WCA March 2020

From the Americas

Component communication An international team of engineers based at Penn State University believes that quasi-particles known as sur- face-plasmon-polariton (SPP) waves, that travel along the interface between a metal and a dielectric material, may bring a solution to the problems caused by miniaturisation of components. Akhlesh Lakhtakia, professor of engineering science and mechanics at Penn State, said, “Microelectronic chips are ubiquitous. Delay time for signal propagation in metal-wire interconnects, electrical loss in metals leading to temperature rise, and cross-talk between neighbouring interconnects arising from miniaturisation and densification, limits the speed of these chips.” Researchers have previously explored ways to solve the problem of connecting various miniaturised components in a “world of shrinking circuits”. While photonics is attractive because of its speed, this approach is problematic because the waveguides for light are bigger than current microelectronic circuits, which makes forming connections between them difficult. In a paper in Scientific Reports , the research team has reported that “signals can possibly be transferred by SPP waves over several tens of micrometres [of air] in microelectronic chips,” and that the “signal can travel long distances without significant loss of fidelity.” They also note that their calculations indicate that SPP waves can transfer information around a concave corner – a common situation in microcircuitry. SPPs are a group phenomenon. The quasi-particles travel along the interface between a conducting metal and a dielectric and appear as a wave on a macroscopic level. Mr Lakhtakia explained that SPPs are what give gold its characteristic shine; under certain conditions, electrons in the metal and polarised charges in the dielectric material can act together to form an SPP wave. Guided by the interface between the two materials, this wave can continue propagating even if a metal wire suffers a break, or if the metal-dielectric interface terminates abruptly. The SPP waves can travel in air for a few tens of micrometres, or the equivalent of 600 14nm-wide transistors laid end-to-end, but they can only travel when in close proximity to the interface between the materials, so they do not produce crosstalk. However, for the time being the theoretical underpinnings of the phenomenon are less defined. Mr Lakhtakia is optimistic for the research: “We are studying problems that were unsolvable ten years ago.” Cleaner transport Lincoln hops on board the skateboard An all-wheel-drive electric Lincoln SUV, due to launch in 2022, will be the first Ford Motor Co vehicle built on a custom electrified chassis that resembles a skateboard. The chassis was developed by the Ford-backed start-up Rivian.

Research More superconductor surprises

BigStockPhoto.com Photographer: Aispl

A new paper in the publication Science shows that Cooper pairs (the electron pairs that allow superconductors to conduct electricity without resistance) are also capable of conducting electricity with resistance. The research team believes the findings describe an entirely new state of matter that will require a new theoretical explanation. “There had been evidence that this metallic state would arise in thin-film superconductors as they were cooled down toward their superconducting temperature, but whether or not that state involved Cooper pairs was an open question,” said Jim Valles, a professor of physics at Brown University and the paper’s corresponding author. Added Mr Valles, “We’ve developed a technique that enables us to test that question and we showed that, indeed, Cooper pairs are responsible for transporting charge in this metallic state. What’s interesting is that no one is quite sure, at a fundamental level, how they do that, so this finding will require some more theoretical and experimental work to understand exactly what’s happening.” Mr Valles, working with Brown engineering and physics professor Jimmy Xu, has previously shown that Cooper pairs can produce insulating states as well as superconductivity. In very thin materials, instead of coordinating their movements with other sets of Cooper pairs in a way that reduces electrical resistance, the pairs stay in place, “stranded on tiny islands within a material and unable to jump to the next island.” For the latest study, Mr Valles, Mr Xu and colleagues in China looked for Cooper pairs in a non-superconducting metallic state using a technique similar to the one that revealed Cooper pair insulators. This technique involves patterning a thin-film superconductor – in this case, the high-temperature superconductor yttrium barium copper oxide (YBCO) – with arrays of tiny holes. When the material has a current running through it and is exposed to a magnetic field, charge carriers in the material will orbit the holes like water circling a drain. “We can measure the frequency at which these charges circle,” Mr Valles said. “In this case, we found that the frequency is consistent with there being two electrons going around at a time instead of just one. So we can conclude that the charge carriers in this state are Cooper pairs, and not single electrons.” That this phenomenon was detected in a high-temperature superconductor makes future research, employing spectroscopy and other techniques, more practical. YBCO starts superconducting at around -181°C, and the metallic phase starts at temperatures just above that – much warmer than other superconductors at -273°C. In time, researchers believe, it might be possible to harness this metal state for the development of new types of electronic devices. “Science is built on discoveries,” Mr Xu said, “and it’s great to have discovered something completely new.”

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