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Flash Physics: Supernova appears in four places, spins galore in germanium, Chinese rocket docks with space lab

24 Apr 2017 Sarah Tesh

Flash Physics is our daily pick of the latest need-to-know developments from the global physics community selected by Physics World‘s team of editors and reporters

Weak current: spins accumulate in germanium


Supernova appears in four places at once

Astronomers have observed a single supernova appearing at four different locations in the sky. The multiple images are caused by gravitational lensing – a phenomenon first predicted by Albert Einstein as part of his general theory of relativity. It occurs when the gravity of a huge cosmic object bends and magnifies the light of a more distant object. In this case, a galaxy in between the type Ia supernova (called iPTF16geu) and Earth is magnifying the star’s light more than 50 times over and splitting it to create the four separate images. As type Ia supernovae have a well-known intrinsic brightness, the increased signal drew the attention of astronomers when it was seen by the intermediate Palomar Transient Factory (iPTF) – a wide-field sky survey based at Palomar Observatory in the US. Ariel Goobar from Stockholm University in Sweden and colleagues were able to resolve iPTF16geu’s separated images using the Hubble Space Telescope, the W.M. Keck Observatory in Hawaii and the Very Large Telescope in Chile. “Normally, when we view a lensed object we don’t know the intrinsic brightness of that object, but with type Ia supernovae, we do,” says Goobar, “This will allow us to better quantify and understand the phenomenon of gravitational lensing.” Type Ia supernovae, often referred to as “standard candles”, are used to study the expansion rate of the universe. The astronomers hope that iPTF and the network of scientists and telescopes called the Global Relay of Observatories Watching Transients Happen (GROWTH), will help discover similarly lensed type Ia supernovae. The work is presented in Science.

Germanium is a surprisingly good source of spins

Germanium is much better at producing electron spins than had been previously thought – according to Federico Bottegoni and colleagues at the Polytechnic University of Milan in Italy and the University of Grenoble-Alps in France. The discovery could be a boost to researchers trying to develop spintronic devices, which use the spin of the electron to store and process information in much more efficient ways than possible with conventional electronics. The spins are generated by the spin Hall effect (SHE), which involves an electrical current flowing through a material in which the electrons experience an interaction between their spin and orbital angular momentum. Under certain conditions, spin-up electrons will veer off in one direction and spin-down electrons in the opposite direction. The overall effect is a spin current that flows in a direction perpendicular to the electrical current. This leads to an accumulation of spin-up and spin-down electrons at opposite edges of the material. Previous studies of the SHE in germanium revealed very small spin currents, which is why the material had been overlooked for spintronics. Once a spin current is created in germanium, however, it is longer-lived than spin currents in other materials. Bottegoni and colleagues realized that large amounts of spins could accumulate in germanium, despite the relatively weak spin current. When they tested this in the lab, they found that the accumulated spin density in germanium is about 100 times greater than that seen in indium gallium arsenide and on a par with gallium arsenide. Unlike these two compound semiconductors, germanium is compatible with silicon – which makes it more attractive for practical spintronic applications. The research is described in Physical Review Letters.

China takes key step towards permanent space station

China has successfully docked a cargo craft with its Tiangong-2 space lab, taking a major step towards establishing a permanent space station by 2022. On Thursday China launched the Tianzhou-1 cargo resupply spacecraft and two days later it successfully docked with Tiangong-2 in an automated manoeuvre. Tianzhou-1 can carry six tonnes of goods and two tonnes of fuel, although it did not have any actual supplies as there are no astronauts aboard Tiangong-2. The cargo vessel and Tiangong-2 will now have two further docks, including one that will be accelerated to take six hours rather than the usual two days. Tiangong-2 was launched in September 2016 and a month later two Chinese astronauts spent a month aboard the lab in what was the country’s longest-ever manned space mission. It is unlikely that Tiangong-2 will be occupied again and instead China will begin launching the Tiangong-3 station from 2018.


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