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Chinese-American physicist cleared of spying allegations

The US Department of Justice has announced it will not prosecute Xiaoxing Xi – a former chair of Temple University’s physics department – for sharing information on sensitive technology with Chinese colleagues. The decision comes after prosecutors realized that the blueprints they had used as evidence did not describe the technology at issue.

   Xi is a naturalized American citizen of Chinese ancestry, who specializes in thin-film materials and had been accused of sharing information about a commercial product called a “pocket heater”. The device is made by US-based Superconductor Technologies Inc. (STI), and can be used to manufacture magnesium-diboride superconducting thin films. Xi had previously signed an agreement with STI to keep the design of the device secret.

   

E-mail exchanges

 

Xi was arrested and handcuffed at his home by armed FBI agents in May 2015, on the basis of evidence that included four e-mail exchanges during 2010 with “an associate in China”. According to prosecutors, the exchange discussed means to “advance the field of superconductivy in China” and included drawings of the pocket heater. Responding to the arrest, Temple University relieved Xi of his physics chairmanship, although he was able to remain in the physics department.

   

 I am relieved that the justice system has done its work and the threat of criminal prosecution has been lifted
  Xiaoxing Xi, Temple University  

   

The case fell apart, however, when leading authorities in thin-film materials, including a co-inventor of the pocket heater, Ward Ruby, convinced prosecutors that the drawings did not describe the pocket heater. “It is an extremely technical area. That’s the problem,” says Xi’s lawyer Peter Zeidenberg. “I think what happened is that certain assumptions were made that were incorrect.” The e-mail messages, Zeidenberg continued, “represented the kind of international academic collaboration that governments and universities encourage”. In a statement, Xi says that he is “relieved that the justice system has done its work and the threat of criminal prosecution has been lifted”.

   

Similar cases

 

The collapse of Xi’s prosecution comes six months after government lawyers dropped criminal charges against another naturalized American of Chinese ancestry. They charged National Weather Service hydrologist Sherry Chen, a specialist in modelling water flows in rivers, with unlawfully downloading information on US waterways for a Chinese official. The information turned out to be publicly available.

   Another naturalized Chinese scientist to have come under suspicion was Wen Ho Lee – a physicist at the Los Alamos National Laboratory – who in 1999 was charged with spying for China and spent nine months in solitary confinement before the case was dropped. “We now have another example of apparent discriminatory arrest and discriminatory charging by federal officials,” California Democratic Congressman Ted Lieu commented in response to Xi’s case.

   

‘Ridiculous charges’

 

Xi is now free to resume his research and teaching at Temple University. “As far as we’re concerned, nothing has changed,” Jim Napolitano, who replaced Xi as physics department head, told Physics World. “We set things up so that he could devote all of his energies to fight these ridiculous charges.” Napolitano emphasizes that the physics department and the entire Temple University community have supported Xi since his arrest. Xi has not yet returned to the physics department, but Napolitano expects him to do so soon. “He has a very strong group of graduate students who have kept things going,” he says. “They are getting ready to receive their PhDs in December.”

   Xi describes his research in the video above, which was released earlier this year.

 

Looking back on the H-bomb

Ken Ford is one of the very few surviving witnesses to the birth of the US hydrogen bomb. His book, mixing physics and history with personal memoir, has been received with great interest, partly because of a dispute with the US Department of Energy (DOE) over classification. In summary, the DOE didn’t want you to read parts of Ford’s book, although the author states plainly that it “contains nothing whose dissemination could possibly harm the United States or help some other country seeking to design and build an H-bomb”. Indeed, although his account of bomb design is detailed and intriguing, readers will find it consistent with several others in the public domain.

Ford explains that he was persuaded to join the nuclear weapons research laboratory at Los Alamos in 1950 by the physicists John Wheeler (who was, at the time, his PhD supervisor at Princeton University) and Edward Teller. Wheeler and Teller felt the pressure of Cold War rivalry keenly, and Teller in particular was the H-bomb’s greatest enthusiast. Ford, on the other hand, appears to have been charmingly naive: “At the time politics, like professional sports, were outside the range of my interests. Yet I had a general feeling that it would be a good thing if teams from Boston won their games and if the United States acquired an H-bomb before the Soviet Union.”

But the race for the new weapon – many times greater in destructive power than the existing atomic bomb – was not going well. The key challenge was to use a fission atomic bomb to ignite and sustain a fusion reaction in heavy hydrogen, and since the Second World War, Los Alamos scientists had been pursuing (with more or less enthusiasm) a plan for doing this based on a design known as the “classical Super” or “runaway Super”. But calculations showed more and more conclusively that too much energy would be lost: the classical Super wouldn’t work. Then, in February 1951, a conversation between Teller and fellow scientist Stanis?aw Ulam suggested a new direction. Radiation from the fission bomb, if confined and channelled correctly, could implode the fusion fuel, compressing it sufficiently to sustain a reaction yielding enormous amounts of energy. This new “equilibrium Super” idea led to success.

The relative contributions of Teller and Ulam have been hotly disputed, and Ford gives a clear and sympathetic account of claim and counter-claim. In many popular versions of the story, the driven Teller is the arch-villain. Ford, however, recognizing that Teller and Ulam “were not soul mates”, got on with both men. Perhaps as a result, he is refreshingly even-handed. (Although Teller, surely, deserves most of the credit for America’s H-bomb, if only for the sheer force of will with which he kept the programme alive.)

Ford goes on to describe his contribution to modelling calculations ahead of the test of the “Ivy Mike” thermonuclear device in November 1952. Here and elsewhere, Ford’s account is vivid and appealing, illustrating the informality and also the uncertainty of the work. Personal memories are the book’s greatest strength. In one such anecdote, Ford describes Wheeler writing and drawing on a blackboard simultaneously and ambidextrously. He tells us about job offers made without much thought to budgets or authorizations. He describes falling in love with the New Mexico countryside, hiking and driving at weekends, and watching Teller and Nobel laureate Enrico Fermi “fiercely” playing Parcheesi (the American equivalent of Ludo).

Upon moving back to Princeton in 1951, Ford found his office located in a corrugated metal shack that had previously housed laboratory animals. When the potential of slide-rules and “computers” (junior, generally female laboratory staff employed for number-crunching and drawing up spreadsheets in long-hand) was exhausted, Ford needed time on the cutting-edge IT of the day. The MANIAC computer at Princeton itself was “a prototype Aston Martin … fast, sleek … and unreliable” and thus not good enough for work on the H-bomb. For parts of 1951 and 1952, therefore, Ford put in long night shifts with other electronic computers, first at an IBM office in New York, then at the Bureau of Standards in Washington, DC. Here, having cautiously checked the first calculations made on the SEAC computer by hand, he became a fan: “I gained a true affection for the SEAC…my night-time companion for the many weeks of a long hot summer.” The likely progress of the fusion reaction could be modelled only in one dimension at a time, either radially or axially, and Ford was well pleased when his prediction for the yield of the Ivy Mike device was within 30% of the observed figure.

The Ivy Mike test, Ford writes, “produced in me that odd combination of euphoria and dread that other nuclear weaponeers have no doubt also experienced”. Following the test, he returned to his PhD dissertation and never worked on nuclear weapons again. Many years later, disillusioned by the Vietnam War, he announced his unwillingness to return to any secret work, although in 1968 he found himself back at Los Alamos doing unclassified research. By this time, he had enough children to qualify for housing with a fitted bathtub, and he lived next door to Ulam and his family on so-called “Bathtub Row”.

Ford doesn’t glorify, or apologize for, his work on the H-bomb. He simply tells it as it was. As a result, this is an engagingly human glimpse into the world of physics in the US in the early 1950s.

  • 2015 World Scientific £38/$58hb £16/$24pb 250pp

An insidious and ubiquitous menace

When the US Department of Defense describes something as a “pervasive menace”, one that can tear apart aeroplanes, shut down oil pipelines and render nuclear weapons useless, most people will react by picturing a terrorist organization or hostile foreign power. The reality, though, is both more mundane and far more damaging. That destructive force is corrosion and it is the subject of science journalist Jonathan Waldman’s new book Rust: the Longest War.

The financial cost of corrosion is higher than all the more spectacular natural disasters combined, amounting to $437bn per year in the US alone, yet its slow, undramatic nature allows it to operate beneath our consciousness. Waldman recognizes that, consequently, rust is not necessarily guaranteed to hold the attention of a general audience, and he has attempted to address this problem by producing a book that is more a collection of short stories, linked by a central theme, than a single monolithic text. As a result we are treated to a tour of the field, touching on issues as diverse as the discovery of stainless steel and the maintenance of pipelines, and personalities as varied as an art photographer and a driven scientist.

The science of coatings and surfaces – the primary weapon in the fight against corrosion – is a subtle and often under-appreciated one, and the importance of getting the barrier right emerges in several chapters. Nowhere is this more clearly illustrated than in the chapter discussing the Statue of Liberty. Superficially, New York City’s famous monument appeared to be sound, yet by the 1980s substantial rot had already taken hold underneath. Rather than protecting the statue, layers of coal tar and paint had instead contributed to its decay, and attempts to clean it with bicarbonate of soda were equally disastrous. Indeed, years of superficial inspection had left its structural integrity in such a parlous state that the iconic torch was in danger of collapse, its frame not “seriously weakened”, as originally thought, but entirely absent. It was only through the tenaciousness of the statue’s caretaker – one of the multitude of unsung heroes who appear in the book – that the problems were recognized and ultimately dealt with.

While the Statue of Liberty is an example of the spectacular in terms of corrosion-related stories, Waldman does not neglect the apparently mundane. The humble aluminium can, he notes, is one of the most engineered products in the world. No surprise when we find that the pH of one popular fizzy drink – and not even the most corrosive – comes in at a nicely acidic value of 2.75. Waldman captures the ingenuity and persistence required to combat corrosion in canning, a process that took the best part of 150 years to develop, leading to the production of cans to tolerances tighter than that required for safety-critical aircraft parts.

Despite this success, canning is still a sensitive area, both in terms of the processes necessary to protect the cans and in terms of the possible long-term health impact of some coatings. Waldman’s background in environmental journalism comes to the fore here as he finds himself accepted, rejected and confusedly admitted to “Beverage Can School” (an industry course on drinks can manufacture) before being barred from “Food Can School”. The chapter illustrates the underlying tension that can sometimes arise between the mindset of scientists and engineers, who appear willing to be open about their work, and that of more senior management, who have their eye on profit and public relations and create the impression that there really is something to hide.

The largest area where corrosion specialists are active is, like the canning industry, somewhat taken for granted. Around half of corrosion engineers (the exact proportion is hard to pin down) work in oil and gas or related industries, where failure due to degradation of infrastructure can lead to catastrophic environmental damage. Inspecting an oil or gas pipeline might seem like a banal and simple job; however, it is anything but – particularly as the key is to detect and locate any damage well before failure occurs. Waldman successfully conveys the sheer magnitude of, and care needed, in such an operation, the problems that need to be overcome and the stress on those involved.

The book is squarely targeted at a non-scientific audience, and the fact that, from the beginning, Waldman labels all forms of corrosion “rust” – openly admitting that this inaccuracy will come “to the horror of engineers” – makes that clear. As a result, the book is often unsatisfying, with hints of potentially interesting and readily accessible science just visible beneath the surface. More-over, some of the chapters deviate from Waldman’s main topic to focus almost exclusively on personalities such as art photographer Alyssha Eve Csük (who specializes in photographs of rusting structures) and the Pentagon’s “corrosion ambassador” Dan Dunmire. Waldman’s account of following Csük into a disused steelworks, avoiding security guards and climbing up decrepit staircases and along pipes to get the best picture is certainly interesting, but its relevance to the rest of the book feels tenuous and it might be more at home in the colour supplement of a Sunday newspaper.

The chapters dealing primarily with the characters of individuals whose lives are dominated by corrosion also have an unfortunate tendency to play to stereotypes. It may be true that Harry Brearley (the “father of stainless steel”) was so focused on materials science that he left his wife out of his autobiography, but there is no counterbalancing portrait in Rust of a scientist or engineer as a relatively normal person. (They exist, even among the Physics World readership!) It is also saddening that the only woman given significant attention in the book is an artist rather than a scientist. Although 93% of (US registered) corrosion engineers are men, if authors reinforce stereotypical perceptions, this is not going to change any time soon.

Although the book is written from an American perspective, the author takes time to explain terms for international audiences, which is very much welcome. Less welcome are certain annoying tics in Waldman’s prose. On multiple occasions, his sentences evolve into tedious lists, and it appears, at least to a British reader of a certain age, as if he is channelling the late Clement Freud in an episode of Just a Minute on BBC Radio 4. (For those unfamiliar with the reference, Sir Clement often tried to fill his allotted minute of talking about a given subject “without repetition, hesitation or deviation” by reciting mind-numbingly long lists.) Used sparingly, such a device can convey the sheer range of points at which corrosion intersects with our lives, but the threshold from familiarity to contempt is crossed well before the mid-point of the book is reached. The odd inaccuracy – such as the description of “electrons orbiting…neutrons” rather than nuclei – further frustrates, particularly considering the book’s extremely limited scientific content. A niche for a popular work on corrosion most certainly exists, but we feel that there is just too little science and too much emphasis on personality to make this (nonetheless often absorbing and interesting) whirlwind tour of corrosion the definitive read.

  • 2015 Simon & Schuster £17.79/$26.95hb 304pp

Physicists defend against Trojan-horse quantum hackers

Physicists in the UK have shown how to defend a quantum cryptographic system against Trojan-horse attacks. By targeting the physical hardware used to encode quantum keys, such an attack can bypass the quantum-measurement-based security of the key itself. But the new research shows how three simple optical components can reduce the information obtained in this way to essentially zero.

Quantum key distribution (QKD) involves two parties – a sender, usually known as Alice, and a receiver, called Bob – sharing a secret key to encrypt and decrypt messages. The key is encoded using a string of quantum particles, such as photons, which makes the system immune to attacks by an eavesdropper called Eve. Any attempt by Eve to measure the state of the photons when they are travelling between Alice and Bob will cause a detectable change in those states, thereby revealing Eve’s attempt.

Bright lights

While an ideal QKD system is impenetrable, in practice, Eve can exploit imperfections in the devices used to create, send and receive the key. A Trojan-horse attack involves Eve shining a bright light at either Alice or Bob’s encoder and then measuring the reflection to obtain information about how the photon string has been encoded. In this way, she doesn’t intercept the quantum information itself, and so can remain in the shadows.

In the latest work, Andrew Shields and colleagues at Toshiba’s Cambridge Research Laboratory consider such an attack on a quantum-key distribution system that encodes the key using photons’ phase. Alice uses a phase shifter in one arm of an interferometer to slightly delay a photon pulse travelling through that arm, compared with a pulse travelling through the other, and then sends the resulting pairs of pulses along an optical fibre to Bob, who himself has an interferometer with two output ports. Encryption relies on being able to accurately distinguish a phase shift of 0° from 180° or else 90° from 270°, but not both at the same time.

In this case, Eve’s attack involves shining her light at the phase shifter and then measuring the light that reflects back through the optical fibre. As Shields points out, one way to combat this is to install “active devices” in the system, such as a detector that signals Eve’s presence by registering bright light. However, he says, there is always the chance that Eve could interfere with the detector.

Passive defence

Shields and co-workers instead consider “passive devices”. The idea here is to reduce the intensity of the reflected light to the point where it contains too few photons to convey any useful information – by limiting both the amount of light reaching the phase shifter and the fraction of that light then reflected back. Their scheme involves passing the optical fibre through three kinds of device at the exit of Alice’s transmitter: an attenuator, which reduces each pulse to a single photon; an isolator, which allows only the passage of out-going light; and a filter, which transmits only light with the wavelength of the quantum channel.

To test the efficacy of its scheme, the team calculated whether the addition of the three components is enough to prevent a Trojan-horse attack on existing QKD systems, given that no device in practice ever works perfectly. To do so, the researchers assumed that Eve cannot use light with an intensity greater than that which would burn the fibre, and they also measured the reflectivity of the phase shifter and other components in a real QKD transmitter. “We need to [calculate] the maximum information that Eve can gain from making her measurement,” says Shields. “That is a worst-case scenario. It assumes that she can gain useful information from every photon she measures, but that may be difficult in practice.”

They find that many existing QKD systems operating at very close to their normal bit rate can be protected from Trojan-horse attacks as long as enough isolators are placed in the path of the optical fibre. They say that privacy amplification, which further reduces the amount of information available to Eve simply by shortening the key, only needs to be used when Alice and Bob communicate over significant distances.

Easy to implement

According to Shields, the scheme is easy to put into practice because the necessary attenuators, isolators and filters are all quite cheap and can be bought off the shelf. It will be used in QKD prototypes being developed by Toshiba, he says, but could be incorporated into other existing commercial products.

Vadim Makarov, a “quantum-hacking” expert at the University of Waterloo in Canada, praises the latest research, saying that he is giving the paper describing the work to his students “as an example of how to isolate and treat a security loophole”. But he points out that the Toshiba scheme cannot be used to defend every type of QKD system. Its reliance on isolators, he notes, makes it useless in the case that Alice encodes a beam that has first been sent to her by Bob.

Makarov is also confident that new forms of attack will continue to emerge. “Given that the technology gets updated all of the time to improve performance and offer new functionality,” he says, “its security is not a final state but a continuous process.”

The research is reported in Physical Review X.

  • Two team members (Shields and Zhiliang Yuan) have written a feature article for Physics World about quantum cryptography: “Key to the quantum industry“.

Physics World special report on Mexico is out now

 

By James Dacey

Today is Mexico’s Independence Day, marking the Grito de Dolores – the day in 1810 when the Roman Catholic priest Miguel Hidalgo called on his congregation in the small Mexican town of Dolores to revolt against the Spanish colonial government. This “Cry of Dolores” is seen as the flash point that triggered the Mexican War of Independence.

Modern-day Mexico is still a place with its fair share of turmoil, as the government faces increasing pressure over its inability to deal with drugs, violence and corruption. One area that is starting to look more positive, however, is Mexico’s science base – the administration of president Enrique Peña Nieto has vowed to double Mexico’s investment in science and technology to 1% of GDP and has already sanctioned increases in 2013 and 2014.

To shine a light on what the Mexican physics community is up to, this month sees the publication of a new free-to-read Physics World special report on physics in Mexico. We believe that physicists in Mexico are doing engaging work that deserves to be more widely known. In choosing our coverage for the report, we have not only focused on the challenges for the Mexican community, but also hope to give you a flavour of the rich culture and geography of this most colourful of countries.

(more…)

Are Faraday cages less effective than previously thought?

Is the Faraday cage in your lab less effective than you think? A new study by applied mathematicians at the University of Oxford suggests that the mesh wire cages may not be as good at shielding electromagnetic radiation as previously thought. The team has identified a flaw in the conventional understanding of how the effectiveness of a Faraday cage changes as the wires are moved closer together. Even though engineers might have realized this effect experimentally, this mathematical discovery could improve the design of these cages.

The physicist Michael Faraday came up with the idea of using a conducting wire-mesh cage to block electromagnetic fields in 1836. A familiar modern example of such a cage is the mesh screen on a microwave oven door, which is designed to keep microwaves in while allowing us to watch our food as it cooks. A solid metal cage offers the best protection from electromagnetic waves, but Faraday and many scientists since have assumed that mesh is a good enough approximation of a solid metal cage. Specifically, many had believed that the electric field within the cage rapidly approaches zero as the wires get closer together. In the 1960s, for example, the Nobel laureate Richard Feynman gave a famous series of physics lectures at Caltech that included a short discussion on Faraday cages that has since been adapted in many textbooks and physics lectures. He assumed constant electric charge, and argued that the electric field approaches zero exponentially within the cage.

It takes a lot of time to grow confidence that the intuition [of many people we talked to] is wrong
Nick Trefethen, University of Oxford

Nick Trefethen of the University of Oxford first became interested in Faraday cages in 2013 when he discovered that a thorough mathematical analysis of the cages was lacking in the scientific literature. Trefethen had been working on the trapezoidal quadrature rule – a numerical method for approximating integrals – and he guessed that its equations should also describe the Faraday cage. Because the trapezoidal rule has a property of rapid convergence, he guessed that the Faraday cages would work the same way.

He joined forces with Oxford’s Jon Chapman and David Hewett to study the problem, but it soon became apparent to the team that this rapid convergence to zero electric field does not happen. “We spent months and months fretting about it,” he told physicsworld.com, adding “It takes a lot of time to grow confidence that the intuition [of many people we talked to] is wrong.”

The thick of it

Trefethen and colleagues used a simple 2D numerical model of a Faraday cage that comprised a ring made of individual circular conductors, each of which represents a wire in the mesh. The region outside of the cage contained a source of a static electric field, which applies a constant voltage to the conductors. Their simulations suggested that the strength of the electric field inside the cage is proportional to the logarithm of the radius of the individual wire circles – which corresponds to the thickness of the wires in a Faraday cage. This suggests that Faraday cages made from thick wires are better than those made from thin wires.

They also found that the strength of the internal electric field is proportional to the distance between the wires. Trefethen and colleagues were also able to derive a theorem that gives an upper bound for the internal field strength as a function of separation. This theorem also gives a linear relationship. A key finding of the study is that the drop in field strength is not exponential within a Faraday cage, as suggested by Feynman. Although the simulation was 2D, the team says that the result should also apply to 3D cages.

The team also looked at a continuous model of the mesh cage, which Trefethen says is the main result of the research. These calculations revealed that the Faraday cage does not behave like a normal conductor – rather, it behaves like a surface with a limited capacitance. As a result, it takes energy to push charges onto the wires. This means that a mesh will have a different charge distribution than the ideal Faraday cage, which is a continuous sheet of metal.

Everyday examples

Trefethen points out that there is evidence for the team’s findings in everyday technology. The shielding in microwave oven doors, for example, is usually a solid sheet of metal with holes punched in it – which is essentially a Faraday cage with thick wires. Thinner wires would make it easier to see inside the oven, but are not used by microwave designers – a decision that seems to be in line with the team’s findings. Trefethen also points out that despite the thick wires, microwave ovens are not perfect Faraday cages. That is because if you put your mobile phone – which uses microwaves to communicate – into an oven and call it, there is a good chance that it will ring.

Ali Niknejad is an electrical engineer at the University of California, Berkeley, and acknowledges that there are gaps in our understanding of Faraday cages: “Many of us have done that [phone] test and realized, yes, you know the meshes are not working perfectly.”

Niknejad works with high-frequency electronics including microwave devices, and is interested in shielding computer chips and circuits from interference. He points out that engineers have known for a while that solid metal cages should be used in these applications. He also points out that the Oxford study is an indication that it also happens at low frequencies – specifically in the special case of a static electric field.

Safe emissions

Niknejad also points out that there is no need to worry about emissions from your microwave oven, because engineers will have tested the performance of the mesh cages by making real measurements. Microwave ovens, he adds, have several emission safety requirements.

The research is described in SIAM Review and a preprint of the paper is available from the authors.

Dismaland physics, laboratory photowalks and more

 

By James Dacey and Tushna Commissariat

While it may seem as if we Physics World journalists spend our evenings leafing through Newton’s Principia Mathematica or deriving the Dirac equation from first principles, on Wednesday night this week, a few of us visited Dismaland – the pop-up “bemusement park” curated by the elusive British street artist Banksy. Located in the seaside town of Weston-super-Mare – a few miles south-west of the Physics World Bristol HQ – Dismaland offers a darker and more politically motivated alternative to Mickey Mouse and his friends. While our visit was not work-related, there were a few unexpected physics references that we couldn’t help but spot. First we stumbled across “The Astronauts’ Caravan”, a humorous take on the flight simulators used by NASA (see video above).

Created in 2011 by artists and engineers Tim Hunkin and Andy Plant, the outwardly unimpressive-looking theme-park ride is a compact version of the Victorian “haunted swing” illusion. We won’t spoil the magic by explaining the mechanics of the ride here, but you can read this blog by Hunkin where he explains exactly how he and Plant built their spinning caravan and if you can’t visit Dismaland, then watch the video to see what it looks like from the inside.

(more…)

Solar wind casts a reddish hue over rocky objects

The slow darkening of the surface of rocky objects in space may be caused by the bombardment of particles in the solar wind. This is the conclusion of a new study by researchers in the US, who have simulated the effects of the solar wind by bombarding samples of rock with hydrogen and helium ions.

Physical and chemical weathering has a profound influence on the surface of the Earth, and it turns out that weathering also occurs on the surfaces of bodies in space. Space-weathering processes may have a variety of causes, including meteorite and micrometeorite impacts, cosmic rays, ultraviolet radiation and exposure to the solar wind.

Prolonged space weathering has a curious effect. The surfaces of weathered, airless bodies – like the Moon, Mercury and asteroids – gradually become darker and redder over time. Seen in lunar samples, this colouration appears to be caused by the formation at the surface of tiny particles of iron with grain sizes less than 50 nm. Exactly how these tiny iron particles form, however, has been the subject of much debate. One popular theory is the vaporization and recondensation of solid material caused by micrometeorite impacts. However, it is unclear how such a process could operate on small bodies, which experience lower-impact speeds.

Wind worn

In a new study, Kimberly Kuhlman of the Planetary Science Institute in Arizona and colleagues at the University of Wisconsin-Madison have instead explored whether the iron particles could be created by exposure to the solar wind – charged particles that stream from the Sun at speeds of several hundred kilometres per second. The team studied the effects of such charged particles on enstatite, which is a magnesium-rich orthopyroxene mineral that is common in the solar system. It is often found in meteorites and is abundant in the Earth’s mantle.

Kuhlman and colleagues began with wafers of enstatite – approximately 1 cm in diameter – that were highly polished and mounted on a larger slice of silicon. The samples were then bombarded with energetic hydrogen and helium ions using plasma-source ion implantation. This involves placing the target samples within a vacuum chamber, through which hydrogen and helium gas is allowed to flow. The gases are ionized by tungsten filaments and high-voltage pulses are used to accelerate the ions into the samples at energies comparable to the solar wind.

Following the bombardment, the researchers looked for changes to the composition of the outer 20 nm of the samples using scanning transmission electron microscopy. Sure enough, Kuhlman and colleagues found that the simulated solar wind caused the formation of iron nanoparticles.

Broken bonds

The researchers believe that the nanoparticles form because ion implantation disrupts the atomic lattice of the mineral, creating broken bonds. These are free to react with hydrogen, creating molecules of water and hydroxide. These molecules then escape the damaged lattice, leaving behind iron in a chemically reduced state. The iron then nucleates into nanoparticles.

“This continuing work will allow us to estimate the rate at which these nanophase iron particles form as a consequence of exposure to the solar wind,” says Kuhlman. She says that this information will help scientists to use remote sensing to infer the age of objects in the solar system, which in turn will inform our understanding of a wide range of physical processes that occur in space.

Simone Marchi of the Southwest Research Institute, who was not involved in this work, says that the study suggest that the solar wind is the best candidate for explaining nanophase iron formation in asteroids, outweighing the effects of micrometeorite impacts. Marchi adds, however, that other important aspects, such as the timescale involved in the formation of small iron particles, remain to be investigated in detail.

Cateline Lantz – an astrophysicist at the Observatoire de Paris – told physicsworld.com that the research is important because it has the potential to broaden our understanding of space weathering. “Space weathering is now well understood on the Moon and some asteroid types, but the dark primitive surfaces are still puzzling,” she says.

The research is described in Planetary and Space Science.

Kirigami solar cells follow the Sun

 

The ancient Japanese art of kirigami, or paper cutting, has been used by researchers in the US to improve the efficiency of solar-panel tracking systems. The researchers cut a pattern in thin-film gallium-arsenide solar cells, which causes the cells to tilt when stretched. The system is an improvement on existing solar-tracking equipment, which is bulky, expensive and generally beyond the reach of household solar arrays. The team says that its new design could easily be deployed on individual houses as well as in larger arrays, and, as an added bonus, also improves the optical and mechanical properties of the solar cell.

Flat-panelled solar-cell arrays are most effective when sunlight is directly incident on their surface. Solar trackers are used to orient such arrays, along one or two axes, allowing them to follow the Sun as its position in the sky changes during the course of a day and throughout the year.

Cumbersome trackers

Depending on the geographic location of a solar array, and whether it has one or two tracking axes, a conventional tracker could boost yearly energy generation by 20–40%, compared with a static array. But despite these promising figures, such systems have not been widely implemented because of the high costs, added weight and additional space that they require. Indeed, the additional components required for tracking account for nearly 12% of the total cost of the system, and while this number increases by about 1% annually, the price of actual solar cells is dropping. Also, thanks to the tracker’s size, they cannot be used on the roofs of most homes.

To overcome these problems, Max Shtein and colleagues at the University of Michigan in Ann Arbor used a laser to cut a 2D pattern into gallium-arsenide solar cells. By stretching these patterned cells, the researchers can produce tilted solar-cell arrays in 3D. While the cell panel remains flat, the array elements pop up when stretched.

“All in all, we’re getting about a 30% improvement in the amount of energy harvested across the course of a simulated day, say in Arizona, for a given amount of semiconductor used, compared with stationary panels,” says Shtein. “It basically matches what conventional trackers can do in boosting energy production, but with considerably less bulk.”

Optimal harvest

While the cuts do reduce the area of the array available for sunlight harvesting, it is by a very tiny amount, and Shtein explains that the corners of the cuts are also rounded off to reduce stress in the structure, further reducing the area. By adjusting the strain on the stretched solar cells, the team was able to optimize the cells’ optical and mechanical properties. The researchers found that longer cuts that are spaced closer together made for less pulling effort, and that the degree of tilt is proportional to the amount of pull. From a practical point-of-view, these kirigami-enhanced cells could be placed within a double-pane enclosure to make them more weatherproof, and could be reinforced via tensioned support cables in large arrays to prevent them from sagging.

Although the team’s technique is still in the design phase and further research is needed, it offers a lightweight, scalable and cheap alternative to solar tracking, thereby maximizing the efficiency of such solar cells. Shtein told physicsworld.com that the kirigami approach could be extended to other thin-film or flexible solar cells. “That’s not to say there won’t be integration challenges – plenty of development to do there – but the basic idea should be the same,” he says. According to the researchers, their design opens up new markets for solar tracking, including widespread rooftop, mobile and space-borne installations.

The research is published in Nature Communications.

Physics logos

A recent controversy in the US about whether the Confederate flag should be displayed in public places has ignited a furious discussion about the meaning and purpose of symbols. The flag in question is one variant of several emblems of the Confederate states whose secession from the Union ignited the American Civil War. How is it possible for a logo whose elements are so simple – white stars inside a blue St Andrews Cross against a red background – to have such a variety of meanings?

Logos are everywhere, and a quick Internet image search turns up physics logos of all sorts. But what is their meaning and purpose? What aspects of physics do they promote? What is their value?

Many physics logos are simple and rather predictable, using a dot on a sweeping line to suggest an orb and orbit, either a particle or planet. These include the logos of the Brookhaven National Laboratory and the American Physical Society.

Other logos exploit the particle–planet ambiguity more imaginatively to incorporate aspects of physics or specific fields of research. The logo of physics and astronomy department at McMaster University in Canada, for example, also neatly suggests a specific planet, Saturn, while its yin-yang image invokes the complementarity of physics and astronomy.

Several physics logos suggest explosions or particle collisions, resembling a chandelier of the style that designers and architects call “mid-century modern”. A few logos manage to suggest both the microscopic and macroscopic nature of physics, such as one used by the engineering-physics programme at the University of British Columbia, which shows an orbit around a gear.

The Princeton Plasma Physics Laboratory has a modern, stylized logo. The graphic is ambiguous, and suggests an eye or a vortex of whirling plasma currents. It’s pleasing to look at and represents something relevant to physics.

Still other logos call attention to some specific and recognizable structural element instrument of the organization or the subject. The CERN particle-physics lab’s classic logo, which has a lot of lines but is nevertheless graphically simple, suggests an accelerator and manages to look almost 3D. The logo of the International Year of Light (IYL) communicates what it stands for, but almost too explicitly, with an image of the Sun surrounded by array of flags bearing colours of the spectrum.

The Sigma Pi Sigma (ΣΠΣ) society, which honours “outstanding scholarship in physics”, has an ambitious logo whose meaning is more opaque to outsiders than most – and surely even to many students, for the elements refer to century-old instruments. The overall shape of the logo resembles a voltmeter, which is “a symbol of the accuracy necessary for an experimental science”, according to a ΣΠΣ webpage, while the dynamo powering the light bulb inside “represents the creative energy needed to produce the illumination of knowledge”.

At the other extreme is that of the Institute of Physics, which publishes Physics World. It lacks any physics-related elements, confidently using a simple trio of only three sans-serif bold letters to suggest that the organization is distinguished enough not to need any representations.

The critical point

The logos I came across promote a variety of physics aspects: microworld and macroworld, theory and experiment, research and instrumentation. Still, I expected to find a wider range of elements used. Why aren’t there more pendulums, for instance, given that it’s a physics tool that has been in continuous service for over four centuries? Do so few measuring instruments turn up in physics logos because the simplest to represent – a pan and scale – has been co-opted by justice? Why aren’t there more logos incorporating light?

“Have a design contest” is rule number one of a website called “How not to design a logo”, which makes me reluctant to ask for new designs.

I am, though, interested to discover clever examples that I may have overlooked. Send them to me and say why you think they are interesting from a design and a physics perspective, and I’ll write about the results in a future column.

In fact, is it possible to encapsulate what physics is about in a single logo? Or does physics have so many meanings that this is impossible?

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