Science

Illuminating spin: magneto-optical Kerr microscopy reveals spin Hall effect Physicists in Switzerland and Sweden say they are the first to use an optical technique to make direct measurements of the spin accumulation associated with the spin Hall effect. When an electrical current passes through a thin strip made of certain materials, spin-up electrons tend to accumulate at one edge of the strip and spin-down electrons at the opposite edge of the strip. Called the spin Hall effect, it is the result of the spin–orbit interaction between the intrinsic spin of the electron and the magnetic field created by its relative motion to the ions that make up the material. This spin accumulation at the edges is usually measured indirectly by placing a magnetic material next to the strip – but this can have detrimental effects on the measurement. Polarization rotation Now, Pietro Gambardella and colleagues at ETH Zürich and Uppsala University have used an established …

New state: are nematicity and superconductivity related? An unusual type of collective motion among electrons has been spotted by physicists in Germany and the US. Called electronic nematicity, the effect is seen when an antiferromagnetic crystal is exposed to high magnetic fields. The researchers say that their work could help disentangle the phenomenon from that of superconductivity and in doing so contribute to the search for new high-temperature superconductors. Ever since high-temperature superconductivity was discovered in 1986, physicists have been searching for materials with ever higher superconducting transition temperatures. Their quest, however, has been hampered by the absence of a unifying theory to describe the phenomenon. In conventional superconductors, the Bardeen–Cooper–Schrieffer theory tells us that electrons moving through a crystal lattice can undergo a collective motion mediated by lattice vibrations. This causes electrons to link up into pairs and form a superconducting state in which current flows without resistance. But that mechanism cannot explain the …

Double vision: the strong lensing system with magnetic galaxy between two images of a more distant quasar The magnetic field of a galaxy five billion light-years from Earth has been detected by astronomers. This is the most distant galaxy known to have a coherent magnetic field and its measurement suggests that the magnetic fields of galaxies such as the Milky Way emerge early in their lifetimes. Galaxies have magnetic fields that are typically a million times weaker than Earth’s magnetic field. While these fields are very difficult to measure, studying the magnetic properties of galaxies could provide important clues about how magnetism in the universe formed and evolved to its present state. Very long time A popular theory suggests that galactic magnetic fields begin as weak and tangled field lines with little coherent structure. Then, over a very long time scale, the magnetic fields organize themselves into coherent large-scale structures. This latest measurement was made …

Ring of light: gravitational lensing of a distant source of blue light A new and much faster way of analysing the gravitational lensing of light has been developed by physicists at Stanford University in the US. The technique is 10 million times faster than conventional methods and uses artificial neural networks. It could make it possible for astronomers to rapidly analyse images of tens of thousands of gravitational lenses that are expected to be discovered by the next generation of telescopes. Gravitational lensing occurs when light from a distant object is bent by the gravitational field of a galaxy or other massive object that the light passes on its journey to Earth. This leads to the distortion of the image of the distant object – often as multiple images of the distant object or a ring of light. Careful analysis of the distortion can reveal important information about the distribution of matter and dark matter in …

Spinning a yarn: twistrons could soon power remote sensors Scientists have been trying for many years to develop new ways of converting mechanical energy from the environment into electrical energy. Now, researchers in the US and South Korea have found that, at very small scales, they can generate record levels of power by twisting yarns made from carbon nanotubes in contact with an electrolyte. They say that their “twistron” devices are well suited to powering remote sensors in industrial and other equipment, as well as potentially being able to tap energy from ocean waves and bodily movement. Mechanical energy is usually converted into electrical energy using generators, devices that exploit the principle of electromagnetic induction to create a current by moving a conductor in a magnetic field. However, while able to approach 100% efficiency at large scales, generators become increasingly hard to build, and as such inefficient, when they are just a few millimetres or …

Spin states: simulation of merging black holes All the gravitational waves so far detected by LIGO have come from mergers of binary black holes, but how such black holes form and pair up is uncertain. Now, researchers in the US and UK have shown that the waves produced when black holes merge can provide significant information about how the pairs formed. The researchers suggest we should have a definitive answer about the origins of black-hole binaries within a few years. Several theoretical models predict how black-hole binary systems could form. In one, both stars in an isolated binary system collapse to form black holes, which are then drawn into a tighter binary black-hole orbit. Binary black holes formed this way would most likely have their intrinsic angular momenta (or spins) aligned along the axis of the binary orbit. “To grow up together and then to shrink their orbits, the stars’ atmospheres and cores have to …

Down periscope: could submarines use quantum encryption? Photon-based qubits and entangled states have been transmitted up to 3 m in sea water by Xian-Min Jin and colleagues at Shanghai Jiao Tong University and the University of Science and Technology of China. While this distance pales in comparison with the 1400 km satellite-to-ground transmission achieved earlier this year by another team in China, the ability to send quantum information through seawater is a significant challenge because the liquid medium is much more absorptive of light than air. Photons make very good qubits (quantum bits of information) because they can travel long distances without interacting with transmission media such as an optical fibre or air. These interactions destroy quantum information and therefore at first glance water should be a poor medium for qubits because it is much more absorptive of light than optical fibres or air. Window of opportunity The team managed to get around this problem by using …

Three’s company: interference patterns for one to four slits (expand for full image) A group of physicists in Austria has directed a beam of large molecules at a series of extremely narrow slits to search for a phenomenon known as “multipath interference”. Although, as expected, they found nothing, they say that the search is well worth the effort given the potential prize on offer – disproving the principle of superposition and with it quantum mechanics as we know it. According to the superposition principle, any two quantum states can always be added together to create a third, valid state. Conversely, it is always possible to split an existing quantum state into two sub-states. The latter characteristic is exploited in many quantum technologies, be it the superposition of a 0 and a 1 in a quantum bit or the splitting and interference of a laser beam to measure tiny variations in position or acceleration. To investigate …

Variable relationship: the location of Mars’ bow-shock changes Mars has a relatively large eccentricity as it orbits the Sun and this has a significant effect on how the solar wind is deflected by the Red Planet, according to an international team of astronomers. Benjamin Hall of Lancaster University in the UK and colleagues have discovered that the distance between the planet’s bow shock and Mars itself varies by 11% and suggest it is linked to the solar extreme ultraviolet (EUV) irradiation. Like a boat When a boat travels across water, its bow (the front) slows and diverts water around the vessel, creating a wave. On a much larger scale, a similar phenomenon occurs as a planet’s magnetosphere diverts highly energetic particles carried in the interplanetary solar wind. The interaction creates a curved wave – dubbed a bow shock – upstream of the planet as it travels through space. Measurements of Mars’ bow shock have been …

Timely discovery: dynamical quantum phase transitions A type of quantum phase transition first predicted in 2013 has been seen by three independent teams of physicists. All three experiments involved systems of interacting ultracold atoms or ions and the observations could lead to a better understanding of the collective behaviour of quantum matter. Phase transitions occur when matter transforms spontaneously from one state to another – from solid to liquid at the melting point of water, for example. Classical phase transitions are associated with thermal fluctuations in a system such as the random motion of water molecules. In contrast, quantum phase transitions involve fluctuations that arise from short-term changes in energy as described by Heisenberg’s uncertainty principle. As a result, quantum phase transitions tend to occur at absolute zero or very low temperatures, where quantum fluctuations dominate over thermal fluctuations. Time passages Phase transitions are usually studied when a system is at or near thermal equilibrium, …

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