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Chirality affects current flow in graphene transistors

The handedness or “chirality” of electrons affects how current flows in graphene transistors, according to new work done by researchers in the UK and Russia. The team’s findings could help to make better graphene-based electronic devices and could even lead to a new technology, dubbed “chiraltronics”.

Graphene is a sheet of carbon atoms just one atom thick, arranged in a honeycomb lattice. The material is unique in that each electron moves along the sheet relativistically, as if it had no mass, with a speed of 1000 km/s. These electrons are also “chiral” in that they are either “right-handed” or “left-handed” – they are mirror images of each other. The electronic states that they can occupy are also chiral.

The UK–Russia team has now studied these electrons in detail by looking at the way current flows in a simple structure made up of a four-atom-thick layer of boron nitride (BN), sandwiched between two layers of graphene. When a voltage is applied, more electrons can be added to one of the graphene layers, so that it becomes negatively charged, and electrons are removed from the other layer so that it becomes positively charged. The BN barrier layer is thin enough so that electrons can pass between the graphene layers by quantum tunnelling, giving rise to an electrical current.

Quantum ‘selection rule’

Team-member Laurence Eaves at the universities of Nottingham and Manchester explains that in the tunnelling process, electrons obey a quantum “selection rule” – right-handed electrons prefer to enter right-handed states while left-handed electrons prefer to enter left-handed states. These processes determine how strong the tunnel current is in these devices. Processes in which a right-handed electron tunnels into a left-handed state (and vice versa) are rare, and do not contribute significantly to the current.

“The chirality or handedness of our tunnelling electrons shows up clearly when we measure how the current flowing through the graphene transistor changes with applied bias voltage,” explains Eaves. “However, we can more precisely study the effect by applying a strong magnetic field perpendicular to the plane of the graphene layer. This field acts to quantize the electron motion, giving rise to a ‘ladder’ of unequally spaced energy levels,” he adds. The high magnetic-field measurements allowed the researchers to demonstrate that the energy, momentum and spin of the electrons are conserved in the tunnelling process, along with their chirality.

“Electronics is a technology that processes information by controlling the free motion of electrons, while spintronics exploits the spin of an electron as well as its charge,” says Eaves. “It will be interesting to see if the chirality of electrons in graphene-based electronics devices could be exploited in the future to develop a new technology – chiraltronics,” he adds.

The research is published in Nature Physics doi:10.1038/nphys3507.

Immersive art, physics pumpkins, personalizing Thor's hammer and more

 

By Matin Durrani

If you’ve ever been to the Perimeter Institute for Theoretical Physics in Canada, you’ll know that blackboards are everywhere. You can find them in handy little alcoves, in the cafe and even in the institute’s lifts – the idea being that brain-box theorists who have a great idea in their heads can crack off the underlying maths before their thought fizzles into the aether. (Not that there is an aether, of course, but you know what I mean.) Anyway, the institute’s new California-based artist-in-residence Alexa Meade, has taken the idea to a new level, creating a huge 3D living chalkboard to create the “perception-bending art for which she is internationally renowned”.  As you can see from the video above, it brings a whole new dimension to the idea of getting “immersed” into science. You can see more images of Meade’s living installation at Perimeter on Flickr.

This week, China’s president, Xi Jinping, is on a state visit to the UK, and today he toured the new National Graphene Institute (NGI) at the University of Manchester. We reported on the planned tour yesterday, with our story including a special behind-the-scenes video that Physics World recorded on our own recent visit to the NGI in the company of its architect and desinger Tony Ling. But an interesting nugget about the Chinese visit has since emerged: it appears that Kostya Novoselov, the Nobel-prize-winning Manchester physicist who helped to isolate graphene for the first time, has presented President Xi “with a gift of traditional Chinese-style artwork, which Kostya himself had painted using graphene paint”. We’ve yet to see what this objet d’art looks like, but I’m sure it’s lovely.

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Quantifying the success of public engagement

Measuring up – how does one evaluate the success of outreach projects such as the dance Constant Speed? (Courtesy: Anthony Crickway)

By Matin Durrani

Here in the Physics World office our attention was caught last week by a story in the Times Higher Education. It reported on a lecture given by Simon Singh at the 2:AM conference in Amsterdam, in which the broadcaster, author and former particle physicist criticized some projects that are designed to boost the public’s interest in science, but which, he feels, are not value for money.

The story mentioned several projects facing Singh’s ire, one of which was the 2005 dance Constant Speed that was created to mark the centenary of Einstein’s annus mirabilis. It was commissioned by the Institute of Physics, which publishes Physics World, so naturally Singh’s comments piqued my interest.

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Chinese president to visit UK’s graphene hub

China’s president, Xi Jinping, will visit the UK’s new National Graphene Institute (NGI) in Manchester tomorrow on the final day of his first state visit to the UK. Xi will be accompanied by the UK chancellor George Osborne on a tour of the new £61m facility, which is at the University of Manchester.

Award-winning building

Recently crowned “major building project of the year” at the annual British Construction Industry Awards, the NGI is designed to bring researchers from academia and industry together to turn research on graphene and other 2D materials into commercial products. The five-storey, 7600 m2 building opened earlier this year and includes two large cleanrooms, optical labs and open spaces for collaboration. (See the video tour above.)

The building was designed by the London-based architects Jestico + Whiles, who worked closely with the Nobel laureate Konstantin Novoselov to meet the needs of the communities using the facility. The Russian-British researcher and his colleague at Manchester Andre Geim shared the 2010 Nobel Prize for Physics for isolating graphene for the first time and their subsequent studies of the material.

Part of the design ethos has been to create a building with a sense of openness. Many of the internal walls are covered in black PVC so that the scientists can freely share their ideas using special chalk-effect pens. Meanwhile, sections of the subterranean cleanrooms are visible from street level so that the public can catch a glimpse of the work taking place within the NGI.

Strengthening national ties

“We welcome the visit of President Xi Jinping to the University of Manchester,” says Nancy Rothwell, president and vice-chancellor of the University of Manchester. “The university has nearly 4000 Chinese students and 150 Chinese staff, and maintains close links with the Chinese business and academic communities. We are looking forward to showing the president some examples of our world-leading research and commercialization of graphene during his visit.”

The key theme of this Chinese presidential visit to the UK is to strengthen economic ties between the two nations, with the UK government claiming the visit will result in £30bn worth of trade and investment deals. This week has seen the state-owned China General Nuclear Power Corporation (CGN) agree to invest £6bn for a 33.5% stake in a new nuclear power station at Hinkley Point on the south-west coast of England. Meanwhile, the UK’s Engineering and Physical Sciences Research Council (EPSRC) will provide £3m for UK–China research projects focused on low-carbon cities, with matching funding from the National Natural Science Foundation of China (NSFC).

Having addressed Westminster politicians and dined with the Queen on Tuesday, Xi visited Imperial College London on Wednesday to see how China-based researchers are working with Imperial in fields such as nanotechnology, environmental engineering and advanced materials. This was followed by visits to a couple of London-based telecommunications firms and an event organized by University College London’s Institute of Education.

“Trade and investment between our two nations is growing and our people-to-people links are strong,” said the UK Prime Minister David Cameron ahead of the state visit. “This visit will be an opportunity to review all of these things but also to talk about how the UK and China can work together on global issues such as climate change and tackling poverty.”

Such enthusiasm for the visit, however, is not shared by all. On his route to Buckingham Palace on Tuesday, Xi was greeted by protestors as well as supporters. One of the groups protesting was Amnesty International, which was highlighting China’s human-rights record and its stance on Tibet. At a press conference on Wednesday, Xi addressed the issue following a question from a BBC journalist. “China attaches great importance to protection of human rights. We combine the universal value of human rights with China’s reality and we have found a path of human-rights development suited to China’s national conditions,” he said. “Looking round the world we can see that there is always room for improvement.”

Web life: Astrobites

So what is the site about?

Five years ago, a group of astronomy PhD students at Harvard University teamed up to solve a problem they’d encountered when, as eager undergraduates just beginning to dip their toes into research, they attempted to read actual scientific papers for the first time. Like thousands of others before them, they found it a daunting experience – the jargon! the pages of citations! the unfamiliar methods! – and they resolved to do something to make it easier for the next generation of students. The result is Astrobites, a blog where current astronomy postgraduate students post undergraduate-friendly summaries of recent papers.

Who is behind it?

The original Harvard group quickly expanded to include PhD students from other universities across the US and Europe, and the site’s current “daily rotation” has 26 members, each of whom has committed to writing one post per month. All told, more than 60 PhD students have written summaries for Astrobites, while a handful of senior academics have posted as guests.

What are some of the topics covered?

The contributors to Astrobites are a diverse group, with research specialisms that range from planetary science to extragalactic observation and theoretical cosmology. That diversity feeds through to their choices about which papers to cover: a typical week might throw up papers about magnetars (neutron stars with super-powerful magnetic fields), newly observed exoplanets, and an estimate of the number of intelligent civilizations in the universe. “We’re not trying to identify the ‘best’ papers – it’s more a question of what will add the most to our site,” explains Nathan Sanders, one of the original Astrobites contributors and now an administrator on the site.

Anything else I should look for?

In addition to the near-daily summaries of recent papers, the site also has a section devoted to explanations of “classic” papers within astrophysics. A few of these classics are now chiefly of historic interest (such as the Astrobite dedicated to Ptolemy’s treatise on his geocentric model of the universe), but many others feature methods and results that remain relevant today. A good example is a 1987 paper from the Astrophysical Journal (315 L77) in which the researchers used the luminosity spectrum of white dwarf stars to estimate the age of the universe. While the number they arrived at – 10.3 ± 2.2 billion years – is lower than the currently accepted value, Astrobites author Josh Fuchs explains, “The debate and process of determining the age of the universe is a good reminder of the workings of science. Multiple independent methods gave different results, which motivated astronomers to keep searching for a believable number.” Also of note is the fact that the Astrobites concept has been adopted by several other scientific fields: there’s an Oceanbites for ocean science and a Particlebites for particle physics, to name just two.

Can I get involved?

If you are a PhD student in astronomy, astrophysics or a related field, then yes, potentially. The site runs a “hiring call” every autumn when would-be contributors are asked to submit a short example post and some information about themselves; the most recent call began on 15 September, so if you’re quick, you might just make it in. They usually have more applicants than they can accept into their regular daily rota of contributors, and Sanders attributes this to the site’s “dual benefit”: reading the summaries is beneficial for undergraduates, but writing them gives graduate students valuable communication experience. “It looks a lot like teaching, which is what some of us are trying to do as lecturers 5–10 years in the future,” he explains. And speaking of teaching, Sanders told Physics World that the Astrobites crew is working on a spin-out site in which astronomy undergraduates write summaries for younger students. Watch this (outer) space.

Spectroscopy technique offers a new way to define temperature

A special type of laser spectroscopy has been used by researchers in Australia to measure the velocities of atoms in caesium vapour. The technique could allow researchers to infer both the temperature of the vapour and the lifetimes and energy separation of the atomic states. It can also be used to measure Boltzmann’s constant, thereby helping to redefine the kelvin relative to universal physical processes.

Today, the temperature of an object in kelvin is defined relative to the triple point of water – the point at which ice, liquid and steam exist in equilibrium. In 2018 the kelvin is to be redefined in terms of the physics underlying temperature, which is “fundamentally a measure of the energy of the atoms and molecules in an object”, says metrology expert Michael de Podesta of the National Physical Laboratory in Teddington, UK. “We’re going to specify a value of Boltzmann’s constant, which is a certain number of joules per degree. That will tell you fundamentally that, if an object has this much energy of motion, then its temperature is this.”

Transition widths

Several groups are attempting to measure Boltzmann’s constant in different systems. The best measurement to date has an uncertainty of less than one part per million. It was made in 2013 by a team led by De Podesta, who used the speed of sound in argon gas to deduce the constant. In the new research, a team of physicists at several Australian universities used a different technique called Doppler-broadening thermometry, which relies on the spectral width of specific atomic transitions.

The researchers focused on two absorption lines in the caesium spectrum, corresponding to the same atomic transition but separated by the hyperfine splitting of the excited states. These lines are broadened by two underlying effects: the intrinsic uncertainty of the state’s energy as defined by Heisenberg’s uncertainty principle – which leads to a Lorentzian distribution – and the fact that, if an atom is moving towards or away from the laser, it sees a Doppler-shifted laser frequency. This latter effect provides a Gaussian variation in the frequencies to which the laser responds. The hotter the sample becomes, the faster the particles are moving, so this Doppler shift becomes more significant and the peaks become broader. By measuring the relationship between peak width and temperature, one can deduce the value of Boltzmann’s constant.

Distinct deviations

The researchers used a gas of ultra-low-density caesium atoms in a vacuum chamber and probed it with a cavity-stabilized microwave laser. They measured the transmission through the cell and recorded the two distinct dips at the positions of the absorption lines. Textbooks have previously modelled the line width as a simple combination of Gaussian and Lorentz distributions, but the precision of the researchers’ measurements revealed small yet significant deviations from this model. Most noticeable, at the level of hundreds of parts per million, were deviations caused by changes in the population statistics of the caesium gas by the laser. “There’s an analogy with speed traps,” explains team member Tom Stace of the University of Queensland. “We’re measuring the speed of the atoms with a Doppler technique. If you look at the traffic just after a speed trap, the cars are all going a little bit slower than they would otherwise have been.” A further, smaller correction arose from unavoidable reflections inside the laser called etalons.

Having corrected for these effects, the researchers extracted and separated the Gaussian component caused by temperature with unprecedented precision, producing an estimate of Boltzmann’s constant consistent with other measurements that has a precision of six parts per million and an uncertainty of 71 parts per million. They are working to improve this further. They also measured the hyperfine splitting of the state with the lowest uncertainty ever recorded. The largest uncertainty in the measurements after the researchers had performed their corrections was the width of the Lorentzian contribution caused by the state’s finite lifetime. The ability to make this measurement could lead to a more precise method for calculating this lifetime. “We can fit [the lifetime width and the thermal width] and derive their values independently from a single measurement,” says Andre Luiten, from the University of Adelaide, who led the work.

De Podesta is impressed, saying the work is important both to fundamental spectroscopy and because “it’s another measurement of Boltzmann’s constant using completely different physics”. “It doesn’t look like the uncertainty will quite be low enough for the measurement to be a significant contributor to the final value, but it adds to the firmness of the foundations,” he says.

The research is described in Nature Communications.

How to win a Nobel prize

By Hamish Johnston

Eureka moment: Takaaki Kajita’s Nobel journey began when he was improving software. (Courtesy: Takaaki Kajita)

This morning I had the pleasure of speaking with Takaaki Kajita, who shared this year’s Nobel Prize for Physics. He won for discovering that some of the muon neutrinos produced by cosmic-ray collisions in the atmosphere change flavour as they travel to Earth. This phenomenon, called neutrino oscillation, tells us that neutrinos have mass – something that was not initially included in the Standard Model of particle physics.

From his office at the University of Tokyo, Kajita told me that the story began in 1986 when he was working on a proton-decay experiment at the Kamioka underground lab in Japan. He was trying to improve some software that was designed to discriminate between electrons and muons created within the detector. He noticed that there were fewer events associated with muon neutrinos than expected. Muon neutrinos are created in the atmosphere when cosmic rays collide with air molecules and a possible explanation for the deficit was that some of the muon neutrinos were oscillating into electron neutrinos on their journey to the detector. Looking back, however, Kajita told me that his initial reaction to the deficit was that he must have made a mistake in his analysis.

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The rise of neutron scattering

ISIS is Europe’s only pulsed source of neutrons and is visited by more than 3000 users from over 30 different countries each year. In 2008 the facility turned on its second target station, which gave the centre space for a further 18 instruments on top of the 20 that are housed in the first target station. Recently, construction was completed of 11 of those 18 instruments, meaning ISIS is just seven short of full capacity.

Many of the new instruments at ISIS will be used by industry – a growth area for neutron-scattering research. Physics World was given a tour of ISIS by Chris Frost, who is industry liaison manager as well as an instrument scientist on ChipIR – a new industry-focused instrument. In this podcast, Frost also outlines how ChipIR can help the aerospace industry mitigate problems when electronic components are hit by neutrons. He also touches on why ISIS will continue to be a major player in neutron scattering even when the €1.8bn European Spallation Source in Lund, Sweden, turns on in 2020.

This podcast was produced in conjunction with a Physics World focus issue on neutron science that was published in October.

Lasers burn holes in quantum security systems

A new way to hack quantum-cryptography systems has been unveiled by physicists in Canada. The method involves using a powerful laser to physically damage the optical equipment used to send and receive secret keys in “quantum key distribution” (QKD) systems. QKD systems are already in commercial use, and this latest disruption comes as quantum-cryptography experts have already modified their systems to make them immune to other eavesdropping techniques.

QKD uses the laws of quantum mechanics to guarantee complete security when two people exchange a cryptographic key. This secret key then allows them to exchange information using conventional communications. The sender and receiver – usually called Alice and Bob, respectively – share a secret key made up of a series of quantum states that an eavesdropper, Eve, is in principle unable to intercept without altering those states and thereby revealing her presence.

Unconditional security?

In practice, however, the security of QKD is impaired by physical limitations of the sources, receivers and other hardware used to implement it. According to Vadim Makarov of the University of Waterloo and colleagues, many scientists assume that as long as the technical shortcomings of this equipment are properly characterized, then QKD can “provide unconditional security”. But the team has shown that even in perfectly understood systems, an eavesdropper can create “loopholes on demand” to steal quantum keys.

Makarov and colleagues have worked out how to create such loopholes in two kinds of QKD system: those using fibre-optic cables and others that send quantum information through free space. The fibre system was based on equipment manufactured by Swiss company ID Quantique, and the team subjected it to a “Trojan horse” attack. This involves Eve shining a bright light at Alice and then measuring the reflected light to try and work out how Alice is encoding a series of photons sent to her by Bob that will constitute the secret key.

Burning bright

Trojan-horse attacks can be prevented if Alice sets up a detector to measure the energy of the incoming photons, which sounds an alarm if the energy is too great. To get around this measure, the team shone an infrared laser at Alice’s photodetector for up to 30 s after disconnecting the fibre channel, which they it during one of the system’s several extended periods of recalibration. The researchers discovered that they could burn a hole in the photodiode detector and render it either partially or completely insensitive to light – the latter requiring at least 1.7 W of laser power. They repeated the experiment using six detectors, and found that in each case “the damage was sufficient to permanently open the system up to the Trojan-horse attack,” although they add that only in half of those trials did QKD continue uninterrupted after reconnecting the fibre between Alice and Bob.

The group also used the same laser to weaken the security of “quantum coin tossing”, which allows two mutually distrustful people to make a decision by metaphorically tossing a coin, even when physically separated. In this case, the impaired sensitivity of the photodetector can increase Bob’s odds of being able to successfully cheat above what is possible with classical technology, so rendering the quantum system redundant.

Faking states

In the case of free-space cryptography, Makarov and colleagues showed that they could enable a “faked-state attack”. Alice and Bob share a key encoded using photon polarization, while Eve inserts a device into the polarized beam that very slightly tilts the beam so that it misses the core of three of the four fibres leading to Bob’s polarization detectors. This allows Eve to control which detectors are used to measure which photons, and by doing so to steal the key unnoticed.

This attack can be prevented by placing a pinhole inside Bob’s receiver – an arrangement that limits the angles over which the incoming beam can enter the device. But Makarov and colleagues were able to enlarge the size of the pinhole by exposing it to a 10 second pulse from a 3.6 W near-infrared laser. Enlarging the hole’s diameter from 25 μm to about 150 μm, the researchers were able to tilt the beam enough to enable any eavesdroppers to steal the secret key.

This work goes further than an experiment reported last year by Makarov and an international group of scientists, because it targets two complete systems and does so without impeding their operation (the earlier research, in contrast, damaged a single isolated component). The Canadian group says that the new results should force scientists to “think again” about how to assess the security of quantum-cryptographic devices, arguing that testing against laser damage and other optical attacks will become “an obligatory part of security assurance for future quantum communications”. It adds that related technologies, such as a type of cloud computing to share the processing power of future quantum computers, might also be vulnerable to laser damage.

Better detectors

Norbert Lütkenhaus of the University of Waterloo, who was not involved in the current work, says this idea of actively damaging QKD components was “not previously on the radar screen” of scientists working on quantum-communication technologies. He believes that countermeasures are possible, suggesting that an additional detector could be installed to register the light from any damaging laser beams. But he points out that manufacturers will need to ensure that their new detectors are themselves resistant to any potential attack, arguing that improving “best engineering practice” is the way to do that. As with other cryptographic technologies, he says, the development of QKD is “always a cat and mouse game”.

A paper describing the research has been uploaded to the arXiv server.

Physicists crack mystery of the spectacular stones of the Giant’s Causeway

The formation of the spectacular hexagonal stone columns at Ireland’s Giant’s Causeway and similar structures around the world can be explained by two new models of how stone fractures. That is the claim of researchers in Germany who have created models to describe how the hexagonal columns emerge from an initial rectangular pattern of cracks in cooling lava. Beyond addressing a question that has intrigued geologists for centuries, the new models may also help in the study of cracking in other materials, such as cooling ceramics.

Located on the north coast of County Antrim, the Giant’s Causeway is renowned for its hexagonal columns of basalt, formed from an extensive lava plateau that was erupted around 55 million years ago. While local legend says the spectacular feature was built by the giant Finn MacCool, geologists know that the interlocking columns are a result of the lava shrinking as it cools – with the surface of the solidifying rock contracting faster than the material beneath. This results in stresses, which are relieved by cracks that spread from the surface downwards.

Why hexagons emerge, however, is not well understood because the cracks first form a rectangular pattern. According to team member Martin Hofmann of the Technische Universität Dresden, the rectangular pattern occurs because the maximum amount of energy is released from the cooling material when cracks develop at 90° to each other.

Y-junctions emerge

As the lava cools, however, the initially rectangular columns gradually transform into more hexagonal shapes, with the T-junctions of the fledgling fracture patterns evolving into Y-junctions over time. This is also seen in laboratory experiments with solidifying starch, which undergoes a similar transition in fracture patterns.

In their new study, Hofmann and colleagues explore how these fracture patterns evolve using two 3D models based on the theory of linear elastic fracture mechanics. This approach describes how crack patterns evolve in a uniform lava layer while ensuring that the optimum amount of energy is released in the process. This new approach, Hofmann says, “sets itself apart by its proximity to the mechanics of the actual process of this pattern shift”. The first of the two models takes a purely analytical approach, whereas the second is based on a 3D finite-element numerical simulation.

Both models trace the development of the joints from the initial cracking to the point at which the cracks extend all the way through the cooling lava body. The models suggest that the transition from T- to Y-junctions maximizes the energy released at each crack face. This, says the team, occurs when the growth of the fracture pattern goes from being dominated by the growth of individual cracks to a collective process of crack development throughout the material.

Completing the picture

“The necessary ingredients for the formation of basalt columns are fitting in place,” says Eduardo Jagla, a researcher at the Centro Atómico Bariloche in Argentina, who was not involved in this study. While the favourable energetics of the switch between T- and Y-junctions was already clear, he says, the numerical demonstration that fracture mechanics does indeed predict this transition helps to complete our understanding of why the hexagonal patterns emerge.

György Hetényi – a geophysicist at the ETH Zürich – agrees, calling the new model a “step forward”. Hetényi cautions, however, that there are other factors beyond pure fracture mechanics – including rock type, crystallization order and geological environment – that also need to be considered when studying column formation.

As well as helping to explain the fracturing process in solidifying lava, the researchers say that their new model could also be applied to the analysis of crack formation on drying lakebeds, as well as to help prevent or limit the cracking of ceramics as they cool.

The research is described in Physical Review Letters.

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