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Flash Physics: Room-temperature superconductor, well-known fundamental constants, photosynthesis in action

24 Nov 2016 Hamish Johnston

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

In the balance: the kilogram can be defined using a watt balance

Superconducting transition spotted well above room temperature

An abrupt transition in the electrical resistance of graphite at 350 K could be a signature of superconductivity occurring well above room temperature (293 K). That is the claim of Pablo Esquinazi and colleagues at the University of Leipzig in Germany and also in Brazil and Australia. The effect was spotted in samples of natural graphite that came from a mine in Brazil. While claims of room-temperature superconductivity in graphite have been made several times over the past 40 years, this is the first time that the transition temperature has been measured, according to Esquinazi. The team found that the transition went away when the graphite was exposed to a magnetic field – something that is indicative of superconductivity. The team believes that individual grains within their samples are tiny superconductors and the spaces between the gaps act as Josephson junctions that allow supercurrents to flow from one grain to another. X-ray diffraction studies suggest that the grains have atomic structures that could support superconductivity, says Esquinazi. The research is described in New Journal of Physics.

Measurements of fundamental constants are good enough to revamp SI units

The fundamental physical constants have been measured with sufficient precision to allow the values to be used to redefine the International System of Units (SI), according to scientists at NIST in Gaithersburg, Maryland. These constants include the speed of light, the Planck and Boltzmann constants and the electrical charge of the electron. Metrologists are in the process of creating a completely new way of defining SI units – such as the metre, kilogram and second – in terms of the fundamental constants. This is unlike the current definition, which relies in part on artefacts such as the standard kilogram that is stored in Paris – and losing mass over time. In the new system, the Planck constant – which is now known to 12 parts in one billion – would be used to define the kilogram. Other planned changes involve using the Boltzmann constant (6 parts in 10 million) to define the kelvin, which is currently defined using the triple point of water. “These now ultra-small uncertainties in the constants will allow the General Conference on Weights and Measures to revise the International System of Units so that the seven base units will be exactly defined in terms of fundamental constants,” says NIST’s Donald Burgess.

X-ray laser reveals key steps in photosynthesis

An X-ray free-electron laser at SLAC in the US has been used to observe two important steps in photosynthesis in which water molecules are split to liberate oxygen atoms. The work was done by an international team of scientists that used X-ray pulses just 40 fs in duration to determine the structure of a protein complex called “photosystem II”, which is involved in water splitting. Unlike previous studies of the process, which were done using frozen samples, the measurement was done at room temperature. The team was able to observe the steps in the four-step cycle by first firing pulses of green laser light at the liquid sample to initiate the splitting. Then, the X-ray pulses are used to measure the structure of photosystem II as the splitting proceeds. The team hopes its measurements will shed light on how water is split by a complex in photosystem II that contains manganese and calcium. “Learning how exactly this water-splitting process works will be a breakthrough in our understanding, and it can help in the development of solar fuels and renewable energy,” explains team member Vittal Yachandra of Berkeley Lab. The research is reported in Nature.

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