £10 million, magnetic switch controls heat
Dec 16, 2016 16 comments
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Half-quantum vortices spotted in superfluid helium-3 at long last
The first observation of half-quantum vortices (HQVs) in superfluid helium-3 has been made by physicists at Aalto University in Finland and the P L Kapitza Institute in Russia. A superfluid vortex is a point-like object around which superfluid flows. The flow is quantized in units of h/m, where h is Planck's constant and m is the mass of the constituent particles of the superfluid. HQVs can occur when the constituent particles are bound pairs of more fundamental particles. This is the case for helium-3, which when cooled forms "Cooper pairs" of atoms which behave collectively as a superfluid that will keep flowing without dissipating energy. The Cooper pairs have both spin and orbital angular momentum and interactions between these two quantities can result in HQVs when the superfluid is in a confined environment. This was first predicted in superfluid helium-3 in 1976 and has been seen in several physical systems including high-temperature superconductors, where pairs of electrons form HQVs. Now, Aalto's Samuli Autti and colleagues have confined superfluid helium-3 in a material called nafen, which contains a forest of aligned strands that are separated by about 35 nm. The helium fills the spaces between the strands and the team creates vortices by rotating the sample around the axis defined by the strands. While they could not see the HQVs directly, they used nuclear magnetic resonance to measure the rotation via a signal generated by the magnetic moments of the helium nuclei. "In the future, our discovery will provide access to the cores of half-quantum vortices, hosting isolated Majorana modes – exotic solitary particles," says Autti. "Understanding these modes is essential for the progress of quantum information processing [and] building a quantum computer." The work is described in Physical Review Letters.
Magnetic switch controls heat
A new magnetic switch that controls the flow of heat has been made by Joäo Ventura and colleagues at the University of Porto in Portugal. At its heart is a cylindrical container that is about three quarters full of a magnetic nanofluid – a substance that contains nanometre-sized magnetic particles and flows in the presence of an applied magnetic field. The side of the container is thermally insulated and the top and bottom are capped with material that is a good conductor of heat. In the off position, the nanofluid absorbs heat from the bottom of the cylinder but most of this heat is unable to pass through the gap at the top. To put the switch in the on position, a magnetic field is applied along the axis of the cylinder. This causes the nanofluid to jump up and come into contact with the top of the cylinder – allowing heat to transfer from the nanofluid to the top of the cylinder. The team made two prototype switches – one with a 1 cm-tall cylinder and the other 3 cm – that were tested with a temperature gradient of 35°C. They found that the smaller switch operated efficiently at switching frequencies of up to 30 Hz, while the larger device started to flag at about 10 Hz. The team was also able to control the temperature of an LED using one of their switches. The device is described in Nano Energy and could someday be used to ensure that fuel cells, solar cells and other devices operate at their optimal temperatures.
UK fusion-energy company gets £10m boost
The UK-based company Tokamak Energy will receive £10m in additional investment from Legal & General Capital and the private investor David Harding. Based in Oxfordshire, the firm is building a compact tokamak fusion reactor with the aim of confining and heating a plasma so that nuclear fusion can occur. According to research published by the company, a facility based on their technology should be capable of delivering 100 MW of electricity, which is about one tenth of the output of a conventional fusion reactor. Tokamak says that its next prototype reactor should be built by March 2017 and that it expects to heat plasmas to 100 million degrees by the end of next year. "The world-class facilities in Oxfordshire and 50 years of solid scientific progress with tokamaks have laid the groundwork for a UK fusion industry, and our latest major investment from UK backers demonstrates further recognition of fusion as the most exciting opportunity available to investors anywhere," said David Kingham, chief executive officer at Tokamak Energy.
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Hamish Johnston is editor of physicsworld.com