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Global map of tiny ‘third-degree tides’ made using satellite observations

Third-degree tides – tiny sea-level fluctuations once known only locally from measurements made by tidal gauges – have been mapped out across the globe by geophysicist Richard Ray at NASA’s Goddard Space Flight Center in Maryland, US. As well as helping to refine ocean tidal predictions, the work could find application in geodesy and in understanding the behaviour of the Earth’s crust and mantle, which tides affect.

In simple terms, tides occur because the Moon’s gravitational tug causes the oceans to bulge out in two places – one positioned beneath the Moon and the other on the opposite side of the Earth. The gravitational potential can be expressed mathematically in terms of latitude and longitude using spherical harmonic functions.

To model the Earth’s dominant tidal patterns, explains Ray, “it suffices to use just the three spherical harmonic functions of degree two. However, that is only a first approximation.” In reality, the tidal bulges are ever-so-slightly asymmetrical, with one side larger than the other.  “To express that mathematically, we need higher degree spherical harmonic functions. Specifically, here we use functions of degree three,” says Ray.

Swamped in the data

The “third-degree” tides that arise from the tiny asymmetry are very small and their signal is easily swamped in tidal data – both by measurement noise and genuine non-tidal oceanographic signals.

“The tides are fairly obscure, because they are so small, but they can be detected in coastal tide gauges if the time series is long enough, say 10 years or longer,” Ray explains. “Away from these isolated tide gauges, little information about them has existed; no one had ever seen a global map of these waves from direct measurements.”

Today, maps of second-degree tides on the open ocean are made today using satellite altimetry observations. Records have been collected for some three decades, so researchers like Ray can search for third-degree tides. “The nice thing about going fishing for a time-coherent signal – like tides – is that after lots of averaging, even a tiny signal can eventually begin to emerge from the background noise.”

In his study, Ray worked with altimetry data collected by the TOPEX/Poseidon and Jason satellites, which operated from 1992–2005 and 2001–present, respectively. Ray used an averaging process that involved using a least-squares fit to sine waves of known frequency, which revealed the tidal amplitude and phase at each ocean location.

Distinctive pattern

The findings revealed a pattern of third-degree tides that was quite distinct from their second-degree counterparts. Ray says that this is a result of the spherical harmonic forcing being so different in each case. “Depending on the tidal forcing, its frequency, the ocean depth, and the shape of a basin, tidal waves can have certain resonances, some with unusually large amplitudes,” he adds.

The maps revealed that in the Atlantic Ocean, for example – where regular, second-degree diurnal (daily) tides are relatively suppressed – third-degree tides are relatively large. They reached even greater amplitudes in the Indian Ocean. In the South Pacific, meanwhile, third-degree tides were suppressed and barely reached 2 mm in height.

Philip Woodworth, a researcher at the UK’s National Oceanographic Centre in Liverpool says, “The largest third-degree tide, called M1, has been mapped previously around parts of the global coastline and at islands using tide gauges, but this is the first time they have been mapped over the whole ocean”.

Edward Zaron at Oregon State University adds, “It is truly astounding when you consider that the millimetre signals are here being extracted using data from satellites flying above the Earth at about 1300 km elevation, and it is a testimony to the precision of the satellite orbit determinations and geodetic sciences in the current age”. He concludes, “Richard is a master of these analysis techniques and their application to satellite altimetry”.

With his initial study complete, Ray is now looking to refine the accuracy of his tidal maps. “The results are still fairly noisy,” he says. “I’d like to apply formal data assimilation methods, which combine theory from fluid dynamics with measurements, to get the best of both.” He adds that the collection of more altimetry data in the future will also yield better results.

The research is described in Science Advances.

Nanotechnology and materials highlights of 2020

Regular readers of Physics World know that we have a penchant for materials and nanotechnology research. I developed my fondness for materials physics when I did a PhD many years ago on the magnetic properties of ultrathin films and my interest has never waned. So here are five of my favourite materials and nanotechnology stories of 2020.

Snake vision inspires pyroelectric material design

Bioinspiration and biomimicry involve studying how living organisms do something and using that insight to develop new technologies. Pit vipers have two special organs on their heads called loreal pits that allow them to “see” the infrared radiation given off by their warm-blooded prey. Now, Pradeep Sharma and colleagues have worked out that the snakes use cells that act as a soft pyroelectric material to convert infrared radiation into electrical signals that can be processed by their nervous systems. As well as potentially solving a longstanding puzzle in snake biology, the work could also aid the development of thermoelectric transducers based on soft, flexible structures rather than stiff crystals.

Perovskites could be platforms for exciton condensates

Is there anything that perovskites cannot do? This family of crystalline materials is usually associated with high-performance solar cells, but perovskites are true wonder materials that are finding increasingly exotic applications. Now, researchers have shown that certain perovskites could be ideal platforms for creating Bose–Einstein condensates (BECs) of excitons. Excitons are quasiparticles that comprise an electron–hole pair (and we do love a quasiparticle at Physics World). BECs are normally made from atomic gases that must be chilled to near absolute zero. However,  Kai Chang and colleagues reckon that exciton BECs in perovskites could exist at a balmy 77 K.

Rippling graphene harvests thermal energy

There is no shortage of clever applications for graphene – a sheet of carbon just one atom thick – but research really fired my imagination this year. The rippling thermal motion of a tiny piece of graphene has been harnessed by a special circuit that delivers low-voltage electrical energy. The system was created by  Paul Thibado and colleagues, who say that if it could be duplicated enough times on a chip, it could deliver “clean, limitless, low-voltage power for small devices”.

Supercurrent goes to the edge

The topological properties of matter have been a very hot topic over the past few years, so it is no surprise that in 2020 physicists have observed “topological superconductivity” for the first time. Nai Phuan Ong and colleagues have measured a robust supercurrent at the edge of a superconductor that is very different to the supercurrent in the material’s bulk. The team does not yet fully understand the reason for why the edge supercurrent remains independent of bulk supercurrent, but they believe it could come from the topologically protected edge states in the material.

Tipsy sludge worms simulate active polymers

Polymer strands are often described as worm-like, so why not use living worms to gain insights into polymer materials? And if you want to alter the behaviour of the worms, there is no better way than to give them a stiff drink. That is exactly what Antoine Deblais and colleagues did – using worms to gain new insights into the properties of poorly understood “active polymer” materials by measuring the viscosity of clusters of sludge worms as they were subjected to shear forces. The wriggling activity of the worms was controlled by adjusting their temperature, and the creatures were temporarily knocked out using alcohol.

Physics World review of 2020: a year we’re glad to say goodbye to

I remember receiving one of those “hilarious” WhatsApp messages in mid-March just as the coronavirus pandemic was taking hold and everyone was going slightly mad. Signed by “The Management”, it declared that it was no longer in anyone’s best interests to proceed with 2020 and that “after careful consideration”, this year had been cancelled. “While we recognize that a lot of hard work has gone into preparing for 2020, it has turned into a bit of a shitshow and we feel it is best to just call it off.”

I laughed, thinking that 2020 couldn’t possibly end up being that bad. Surely COVID-19 would be quickly forgotten and we’d all be back to “normal” in short measure. In fact, 2020 has been an absolute stinker. It’s ranged from the seriously bad – death, illness, lockdown and economic disruption – to the merely frustrating: social distancing, face masks and copious amounts of hand gel.

As if the pandemic wasn’t bad enough, 2020 saw the loss of several distinguished physicists who had had long and successful careers, including the Nobel laureates Philip Anderson (96), Masatoshi Koshiba (94), Arthur Ashkin (98) and Jack Steinberger (99), as well as the eminent mathematical physicist Freeman Dyson (96). We mourned too the destruction of the iconic Arecibo Observatory in Puerto Rico, which dramatically collapsed following structural damage.

It’s hard to put a positive spin on the year that’s mercifully drawing to a close (and I haven’t even mentioned Brexit yet, though there was some last-minute good news in the form of the withdrawal deal agreed by the EU with the UK on Christmas Eve, which means that Britain will fortunately remain part of the vital Horizon Europe research programme).

Positive spin

Still, I’ll try to look on the bright side. First off, as we’ve reported on the Physics World website in our “Physics in the pandemic” blog series, physicists have shown enormous resilience. Conferences, lectures and events have gone online, opening up such occasions for a more diverse mix of people. Kits for lab classes have been posted to students. Old data have been analysed while plans for future experiments have been drawn up. Hi-tech businesses have innovated with a renewed green focus, while medical and biophysicists have taken centre stage.

The importance of making physics more attractive and welcoming to people of all backgrounds has, rightly, been in the spotlight as never before.

The BlackInPhysics Week logoSecond, the importance of making physics more attractive and welcoming to people of all backgrounds has, rightly, been in the spotlight as never before. It was great to see Andrea Ghez’s work on black holes making her the second woman in three years to win the Nobel Prize for Physics, while October saw the first #BlackInPhysics week followed soon after by BlackInNano week. Here at Physics World we were delighted to welcome a new pool of contributing columnists from across the international physics community, bringing fresh voices to Physics World.

Great physics

We should also celebrate the fact that plenty of amazing physics has continued, as made clear through the variety of exciting research featured in the Physics World top 10 Breakthroughs of the Year, Our picks this year ranged from the first room-temperature superconductor (albeit at high pressure) to a super-sensitive X-ray detector. The overall prize went to physicists in the Netherlands and Germany, who created a silicon-based material with a direct band gap that emits light at wavelengths used for optical telecommunications.

There was also the exciting news of the potential discovery of phosphine – a signature of life – in the clouds of Venus. Finding life elsewhere in the cosmos would be a massive development, although we decided not to include the phosphine story in our Breakthroughs of the Year simply because the evidence doesn’t yet seem cast-iron, with some astronomers suggesting the signal was from sulphur dioxide instead.

Beyond physics, the science story of the year was of course the development of vaccines against COVID-19 and the role that modellers and bioscientists played in understanding the spread of the SARS-COV-2 virus.  The pandemic was not the stage that researchers would have wanted to remind the world of the value of science, but scientists have certainly been in the spotlight as never before. And we shouldn’t forget either the historic launch in May of SpaceX’s Dragon rocketship, which was the first privately built rocket to carry people into space, ferrying two NASA astronauts to the International Space Station.

Finally, closer to home, you may remember the special issue of Physics World magazine back in May, which looked at how to reduce, track and recycle all the plastic waste the world uses and discards each year. After a delay due to COVID-19, it was a pleasure and a relief earlier this month that that we were finally able to switch our print magazine from a plastic to a paper wrapper. Paper’s more expensive than the old “polywrap”, but it’s better on environmental grounds and I was pleased we could make the switch.

So as 2020 draws to its undignified close, I’ll end by saying a big “thank you” to all Physics World readers and advertisers for sticking with us this year. We’ve appreciated all your thoughts and contributions – after all, without the work of physicists around the world, there would be no Physics World.

Let’s just hope for a better 2021!

 

A year of quantum highlights

This pandemic-blighted year isn’t going to top anyone’s list of favourites, but looking on the bright side for a moment, 2020 has seen some remarkable advances in quantum science and technology. Here are a few of the highlights from subfields ranging from quantum fundamentals to quantum computing.

The most precise thermometer possible

How precise can a thermometer be? In January, Jukka Pekkola, Bayan Karimi and colleagues at the University of Aalto, Finland, and Lund University in Sweden found the answer by building a nanoscale device that can detect fundamental fluctuations in the electron temperature of a sample. The noise level in their thermometer is so low that they could detect the energy change due to the emission of a single microwave photon – all without disturbing the system. Being able to spot such tiny temperature changes could enable advances in fundamental physics, and this “quantum calorimeter” might also be used to make non-invasive measurements of quantum systems such as qubits in superconducting quantum computers.

Entanglement in “hot and messy” conditions

“Everybody knows” that quantum entanglement is a delicate phenomenon that only survives in ultracold, ultra-low-noise environments. And usually, “everybody” is correct. But in June, physicists at the ICFO in Barcelona, Spain used a technique called a quantum non-demolition measurement to show that at least 1.52 × 1013 out of the 5.32 × 1013 rubidium atoms in their 450 K sample were, in fact, entangled. The team, led by Morgan Mitchell and Jia Kong, also showed that this entanglement was non-local, meaning that it involved atoms that were not close to each other. As well as challenging assumptions about what entanglement looks like, the finding could be important for sensing technologies such as vapour-phase spin-exchange-relaxation-free (SERF) magnetometers that are based on hot, dense clouds of atoms.

zoom-1-light
The first quantum phase battery. (Courtesy: Andrea Iorio)

The first quantum phase battery 

As quantum circuits become more complex, so, too, do the elements within them. In June, physicists at the NEST-CNR Nanoscience Institute in Pisa and the University of Salerno, Italy demonstrated the first quantum phase battery: a device that provides a persistent phase bias to the wavefunction of a quantum circuit, similar to the way that a conventional battery provides a persistent voltage bias to an electrical circuit. The device that Francesco Giazotto, Elia Strambini, Andrea Iorio and colleagues built out of InAs nanowires and superconducting Al leads was based on a theoretical concept developed only five years ago by physicists in Spain – a speedy turnaround that illustrates just how fast this field is progressing.

Measuring quantum tunnelling time 

How long does a particle take to tunnel through an energy barrier? To the physicists in the first “golden age” of quantum mechanics, who stumbled across tunnelling while playing around with the Schrödinger equation in the mid-1920s, the question would have seemed outlandish. Such is the progress in quantum fundamentals, however, that we now have an answer. In July, physicists led by Aephraim Steinberg of the University of Toronto, Canada, found that ultracold rubidium-87 atoms spent 0.62 ms tunnelling through a barrier 10 000 times wider than their diameter. While Steinberg acknowledges that his team’s definition of tunnelling time is not the only one available, their experiment sheds much-needed light on a phenomenon that remains poorly understood despite lying at the heart of practical technologies such as scanning tunnelling microscopes and flash memories.

Quantum advantage in an optical circuit

In September 2019, quantum computing experts at Google announced that they had used their Sycamore processor to solve a problem more than a billion times faster than a classical supercomputer. Within weeks, competing experts at IBM were pouring cold water over the claim, suggesting that the upgrade was more like a factor of 1000 (still impressive). Late in 2020, the quest for “quantum advantage” hit the headlines again as researchers led by Jian-Wei Pan and Chao-Yang Lu at the University of Science and Technology of China in Hefei announced that they had performed a quantum computation called Gaussian boson sampling 100 trillion times faster than a supercomputer could. Notably, Pan and Lu constructed their quantum circuit using optical elements rather than superconducting ones. The result is a work of art as well as science, with 100 inputs and 100 outputs generated by some 300 beam splitters and 75 mirrors arranged in a random manner.

Gaussian boson sampling
Quantum advantage: the Gaussian boson sampling experiment at the University of Science and Technology of China. (Courtesy: Chao-Yang Lu)

Whether such a system can be scaled up is an open question, but it’s also a question that isn’t unique to optical technologies. In a year that many of us would love to forget (and certainly don’t want to relive), developments like this – like the others on our list – are worth cheering.

Hydrogen sensor is inspired by butterfly wings

Photonic nanostructures found on the wings of some butterflies have inspired researchers in Australia to create a new and highly accurate sensor for measuring hydrogen gas. The device operates at room temperature and was made by a team led by Yilas Sabri and Ahmad Kandjani at RMIT University in Melbourne. The sensor could play a role in the safe industrial storage of hydrogen fuel and the research could also lead to the development of new techniques for non-invasive medical diagnoses.

As a promising source of renewable energy, increasing amounts of hydrogen gas are now being stored at large facilities around the world. Because of the extreme flammability of this gas, there is a need for highly accurate sensors that can detect even the smallest traces of hydrogen that has leaked into the air. Today’s commercially available sensors measure changes in electrical resistance in metal-oxide layers as they interact with hydrogen. However, these devices require temperatures of over 150 °C to operate and are also sensitive to other types of gas – limiting their potential for industrial applications.

Sabri and Kandjani’s team took a more sophisticated approach in their study; where instead of heat, hydrogen detection in their sensors is assisted by light. Their design employs photonic crystals: optical nanostructures that can be manufactured, but also appear in nature. In this case, the team was inspired by the wings of some butterflies – which have orderly patterns of tiny bumps that make the wings extremely good at absorbing light. To mimic this structure, the researchers fabricated a lattice of hollow titanium dioxide nanospheres, which they deposited onto an electronic chip. They then coated the device with a titanium palladium composite to enhance its sensitivity.

Explosion alarm

When activated by light, the surface of this sensor reacts hydrogen gas with oxygen to create water. The presence of water changes the sensor’s electrical resistance, providing a precise measure of the amount of hydrogen in the air. Operating at room temperature, the sensor can measure concentrations in the 10-40,000 parts-per-million range. It can therefore sound the alarm when the concentration of the gas is high enough to be an explosion risk. The device can discriminate between hydrogen and other gases with a selectivity that exceeds 93%.

The sensor was made using established fabrication processes so the team is confident that production could easily be scaled up for widespread using – including in hydrogen fuel cells. Furthermore, the ability of the sensors to detect low levels of hydrogen make them suitable for medical applications. By detecting the gas produced by gastrointestinal disorders in a patients’ breath, clinicians could carry out non-invasive diagnoses and monitoring procedures far more easily.

The research is reported in ACS Sensors.

Midwinter looking bleak? Have some festive cheer with this physics quiz

1. It’s dark at the North Pole in winter. Fortunately, Santa’s reindeer have exceptional eyesight. Which wavelengths of light can they see? A Ultraviolet B Infrared C Microwave D All of the above

2. In 2019, researchers at the University of Manchester calculated that these same reindeer would burn 5.4 x 1013 J of energy per second while pulling Santa’s sleigh on its all-night supersonic Christmas flight. How many carrots would they need to consume to refuel? A 9.1 x 1012 B 1.2 x 1015 C 7.5 x 1018 D 5.8 x 1021

3. Speaking of amazing animal feats, in July this year, a team of researchers calculated that the rectal pressure of a certain snow-dwelling creature can be as much as 28.2 kPa – enabling it to fire off its faeces at nearly 8 km/hr. What is it? A Polar bear B Penguin C Arctic fox D Partridge (in a pear tree)

4. The Jewish festival of Hanukkah (celebrated this year from 10–18 December) commemorates an occasion in the 2nd century BCE when a single jar (1 Greek amphora ~ 38.3 L) of sacred olive oil burned for eight days instead of one. What was the approximate carbon footprint of this miracle? A 117 kg CO2 B 306 kg CO2 C 720 kg CO2 D 948 kg CO2

5. Which famous physicist was (according to the old Julian calendar) born on Christmas Day? A Galileo Galilei B Johannes Kepler C Isaac Newton D Gottfried Wilhelm Leibniz

6. Physicists love to quantify things – six quarks, four fundamental forces, three flavours of neutrino, etc. etc. But how many “hallelujahs” are there in Handel’s Hallelujah Chorus? A 18 B 23 C 54 D 72

7. If a holiday fruitcake were as dense as a white dwarf star, what would be its approximate mass? A 1 x 104 kg B 1 x 106 kg C 1 x 108 kg D 1 x 1010kg

8. If you’re dreaming of a white Christmas, but the (dry bulb) temperature outside is hovering around 2 °C, how low must the humidity fall before snow can be made artificially? A 5% B 25% C 40% D 50%

9. Which scientist was the first person to manufacture an artificial snowflake? A Wilson “Snowflake” Bentley B Nathan Myhrvold C Ukichiro Nakaya D Frank Zamboni

10. And finally, what was the diameter of the largest natural snowflake ever recorded? A 6 cm B 17 cm C 24 cm D 38 cm

Stuck on the questions? We’ll provide the answers in the new year.

 

Update: As promised, here are the answers: 1 A 2 C 3 B 4 C 5 C 6 D 7 B 8 B 9 C 10 D

The best of physics in books, TV and film in 2020

In this episode, Andrew Glester is joined by Physics World journalists to discuss some of 2020’s best physics books, along with their favourite examples of physics featuring in television and film this year. For more information about all of the media discussed, you can revisit these reviews that have appeared in Physics World during 2020.

Magnetic coating gives life to millirobots

It is thrilling to live at a time when robots the size of small insects, known as millirobots, can open up new avenues of research. Such millirobots can be finely tuned to exhibit real-life locomotive behaviour, such as crawling and walking, and find use in biomedical applications.

The millirobots’ existence is thanks to a joint effort between two research groups in China. Their research, published in Science Robotics, took advantage of recent materials developments to employ magnetic fields as a driving mechanism. This approach enabled the team to design a variety of one-, two- and three-dimensional objects that, when coated with a magnetically drivable film, can simply be actuated by a magnetic field. More surprisingly, the novel millirobots can also be disintegrated upon command, using an oscillating magnetic field in an aqueous environment.

Millirobot composition

The two teams of researchers, led by Yajing Shen from the City University of Hong Kong and Xinyu Wu at the Shenzhen Institutes of Advanced Technology at the Chinese Academy of Sciences, created the millirobots by coating the surface of a target object with an adhesive agglutinate magnetic spray (M-spray). Considering the required degree of control over the robots and their scale, the researchers thought that this adhesion strategy could efficiently overcome the millirobots’ deformability constraints, irrespective of the size or shape of the targets.

The M-spray consists of polyvinyl alcohol (PVA), gluten and iron particles. The PVA and gluten provide the self-adhesive ability of the M-spray (referred to as M-skin). The iron particles, meanwhile, provide the magnetic component, responding to the direction and strength of an applied magnetic field that acts as the driving mechanism. The M-spray can create a film that’s thin enough (100 to 250 µm) to not interfere with the target’s original size, structure or morphology.

The researchers tested their devices under different magnetic field strengths (0 to 200 mT), demonstrating that their inanimate designs could be transformed into walking, rolling, crawling and flipping millirobots. What’s more, the researchers can reprogramme the millirobots’ navigation ability on demand. This reprogramming depends upon the direction and strength of the applied magnetic field, together with the distribution and alignment direction of the magnetic particles.

Potential for biomedical application

Since all the components of M-spray, namely PVA, gluten and iron particles, are biocompatible, the researchers sought to test the feasibility of their millirobots for biomedical applications, such as drug delivery. For this, the team performed in vivo experiments in anaesthetized rabbits, using radiology imaging to track the route of a drug encapsulated in an M-spray-coated capsule. When the capsule reached the target site, the researchers disintegrated the M-spray coating by applying an oscillating magnetic field. As its raw materials are biocompatible, the disintegrated coating can be absorbed and excreted by the body with little consequence.

To increase the stability of the M-spray-coated drug-delivery capsule in highly acidic environments, the researchers plan to replace the iron particles with nickel particles in the future. Preliminary findings have shown that this can prolong the stability of the capsule from eight to 30 minutes.

“Our experiment results indicated that different millirobots could be constructed with the M-spray adapting to various environments, surface conditions and obstacles. We hope this construction strategy contributes to the development and application of millirobots in different fields, such as active transportation, moveable sensor and devices, particularly for the tasks in limited space,” says Shen.

Following the first stars

With Christmas looming just around the corner, it will soon be time for one of my favourite festive traditions: considering the physics of Santa Claus. Not a year seems to go by without some enchanting new theory on how Saint Nick manages to pull a fast one on the laws of physics, delivering gifts to all good little children without running out of time or resorting to speeds that would vaporize poor Prancer and Vixen. Naturally, he has an ion shield, operates in 11 dimensions and owns a teleportation device. I confess, however, that I had given little thought to the physics of compiling the naughty list – but was relieved to learn that, as reindeers can see in the ultraviolet end of the spectrum, Santa is probably at least able to spot counterfeit notes and narcotics, which takes care of the most hardened end of child miscreants.

This amusing fact about reindeers’ UV vision crops up as one of many fun asides in physicist Emma Chapman’s effervescent new book First Light: Switching on Stars at the Dawn of Time. The subject of the work is the earliest type of star, rather confusingly dubbed by astronomers as “population III” stars. These giant bodies of helium and hydrogen, consisting of no heavier metals, were several times larger than the Sun and burnt thousands of times brighter. Chapman takes us on a tour of the pursuit of these elusive cosmic antiques, from the basics of star formation and evolution, to what makes population III stars special, all the way to how pioneering astrophysicists are working to locate them today.

Aside from a Magi-like interest in the stars, you may very well be asking: what does the winter holiday season have to do with our universe’s adolescence? You’d be surprised. First Light may be her first book, but Chapman is quite the master of the elaborate structural metaphor, with many of her chapters framed around an overarching anecdote or comparison. Exploring what she dubs the “cosmic dusk” – the ends of the lives of the population III stars – Chapman leads into a discussion of the James Webb Space Telescope by comparing the intricacy of the project to the juggling act of cooking Christmas dinner for one’s extended family (albeit with vastly more stress and more at stake than the Brussels sprouts). Adding to the picture, she deftly compares the James Webb craft’s large, folding mirror to both an origami swan napkin and a Transformers toy one might have found wrapped under a Christmas tree in the 1980s.

The flow is kept firmly tied to the motif by considering the densities of stellar remnants in terms of Christmas turkeys: “The density of a typical white dwarf is about 1000,000,000 kg/m3,” she notes, the equivalent of the unlucky bird “weighing the same as 3000 elephants”. The unexpected presence of supermassive black holes as early as 690 million years after the Big Bang, meanwhile, is like having teenage nieces and nephews turn up for Christmas looking like they have already reached middle age. Whereas the prospect of witnessing the first stellar deaths, or even the earliest stars themselves, would be “like all our Christmases have come at once”. The festive theme is a lovely through-line for the chapter and serves well not only as an explanatory function but also in forging a secondary narrative among what, in the hands of a less-talented writer, could easily have become a stodgy information dump.

An earlier chapter, meanwhile, pulls off the feat of exploring the impact of the first stars on the environment of the early universe by looking at them through the lens of an episode from the geological record: the so-called Great Oxidation Event. Explaining one esoteric scientific concept by first introducing another from a different field would seem like a practice that definitively belongs in the annals of communication no-nos – and yet Chapman makes it work with aplomb. As she explains, this episode, which took place some 2.4–2 billion years ago, saw a revolution in atmospheric make-up as blue-green algae caused the first significant accumulation of free oxygen in the atmosphere. This irreversibly changed the Earth’s environment, causing a mass extinction of existing life, and paving the way for the development of multicellular life. In a similar fashion, the earliest stars overhauled the early universe – adding heavy elements to a mix that had previously only featured hydrogen and helium. Unlike their cyanobacteria analogues, however, population III stars inadvertently brought about their own demise through the changes they wrought.

Emma Chapman’s authenticity and humour shine through

Throughout First Light, Chapman’s authenticity and humour shine through – whether it comes in the form of a darkly funny anecdote about shooting pigeons (which, no matter where they were released, seemed determined to come home to roost in a square horn antenna at New Jersey’s Bell Telephone Laboratory, leaving undesirable “dielectric deposits” on the equipment) or poking fun at elaborate acronyms like WIMPs (weakly interacting massive particles) and MACHOs (massive astrophysical compact halo objects). In fact, my only real criticism of the work is that, while it starts out very well paced, there are a few sections that are a little information-dense, especially in the second half. These seem to cover more material in less space, but even these parts of the work are far more engagingly presented than they might have been at the hands of a lesser writer.

In short, this is a charming book that was as fun to read as it was informative, making it as ideal for the casual reader as for those with an existing understanding of the field.

  • 2020 Bloomsbury Sigma 288pp £15.29hb

NASA scientists design a nanoscale complementary vacuum field emission transistor

What is a VFET?

A vacuum field emission transistor (VFET), also known as nanoscale vacuum channel transistor, is a device with no semiconductor channel. Instead, it has an empty gap between the source and drain terminals. Electrons tunnel through this empty space.

Vacuum diodes and triodes have long been known and used in numerous applications. Recently, researchers have combined the best of vacuum physics and modern integrated circuit manufacturing to produce VFETs on wafer scale with extremely small dimensions (for example, source-drain distance of less than 50 nm). This is smaller than the mean free path in air at atmospheric pressure. Thus, these small devices work under atmospheric pressure without the need for vacuum; nevertheless, for stable and reliable operation over time, moderate vacuum levels such as few hundred millitorr may be desirable. These VFETs work on small drive voltages such as 2 V, which is unheard of in vacuum electronics.

A wide variety of materials including silicon, silicon carbide, gallium nitride, graphene and carbon nanotubes have been considered as emission sources in constructing devices with either horizontal or vertical configurations.

What is a complementary device and why has it not been possible to create a complementary VFET?

In conventional metal oxide semiconductor field effect transistors (MOSFETs), we have n-type and p-type devices –NMOS and PMOS respectively.  This is readily possible since semiconductors can be doped either way. The availability of these two types allows construction of a CMOS with the two devices working as a pair. When connected to a common input voltage, they work in opposite fashion: when one transistor is on, the other is off. This allows the CMOS to operate using less power.

Complementary operation of VFET has not been possible because there is no semiconductor material in the channel for doping and no possibility to create holes to make a p-type device.  VFET is unipolar since it is electron only.

How does your design overcome these challenges?

The primary (or the only) source of carriers in a vacuum device is electrons, resulting from the field emission in the source electrode. In the absence of holes, we need an external mechanism to invoke complementary operation (see figure below). That mechanism here is the nanoelectromechanical (NEM) actuation of the gate that modulates the vacuum channel length and resultantly the electron transport across the source-drain channel with the gate voltage. A shorter vacuum channel length is formed, and a positive input voltage turns on the n-type device and a negative input voltage turns on the p-type device.

The NEM-driven gate modulation is a successful technology employed in NEMS-relay switches and other low power electronics.

VFET
Complementary operation: schematic illustration of how the CVFET is actuated and the electron emission trajectory. (Courtesy: Meyya Meyyappan)

Are you currently fabricating devices based on this design, do you have any preliminary results?

Not yet. We have so far demonstrated the concept using simulations and studying potential circuits. This works well as in CMOS, providing complementary type transfer and output characteristics. Device fabrication is next.  We have provided a possible process flow to fabricate the devices, and the process steps are very similar to those currently in use in silicon integrated-circuit manufacturing. We expect that the device research community would come up with their own tweaks to both the design and process flow and their choice of material systems.

What are some possible practical applications for the device, if it can be successfully fabricated?

First, compared to electron-only conventional operation of VFET, this complementary operation will enable low static power consumption and high noise immunity, both of which are important in applications using logic circuits. Thus, low-power logic circuit application is one possibility.  Also, the VFET is ideal for radiation-immune electronics needed in space and military applications.

Why are VFETs immune to the effects of radiation?

Radiation strike on conventional solid-state devices creates all sorts of defects on the channel semiconductor and oxide materials. Depending on the type of defects and damage created, the effect can slowly accumulate and lead to malfunctions or can result in a catastrophic device failure. Absence of a semiconductor channel or dielectric material in the VFET makes it immune to radiation.

The design is described in ACS Applied Nano Materials.

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