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The Lancet highlights role of physics in medicine

By Tami Freeman, editor of medicalphysicsweb

Since the birth of medicine 5000 years ago, physics has played a fundamental role in the development of health technologies. Significant contributions to today’s medical methods range from the application of numerous imaging techniques to diagnosis and patient screening, to the wide variety of treatment techniques made possible by the discovery of radiation and radioactivity.

For their part, medical physicists have a particularly important role to play, both in the discovery of new diagnostic and treatment techniques, and in ensuring the safe and effective implementation of new physics-based medical applications.

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In recognition of these facts, the medical journal The Lancet has just published its Physics and Medicine Series, a set of five articles and two comments that highlight the many ways in which physics has revolutionized medical practice. The series, published to coincide with the anniversary of Albert Einstein’s death, calls for medical physics to be promoted as a career choice and recognized as a vocational discipline.

Writing in one of the associated comment articles, Peter Knight (right), president of the Institute of Physics – which publishes physicsworld.com – discusses the “long and happy marriage between physics and medicine”, and puts forward two proposals to keep this relationship thriving in the future.

First is the continued need to support and invest in the physical sciences. Knight notes that most, if not all, of the physics-based techniques and technologies described in the series derive from the discoveries of basic physics research that was undertaken purely to investigate the nature of our world and expand the frontiers of knowledge.

For example, the understanding and manipulation of radiation was made possible by basic research into the structure and evolution of the universe and the building blocks of matter. Knight urges the UK government and other funders to recognize that continued support for that research will deliver corresponding advances in medical technologies in the years to come.

Second, for medical practitioners to fully exploit modern physics-based technologies, it would be hugely beneficial for them to have a sound understanding of the physics involved. As such, Knight suggests that medical schools should consider restoring the requirement for applicants to hold physics-oriented qualifications for entry into medicine.

Another recommendation arising from the series is for closer collaboration and integration between the physical and life sciences, via a new model in which multidisciplinary teams work closely in a shared research environment. Finally, there is a need for every school to aspire to provide high-quality physics education, to ensure a supply of talented scientists who can perform health-related physics research in the future.

“The Lancet‘s Physics and Medicine Series clearly shows the potential to diagnose and treat increasing numbers of patients, with increasing effectiveness, using physics-based techniques,” Knight concluded. “Understanding the physics that underpins these techniques would be a real advantage to medical practitioners, and to their patients.”

‘Magnetic Josephson effect’ seen for the first time

A fundamental prediction of superconductivity theory has been demonstrated in the lab for the first time. An international team of physicists has observed coherent quantum phase slip, a phenomenon similar to the well-known Josephson effect in which magnetic flux takes the place of electric charge. Its discovery has fundamental implications for our understanding of macroscopic quantum systems and could also lead to intriguing applications, including a possible way to produce a qubit in a quantum computer.

In 1962 the British physicist Brian Josephson developed a theory of how superconducting electrons tunnel across a thin insulating layer between two superconductors – a structure now called a Josephson junction. This was quickly verified in the lab and Josephson was awarded the 1973 Nobel Prize for Physics. The Josephson junction has become an important technology in its own right. For example, superconducting quantum interference devices (SQUIDs) that, depending on their design, use either one or two Josephson junctions are among the most sensitive magnetometers to have been invented. The devices have also shown promise as possible quantum bits (qubits) in quantum computers.

Phase slips through a superconductor

In 2006 Hans Mooji and Yuli Nazarov at Delft University in the Netherlands did theoretical work on the quantum tunnelling of magnetic flux between two areas of free space through a thin layer of superconductor. This effect is called coherent quantum phase slip, and Mooji and Nazarov argued that it is an exact analogue of the Josephson effect. This is because, while free space shows no resistance to the flow of magnetic flux, one of the fundamental properties of a superconductor is the Meissner effect, whereby it expels any magnetic field from its interior. It therefore behaves as the magnetic equivalent of an insulator. However, in the subsequent six years, no-one successfully showed whether or not coherent quantum phase slip across a superconductor could actually occur.

Now, Oleg Astafiev and colleagues at the NEC Green Innovation Research Laboratories and the Institute for Physical and Chemical Research in Ibaraki, Japan, are claiming the first experimental observations of coherent quantum phase slip.

The experiment was done on a quantum-mechanical circuit called a Mooji–Harmans qubit – a ring of superconductor that narrows at one point into a very thin nanowire. If coherent quantum phase slip did not occur, magnetic flux inside the ring would be unable to get out, and magnetic flux outside would be unable to get in because of the impermeability of a superconductor to magnetic flux. However, Astafiev’s group observed clear evidence of magnetic interaction between the inside and the outside of the ring while the ring remained in the superconducting state – clear evidence that flux was crossing the nanowire by quantum tunnelling.

‘Two significant features’

Alexey Bezryadin, at the University of Illinois at Urbana-Champaign, believes that the work marks a significant achievement, both in terms of its progress in fundamental physics and its potential for application. “I would say there are two significant features to this work,” he says. “One is that the observation of these coherent quantum phase slips extends the applicability of quantum mechanics to more complex macroscopic systems. The applied aspect is that there are predictions that, if coherent quantum phase slips can exist (and this paper demonstrates that they do), you can use that to build certain useful devices.”

Astafiev agrees: “The phenomenon that we demonstrated is fundamental. As fundamental, I would imagine, as the Josephson effect. Josephson physics has proved very rich and there are many very useful devices based on the Josephson effect.” He believes it should be possible to exploit coherent quantum phase slip to build devices analogous to those based on the Josephson junction. In particular, Astafiev, who has a specific interest in quantum computing, hopes that qubits based on coherent quantum phase slip may not be prone to “charge noise” – a type of noise caused by the presence of an insulator that tends to cause quantum decoherence in Josephson qubits.

The research is published in Nature.

Explore a visual history of science

By James Dacey

From Darwin and his tree of life to Mendeleev and his conception of a periodic table of elements, images and visual metaphors have played a vital role throughout the history of science. Today, the Royal Society has launched a new online picture library to allow people to browse and search its vast collection of images online for the first time. The collection includes paintings, drawings and prints dating back to when the society was founded in the mid-17th century. Here is a selection of the images connected with physics and physicists.

Orrery demonstrating the transit of Venus

This is a mechanical device known as an orrery, designed to show the relative positions and motions of the planets and moons in the solar system. The British instrument maker Benjamin Cole (1695–1766) made this orrery, which is of particular interest this year because it depicts a transit of Venus – a phenomenon that will occur in June for the last time for more than 100 years.

Eyes and head of a grey drone-fly

Flies can be a bit of a nuisance when buzzing around your head, but when viewed under a microscope, these insects are nothing short of hideous. This sketch of the eyes and head of a grey drone-fly was produced by the natural philosopher Robert Hooke and it appeared in the Royal Society’s 1665 publication Micrographia.

The waterwheel and conduit for Scrooby Mill

The Royal Society’s collection also contains a number of engineering and architectural plans, including this design for the waterwheel and conduit for Scrooby Mill in the English county of Nottinghamshire. It was sketched in 1782 by the British civil engineer John Smeaton.

Engraving of a flying fish

Finally, we have this image of a flying fish, which, according to the Royal Society, holds an unlikely place in the history of modern physics. It is part of a set of engravings from a 1686 book Historia Piscium (a History of Fishes) by John Ray and Francis Willughby, for which the Royal Society held high hopes. But after the society had invested all of its available funds, the book went on to be a flop, meaning there was no money left to publish Newton’s Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), which had been “knocking around the office”.

The story goes that a young Edmund Halley – then clerk at the Royal Society – saw the promise in Newton’s work and managed to raise the funds to publish the Principia, providing much of the money from his own pocket. Newton’s book was finally published in 1687, and went on to revolutionize our understanding of the physical world.

First hint of Majorana fermions spotted in nanowires

Physicists in the Netherlands say that they have found the first evidence for the existence of “Majorana fermions” – particles that are their own antiparticles. The researchers claim to have spotted what they call “signatures” of these elusive particles, which were first predicted by the Italian physicist Ettore Majorana in 1937, at the interface between a tiny semiconductor wire and a superconducting electrode. The Majorana fermions spotted in the Netherlands are not, however, fundamental particles but quasiparticles – particle-like entities that emerge from the collective behaviour of electrons in a solid.

As well as backing Majorana’s original prediction, the discovery also agrees with more recent theoretical work that the particle could be lurking within solid-state devices. The latter could be important for the development of quantum computers because Majorana fermions – unlike more familiar “Dirac” fermions, such as electrons – obey “non-Abelian statistics” and so should be resistant to environmental noise. Majorana fermions could, therefore, be able to store and transmit quantum information without being perturbed by the outside world, which is the bane of anyone trying to build a practical quantum computer.

Half and half

The new evidence for Majorana fermions has been obtained by a team led by Leo Kouwenhoven at the Delft University of Technology and the Eindhoven University of Technology that has studied materials known as “topological superconductors”. These are materials that are superconducting in the bulk but are normal metals on their surface.

The team created their topological superconductor by connecting a nanowire of the semiconductor indium antimonide to an ordinary superconductor electrode (niobium antimonide nitride). This creates a topological superconductor in the region of the nanowire that is near to the ordinary superconductor. The other end of the nanowire is connected to a normal electrode made of gold. The device is cooled to temperatures of tens of millikelvin and a magnetic field is applied along the direction of the nanowire.

Persistent peaks

The team then measured the current flowing through the nanowire as a function of voltage – and, in particular, how the current changed in response to changes in voltage. At zero applied magnetic field, two small peaks were observed on either side of zero applied voltage. When the applied magnetic field was increased, the position of these peaks remained in the same position. This also occurred when an electric field was applied to the nanowire.

According to the team, this lack of response by the peaks to magnetic and electric fields can only be explained by the presence of pairs of Majorana fermions at one end of the nanowire. “What is magical about quantum mechanics is that a Majorana particle created in this way is similar to the ones that may be observed in a particle accelerator, although that is very difficult to comprehend,” says Kouwenhoven.

The team acknowledges that its measurements do not confirm the expected topological properties of the Majorana fermions that it has seen – something that would make the particles useful for quantum-computing applications. To do so, the team suggests a number of new experiments to measure other properties of the quasiparticles to establish their non-Abelian nature.

The research is described in Science.

What is your favourite quasiparticle?

By James Dacey

This week, an international group of researchers has hit the headlines by reporting the first-ever observation of a quasiparticle called the “orbiton”. First predicted a decade ago, the orbiton can be thought of as an electron in which the properties of spin and charge have been suppressed. Elsewhere in the news this week, a separate team has spotted a quasiparticle that resembles the elusive Majorana fermion predicted in the 1930s by Italian physicist Ettore Majorana.

Quasiparticles can be thought of as excitations in a solid that behave like tiny particles that obey quantum mechanics; a phonon, for example, is a quantized sound wave that propagates through a crystal.

However, the definition of a quasiparticle is not something that is universally accepted – indeed, some argue that a phonon is not a quasiparticle, by virtue of it being a boson rather than a fermion. Others ask whether these “particles” are in fact real physical entities or whether they are merely useful mathematical concepts for understanding the collective behaviour of real particles within bulk materials.

However you want to think about them, quasiparticles have proved themselves to be very useful. For instance, an entire fleet of electronic devices has been developed over the years thanks to our understanding of “holes”, which are quasiparticles representing the absence of an electron.

In this week’s Facebook poll, we want to know if you hold a particular affinity for any of these quasiparticles.

What is your favourite quasiparticle?

Phonon
Spinon
Hole
Exciton
Wrinklon

Have your say by casting your vote on our Facebook page. And feel free to post a comment to explain your choice or to nominate another quasiparticle not on our list.

In last week’s poll, we entered the realm of quantum mechanics, and we received a fantastic response to the question “What is the trickiest feature of quantum mechanics to get your head round?”. The results were as follows.

Entanglement, aka “spooky action at a distance” 65%
The Heisenberg uncertainty principle 13%
Wave–particle duality 11%
Superposition 6%
The Pauli exclusion principle 4%

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In addition to the votes, there was also a lively discussion on our Facebook page as people shared their experiences of grappling with the ideas of quantum mechanics. One user who goes by the name of Art Hobson wrote “A close look at wave–particle duality reveals that quantum physics is about fields, not particles. The so-called particles are simply excitations (waves) in these fields.”

Another Facebook user, Wendl Thomis, revealed that the feature of quantum mechanics he has most trouble with is the idea of virtual particles. “Virtual particles are postulated to come into and out of existence at every point of space at dizzying rates so that the energy there can fluctuate as quantum mechanics demands. A very non-intuitive idea,” he says.

Thank you for all your participation and we look forward to hearing from you in this week’s poll.

And if you want to learn more about the ideas of quantum mechanics, take a listen to the latest edition in the Physics World books podcast series, in which we discuss the enduring popularity of quantum mechanics in popular-science writing.

Return to Macondo

Monday 20 April 2010 is a day that will live long in the oil industry’s collective memory. For those of us who were working in the industry at the time, it is like the assassination of John F Kennedy, the Challenger space-shuttle disaster or 9/11: you remember where you were and what you were doing. For example, I was in Tunisia, stranded by the Icelandic ash cloud that had shut down much of European airspace, and just about to enter my last month of working for BP before taking up an academic post at Imperial College London. E-mail exchanges confirmed that all was well in the London office, and neither I nor my colleagues knew that it was about to become the blackest day in the company’s 107-year history.

Then BP’s Macondo oil well blew out, causing an explosion that killed 11 workers on the Deepwater Horizon rig and kicking off what would become 152 days of sheer hell for pretty much everyone involved. The images of the Macondo disaster will also live long in the memory. The burning inferno of the Deepwater Horizon before it sank. The flotilla of ships and rigs around the disaster site. The frantic clean-up activity on the beaches surrounding the Gulf of Mexico. But most of all, it is the haunting images of the gusher on the seabed – brought via live video feed to computer and television screens around the world – that will remain with us long after the dust, or rather the oil, has settled.

It may be several years before we fully understand the chain of events that led to the blow-out. In the meantime, though, anyone who wants an honest, uncluttered and considered account of the Macondo disaster should read A Hole at the Bottom of the Sea by Joel Achenbach, a veteran reporter who covered the story for the Washington Post. Pick up this book and you will uncover a heady mix of forensic science and real-life drama that played out in front of a global audience, complete with a handful of genuine heroes. People like Admiral Thad Allen of the US Coast Guard, who stepped up to the plate and gave their all when disaster struck. Engineers like Richard Lynch of BP, with a seemingly intractable problem to solve and the clock endlessly ticking. Achenbach gives readers a real feel for the stress these people were under at the time, as they tried to answer questions such as “How bad is this going to get?” and, even more importantly, “How do we stop it?”.

The tale of the oil industry’s biggest and most highly publicized disaster is told in the style of a classic thriller, with a beginning (the blowout), a middle (the search for the solutions) and an end (the sealing of the well after 152 days). It opens, crucially, with a testament to the 11 men who died, surely the real victims of the Macondo disaster. Throughout the narrative that follows, Achenbach’s riveting account looks beyond the politics and generally uninformed Internet comment and seeks to tell the story behind the cause, solution and aftermath of a modern technological catastrophe.

It quickly became clear that this event was not going to be a typical oil-spill tragedy played out on the beaches. Instead, all the action was taking place a mile below the surface of the Gulf of Mexico – an alien world where technology is king and the human players in surface-based installations must fight a daily battle to keep on top of engineering in extreme deep-water conditions. As Achenbach describes, this is a tough environment. He’s right. I’ve been there, I’ve got the T-shirt.

Achenbach paces his narrative well, interspersing interviews with the principal figures involved in the crisis (including both government and BP officials) and exhaustive research based on an analysis of almost 20,000 pages of unpublished US government e-mails. On reflection, though, some of this material may be slightly too exhaustive. For my taste, he includes a little too much of the politics (particularly at local level) and not really enough of the science behind deep-water drilling. Readers should not expect a major engineering treatise or an in-depth analysis of every last widget that might have failed. Achenbach also uses lots of technical terms and jargon that even I, with my industry background, had to look up. However, the most important concepts, such as “hot stab”, “top kill”, “junk shot” and so on, are described at a sufficient level of detail.

The book is written very much from a US perspective. This is understandable, as that is where the tragedy had the most impact. However, I was encouraged to find that despite its US origins, the book is not a BP-bashing exercise. Unlike much of the world’s media and the US administration at the time, Achenbach presents a remarkably well-balanced description of the events, from the perspective of someone who was reporting on the incident first-hand. The discomfort of BP executives hauled in to testify at House subcommittee hearings is countered by a narrative that describes an administration in crisis and all too eager to look for a “bad guy” to blame. Achenbach also deals effectively with the constructive tension between BP and US government scientists, who worked alongside each other at BP’s Houston offices. This “forced marriage” was often awkward and strained, but it nevertheless eventually delivered a solution.

In the book’s later chapters, Achenbach moves beyond a description of Macondo’s aftermath onto an analysis of its causes. As he astutely points out, there is seldom a single identifiable causal event for major technological disasters. Instead, it is typically a series of relatively insignificant, often unrelated events that come together to form the “perfect storm”. The author suggests 10 direct causes, including a blow-out preventer with maintenance issues and the absence of a cement-bond log test, but to be honest, it is difficult to know where to draw the line.

Perhaps more importantly, Achenbach also asks what lessons can be learned from such an event. He informs us that the oil industry experienced 33 near misses in the Gulf between the 1979 Ixtoc blow-out in Mexico and the 2010 Macondo explosion, and he is right to ask whether BP and others ventured into deep waters without fully understanding or being in control of the technology. But surely, much of human achievement has been won by working at the edge and taking calculated risks. Did we push too far in the search for these highly desirable and valued hydrocarbons?

One thing is certain: an incident like this must not be allowed to happen again. It is important that we all understand the events of 20 April 2010 and the consequences that ensued. For that reason, if you are involved in any way with the petroleum industry – or just someone seeking an honest and well-written account of its darkest day – I urge you to read this book. Perhaps more information about the cause of the blow-out and subsequent explosion will come to light in the future, but in the meantime, we can be satisfied with Achenbach’s tale. When you finish it, you will still have questions, but then, so does the author. In particular, the part played by the mysterious “bladder effect” seems pretty baffling – I certainly found it so. But I will let you read for yourselves to understand the significance of this effect and the central part it may have played in creating the “hole at the bottom of the sea”.

Introducing the ‘orbiton’

Condensed-matter physicists love quasiparticles, and now they have another entity to admire – the “orbiton”. First predicted a decade ago, the orbiton is a collective excitation of electrons in a 1D solid that behaves just like an electron – with orbital angular momentum but with no spin or electric charge. As well as completing the set of three electron-like quasiparticles predicted to exist in a 1D solid, the discovery, made by an international team of physicists, could offer new insights into the origin of high-temperature superconductivity.

Quasiparticles offer physicists a convenient quantum-mechanical description of the collective behaviour of electrons and atoms in solid materials. Perhaps the most famous example is the “hole”, which describes the absence of electrons in a semiconductor in terms of a positively charged electron-like particle.

Three for one

Sometimes a system can be described in terms of several different quasiparticles, each of which manifests a certain property of the constituent “real” particles. For example, an electron in a solid has intrinsic spin, charge and orbital angular momentum. In some special situations, a system of electrons could be described in terms of three quasiparticles, each having just one of these fundamental properties.

This was illustrated beautifully in the mid-1990s in experiments done on SrCuO₂ and Sr₂CuO₃, in which electrons are confined to 1D along chains in a crystal lattice. The researchers used high-resolution photoemission spectroscopy to remove a single electron from the lattice. In the space left, the researchers observed the formation of quasiparticles representing spin and charge – spinons and holons, respectively – which were seen to travel freely through the crystal lattice.

Surprise sighting

Observing the third quasiparticle – the orbiton, or that representing the electron’s orbital degree of freedom – had been considered too difficult. Now, however, Thorsten Schmitt of the Paul Scherrer Institute at the Swiss Light Source in Villigen and an international group of colleagues have performed the feat using the latest resonant inelastic X-ray scattering technology. This technique provides sufficiently high intensity and resolution to single-out electrons in a 1D solid. “We were very surprised to be able to see it,” says Schmitt.

In their experiment, Schmitt and colleagues used the target material Sr₂CuO₃, which contains a 1D chain of copper-oxide groups. In these groups, the ground state of the outer electrons line up with alternating spins, but they can become excited to different states via the scattering of an X-ray photon. When an X-ray photon interacts with the sample, an electron switches spin orientations with its neighbour. This creates a domain wall where the alternating spin arrangement is discontinuous. The researchers observed two distinct excitation energies: one relating to this local disturbance of the spin arrangement – the spinon – and the other relating to a collective response of all the electrons in the copper-oxide chain, which is the orbiton.

“What really surprises me is that the [phenomenon] works just like the case of a spin–charge separation,” says Giniyat Khaliullin, a theorist at the Max-Planck Institute for Solid State Research in Stuttgart, Germany. “In fact, the orbital quantum number of an electron corresponds to [the electron’s] spatial shape, and it was widely believed that in real systems the orbitals strongly couple to the lattice and cannot therefore move coherently. It seems that they can, and that they do indeed behave like a real quasiparticle, an orbiton, that carries information about the spatial shape of electrons.”

Schmitt believes that a knowledge of orbiton quasiparticles may help physicists towards the goal of understanding high-temperature superconductivity, which arises mostly in materials that contain copper-oxide. “In high-temperature superconductivity, many of these interactions are very important,” he says. “It is very heavy stuff – for 25 years researchers have been trying to understand high-temperature superconductivity, and they haven’t cracked it. So any detail one finds, even in easier systems, is important to make progress.”

The research is published in Nature.

New single-photon source could boost quantum cryptography

A simple source of single photons that is powered only by an electric current has been developed by an international team of researchers. The device, a modification of the traditional semiconductor p–n junction called a p–i–n junction, is made from diamond and is one of the first single-photon emitters to work at room temperature that does not need to be pumped with a laser – another research group working independently has demonstrated a similar device. In the short term, the device could help to make “unbreakable” quantum cryptography a more viable endeavour. In the longer term, the researchers hope the devices might open up new avenues for research in quantum computing and other research in quantum-information theory.

Quantum cryptography allows two individuals, conventionally called Alice and Bob, to send a coded message while tracking any interception by an eavesdropper, usually called Eve. Information is encoded into quantum states – say the polarization of photons – and if Eve tries to secretly measure these states as they pass from Alice to Bob, then the laws of quantum mechanics ensure that Eve’s actions are revealed to the correspondents.

While quantum-cryptography schemes have already been used commercially, they rely on the production of single photons – which has proved to be a difficult thing to achieve. One way of doing it is using an extremely weak pulsed laser (in the femtowatt range). However, achieving a stable power output at this level is difficult. Some of the pulses will contain no photons at all, while others pulses could contain two or more photons. The latter is particularly unhelpful because if a pulse contains two or more identical photons, Eve could in principle measure the state of one photon while leaving the other untouched – and Alice and Bob would be blissfully unaware they are being bugged.

Reliable single photons

For this reason, research groups all over the world are developing various schemes for producing reliable streams of single photons. Systems under investigation include those based on individual, self-assembled organic molecules or semiconductor quantum dots. Unfortunately, the most promising systems only work when cooled to cryogenic temperatures, which is not compatible with the commercial development of quantum-communication systems.

An alternative approach that works at room temperature involves a crystal defect in diamond, called a nitrogen vacancy (NV). This occurs when a nitrogen atom replaces a carbon atom in the diamond lattice and a nearest-neighbour carbon atom is missing (the vacancy). NVs can emit single photons if excited by a laser of the right wavelength – a property called photoluminescence. However, this scheme is also seen as impractical because of the need for a bulky and expensive laser.

Sandwiched NV centre

In this latest work, Norikazu Mizuochi and colleagues at the Japan Science and Technology Agency in Saitama, together with international collaborators, have shown how an NV centre can be made to emit single photons by the application of an electric voltage – a process called electroluminescence. They did this by creating a light-emitting diode (LED) structure using doped diamond as the semiconductor material. The device consists of an undoped or “intrinsic” region of super-high-purity diamond that is sandwiched between p- and n-doped diamond layers. The super-high-purity diamond contains the NV centre.

Mizuochi explains that, when electrons and holes are drawn into the intrinsic region by the applied voltage, they excite the nitrogen vacancy, thereby causing electroluminescence and the emission of single photons.

Jean-Francois Roch of the Quantum and Molecular Photonics Laboratory in Paris, who was part of an independent team that developed a similar device using different methods in 2011, told physicsworld.com that this latest work is impressive. “To be honest,” he says, “I must say that [Mizuochi’s] group has done a more thorough analysis than the one we have done.”

The research is described in Nature Photonics.

Dirac cones could exist in bismuth–antimony films

Physicists in the US have done calculations that suggest “Dirac cones” exist in thin films made of bismuth and antinomy. This is an unexpected result because until now such cones have only been seen in graphene and its cousin materials graphynes. Although the predictions have not been tested in the lab – and only apply at ultra-low temperatures – the researchers are hopeful that the films might find use in next-generation electronics devices.

Dirac cones are features in the electronic band structure of a 2D material where the conduction and valence bands meet in a single point at the Fermi level. The bands approach this point in a linear way, which means that the effective kinetic energies of the conduction electrons (and holes) are directly proportional to their momenta. This unusual relationship is normally only seen for photons, which are massless, because the energies of electrons and other particles of matter at non-relativistic velocities usually depend on the square of their momenta. The result is that the electrons in Dirac cones behave as though they are relativistic particles with no rest mass, travelling through the material at extremely high speeds – a property that could be exploited to make ultrafast transistors.

Better than graphene?

Until now, Dirac cones have only been seen in graphene (and more recently “graphynes”), which has two such (unequal) cones, but Shuang Tang and Mildred Dresselhaus at the Massachusetts Institute of Technology have created a mathematical model that suggests single Dirac cones can exist in 2D bismuth–antinomy films. “Not only that, but we expect that the single cone found in bismuth–antimony can do all the things that the graphene [Dirac cones] can do, and better!” says Tang. “For example, the Dirac cones in graphene are isotropic, so the variety of devices that can be made from this material is limited. However, Dirac cones with a wide range of anisotropies can be constructed in bismuth–antimony films, something that could increase the types of potential devices that might be fabricated.”

Bismuth–antimony films with Dirac cones conduct electricity extremely well while having low thermal conductivity, two properties that make them promising thermoelectric materials – substances that convert heat into useful electrical energy. Tang and Dresselhaus say that they could now make quasi-Dirac ones with different bandgaps, which would greatly increase the entropy carried per charge carrier (a measure of thermoelectric performance) in the material without destroying the electrical conductivity. “Basically, for thermoelectrics you need to have a temperature difference across a sample if you want to produce an electric current,” explains Tang. “In this respect, bismuth–antimony films could be especially interesting for applications in space stations and satellites where electricity could be generated by exploiting the difference between the spacecraft’s Sun-facing and shaded sides.”

Electronics applications

According to Tang, the films could also form the base material for next-generation electronic devices. “Electron speeds in devices made of bismuth–antimony would be hundreds of times greater than those in current silicon devices,” he says. “At the same time, the fact that different anisotropies of cones can be elaborated here means that different devices could be made out of the same class of material, which would greatly save on manufacturing costs.”

The MIT group’s calculations have been published in Nano Letters.

Exotic explanation for Pioneer anomaly ruled out

The unusual trajectories of the Pioneer 10 and 11 spacecraft as they leave the solar system are not caused by any exotic new physics but by mundane thermal emissions powered by radioactive decay. That is the verdict of researchers in the US and Canada, who have compared the results of an extremely detailed computer simulation of the thermal forces on one of the craft with the same forces calculated from the trajectory of the mission. The study also suggests that the observed reduction of the extra acceleration over time is the result of how electricity is generated on board the spacecraft and distributed to its scientific instruments.

Physicists have known for more than a decade that the Pioneer 10 and 11 probes are following trajectories that cannot be explained by conventional physics. Known as the “Pioneer anomaly”, both craft seem to be experiencing an extra acceleration towards the Sun as they exit the solar system that is 10 billion times weaker than the Earth’s gravitational pull. Many explanations have been proposed for the origins of this anomalous acceleration, involving everything from the gravitational attraction of dark matter and modifications of Einstein’s general theory of relativity to string theory and/or supersymmetry.

In 2011 a team led by Slava Turyshev of the Jet Propulsion Laboratory in California – and including Viktor Toth, Jordan Ellis and Craig Markwardt – showed that the magnitude of the acceleration is decreasing exponentially with time. Given that for both craft electricity is supplied by a radioisotope thermoelectric generator (RTGs) powered by the heat given off by the radioactive decay of plutonium – an energy source that decays exponentially with time – Turyshev and others suggested that the extra acceleration could be caused by thermal radiation being emitted from the craft in a preferred direction.

The problem with that explanation, however, is that the acceleration of the spacecraft is decaying exponentially with a half-life of about 27 years, whereas the half-life of plutonium-238 is 88 years. So to see if thermal emissions really are driving the anomaly, Turyshev, Toth and Ellis joined forces with three other researchers – Gary Kinsella, Siu-Chun Lee and Shing Lok – to create a detailed computer simulation of the thermal properties of the spacecraft and the directions in which key components emit thermal radiation.

Efficient acceleration

The simulation reveals that the two main sources of thermal emissions on the spacecraft are the RTG itself and the scientific instruments that it powers. These instruments, which are mostly mounted on the back of the spacecraft, face away from the Sun and, according to the simulations, their thermal emissions have a relatively high efficiency of accelerating the spacecraft towards the Sun. The RTG, in contrast, is mounted to one side of the main body of the spacecraft and emits thermal radiation much more evenly in all directions.

The research suggests that knowing the relative contributions of the RTG and the instruments to the anomalous acceleration is key to understanding why the observed decrease in the anomalous acceleration is faster than the decay of plutonium-238. According to Turyshev, the thermocouples at the heart of the RTGs become progressively less efficient at converting heat to electricity – and that this decay occurs with a half-life that is somewhat shorter than 88 years. As the thermocouples deteriorate, less electrical energy is supplied to the instruments, which means that the anomalous acceleration drops faster than expected from radioactive decay alone. Although more heat is dissipated by the RTG as time progresses, this has little effect on the motion of the spacecraft.

Notes and memories

According to Turyshev, the biggest challenge in developing the simulation was the “lack of precise and complete information on the spacecraft”, which was designed and built more than 40 years ago. As a result, the team interviewed engineers who had built the spacecraft and still had notes and memories on the design and materials used. Also crucial to the team’s success was the use of data that were beamed back to Earth during the mission. These included the temperature at several locations on the spacecraft, which allowed the team to evaluate the accuracy of its computer model and also to infer the thermal properties of some of the materials used in the satellite.

The team also performed an independent analysis of the trajectory of Pioneer 10 from which the researchers were also able to extract the relative contributions of the RTG and instruments to the anomalous acceleration. Both the thermal simulations and the trajectory analysis gave similar results, within experimental and computational errors.

It is this agreement between the thermal and trajectory studies that impresses Benny Rievers of the University of Bremen in Germany. With his colleague Claus Lämmerzahl, Rievers has also used computer modelling to show that directional thermal emissions are the likely cause of the Pioneer anomaly. “I think that we now completely understand what is going on with the spacecraft and that the anomaly is completely down to anisotropic heat radiation,” says Rievers.

The work is detailed in a paper on the arXiv preprint server.

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