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Pushing towards the human–Martian frontier

Eye on the sky: NASA astronaut Walter Cunningham during the Apollo 7 mission, in October 1968. (Courtesy: NASA)

 By Tamela Maciel at the National Space Centre in Leicester

Last week, the planet Mars was under the international spotlight once more as NASA scientists announced that liquid water may still be flowing on the surface of the red planet. Also, the much-anticipated film adaptation of The Martian – a 2011 novel by American author Andy Weir – a science-driven story of human survival on Mars, hit the box office.  Mars was also the hot topic at a recent event held at the National Space Centre in Leicester. The guest of honour was Apollo 7 astronaut Walter Cunningham and throughout the hour-long Q&A, he emphasized the need to push the “next frontier” and send humans to Mars.

Cunningham is not a man lacking in confidence or the experience of pushing boundaries. When asked if he ever felt the pressure of the astronaut selection or training process, he said “I thought I could fly anything, any time, anywhere. Was that true? I don’t know. But that’s how I felt.”

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Dark matter may power supernovae

Stellar explosions known as type Ia supernovae could be triggered by dark matter. So says a physicist in the US, who has worked out how certain burnt-out stars can explode even though they lack the mass to generate fusion reactions. According to the new research, the stars ignite because they accumulate so-called asymmetric dark matter, which, if real, could be detectable in a new generation of earthbound experiments.

Asymmetric dark matter, like familiar visible matter, would come in both matter and antimatter varieties. It was proposed on the basis that the density of dark matter in the universe today, as revealed by its gravitational interactions, is only about five times that of normal matter. In cosmological terms, the two matter densities are almost identical, and this suggests a common link between visible and dark matter. That being a very slight imbalance between matter and antimatter, which, following mutual annihilation in the early universe, resulted in the densities observed today.

This similarity does not apply to the current favourite dark-matter particles – weakly interacting massive particles (WIMPs) – which are their own antiparticles and could not have undergone a lopsided annihilation.

Explosive burst

In the latest work, Joseph Bramante of the University of Notre Dame in Indiana looked for evidence of asymmetric dark matter in observations of type Ia supernovae, the “standard candles” that showed the universe’s expansion to be accelerating. Such supernovae are thought to be generated by white dwarfs, the very dense burnt-out remnants of Sun-like stars. Normally, white dwarfs are not massive enough to compress to the point where their internal temperature allows fusion reactions to take place. But astrophysicists believe they can accumulate additional mass by sucking material from nearby stars. They would eventually reach the “Chandrasekhar limit” of about 1.4 solar masses, at which point they would collapse and then blow apart as a result of an explosive burst of fusion energy.

However, as Bramante points out, a 2014 study of the light emitted by a sample of relatively nearby type Ia supernovae showed that many of the associated white dwarfs failed to reach the Chandrasekhar limit, with some weighing as little as 0.9 solar masses. Other researchers have proposed that white dwarfs might merge with one another to ignite supernovae, but Bramante says that the low density of white-dwarf binary systems observed in our galaxy “presents a challenge to that idea”.

Extra energy

Bramante believes that dark matter could provide the extra gravitational energy needed for a supernova explosion. The idea is that over the course of its lifetime, a white dwarf sucks in dark matter from its surroundings, forming a ball of dark matter at its centre that eventually gets so massive it collapses in on itself. As it does so, the dark-matter particles scatter off carbon and oxygen nuclei in the white dwarf, so transforming gravitational potential into heat and allowing the nuclei to fuse, which results in a supernova. However, symmetrical dark-matter particles such as WIMPs would not do this because their mutual annihilation would limit the accumulated mass.

Bramante has calculated that asymmetric dark-matter particles with mass between 1015–1017 eV could trigger fusion in 0.9–1.4 solar-mass white dwarfs. In comparison, a proton has a mass of 109 eV. To back up his idea, Bramante investigated whether there is an inverse correlation between the mass and age of white dwarfs – the idea being that it takes less dark matter to collapse more massive objects. Indeed, by combining data from separate existing studies that compared supernovae light curves (brightness as a function of time) with their age and with their mass, he found that such a correlation is supported at the level of 2.8σ. While this is a strong correlation, it falls well below the 5σ level required of a discovery.

In addition, Bramante says that the mass of the asymmetric particles in this latest research roughly matches that needed to explain why there are fewer pulsars in the centre of the Milky Way than expected. Indeed, Bramante has shown in previous work that dark matter could accumulate inside pulsars and convert them to black holes. He acknowledges that the evidence for both his pulsar and supernova mechanisms is “circumstantial”, and hopes that astronomers can gather data from other supernovae to try and establish a 5σ correlation. He also notes that his ideas would be boosted by evidence of supernovae closer to the centre of galaxies igniting at a younger age – given that dark matter tends to accumulate in galactic centres.

Detection in the lab

However, even then, Bramante points out, people are only likely to take his claim seriously if it is confirmed via direct detection of dark-matter particles in the laboratory. The experiments best placed to do this, he says, are XENON1T at the Gran Sasso lab in Italy, which is due to switch on in an upgraded form this autumn, and LUX-ZEPLIN in South Dakota, US, construction of which is due to get under way next year. “If they find dark matter, it will be clear fairly early on whether it is as heavy as these scenarios predict,” he says.

According to Kevork Abazajian of the University of California, Irvine, the supernova hypothesis will make the already eagerly awaited results from these experiments “even more interesting”. He says that although the latest research does not provide “definitive proof” of dark matter triggering type Ia supernovae, he believes that confirmation from the direct-detection experiments would cause the ignition mechanism to “almost certainly become a standard consideration by type-Ia-supernova modellers”.

The research is published in Physical Review Letters.

Art McDonald and Takaaki Kajita win 2015 Nobel Prize for Physics

The 2015 Nobel Prize for Physics has been awarded to Arthur B McDonald and Takaaki Kajita “for the discovery of neutrino oscillations, which shows that neutrinos have mass”.

The prize is worth SEK 8m (£629,000) and will be shared by the two winners who will receive their medals at a ceremony in Stockholm on 10 December.

Kajita is a Japanese citizen and a member of the Super-Kamiokande collaboration. He is professor of physics at the University of Tokyo. McDonald is a Canadian citizen and director of the Sudbury Neutrino Observatory (SNO). He is emeritus professor of physics at Queen’s University.

Eureka moment

Speaking on the telephone from Canada after the announcement, McDonald said that “There was a eureka moment when we were able to see that neutrinos appeared to change from one type to the other in travelling from the Sun to the Earth.” He added that he is very pleased to have “many colleagues that share this prize with me”. “It is a tremendous accolade for our group,” he said. McDonald also said that he has a very good relationship with Kajita and his Super-Kamiokande colleagues.

Neutrinos are particles with no electrical charge that interact very weakly with matter – making them extremely difficult to detect. Their existence was first predicted in 1930 by Wolfgang Pauli as a “desperate remedy” for discrepancies arising in the study of beta decays. At the time Pauli was convinced that neutrinos would be impossible to detect but he was happily proved wrong in 1956 when Frederick Reines and Clyde Cowan detected antineutrinos emitted by a nuclear reactor, for which the pair went on to win the 1995 Nobel Prize for Physics.

In 1957 Italian physicist Bruno Pontecorvo suggested that multiple types, or “flavours”, of neutrinos exist and that they can change, or “oscillate”, from one to another. Multiple neutrino flavours were confirmed in 1962 when Leon Lederman, Melvin Schwartz and Jack Steinberger at Brookhaven National Laboratory in the US observed the existence of both Pauli’s electron neutrino and also the muon neutrino. A third type of neutrino – the tau – was predicted in 1975 and discovered in 2000.

The concept of neutrino oscillation came to the fore in 1964, when Raymond Davis and John Bahcall found that their solar-neutrino experiment in the Homestake Gold Mine in South Dakota detected only about 30% of the electron neutrinos predicted by a theory developed by Bahcall. This discrepancy could only be explained if neutrinos were oscillating between flavours as they travel from the Sun to the Earth. If oscillation was occurring, then it meant that neutrinos have mass, contrary to what the Standard Model of particle physics predicted.

Ends of the Earth

In 1998 Kajita presented data taken by the Super-Kamiokande experiment that showed that the ratio of electron to muon neutrinos coming from opposite sides of the Earth were different. This meant that these neutrinos – created when cosmic rays interact with nuclei in the upper atmosphere – were changing flavour as they passed through the Earth. This showed for the first time that neutrinos must have mass, albeit only about 0.1 eV.

Then, in 2001 and 2002 McDonald and his colleagues at SNO reported how many of the electron neutrinos produced in the Sun change into muon neutrinos or tau neutrinos as they travel to the Earth. This was possible because SNO could measure the number of neutrinos of all flavours arriving from the Sun as well as the number of electron neutrinos arriving from the Sun. These measurements allowed McDonald and colleagues to both confirm Bahcall’s theoretical prediction of the solar electron neutrino flux and also show that about two-thirds of the solar electron neutrinos change flavour by the time they reach the Earth.

The evidence for neutrino oscillations was further strengthened earlier this year when researchers at the T2K (Tokai to Kamioka) experiment in Japan fired a beam of muon neutrinos 295 km through the ground to Super-Kamiokande. There they detected electron neutrinos with a statistical significance greater than 5σ, confirming that muon neutrinos do indeed oscillate into electron neutrinos.

The discoveries are also triumphs of experimental physics because neutrinos have no electrical charge and interact very rarely with matter. As a result they are extremely difficult to detect. This is why both Super-Kamiokande and SNO are located deep underground, to shield them from cosmic radiation.

Earlier this year, McDonald explained to Physics World why such experiments are built underground:

McDonald was born in 1943 in Sydney, Nova Scotia and did a BSc and an MSc in physics at Dalhousie University in Halifax. He then moved to the US, where he completed a PhD at Caltech in 1969 before returning to Canada to work at Atomic Energy of Canada’s Chalk River Laboratories until 1982. After a seven-year stint at Princeton University, he joined Queen’s University in Kingston, Ontario in 1989, when he also became director of SNO.

Nick Jelley of the University of Oxford, who worked on SNO, describes McDonald as “a great person to work with and a great leader”. He told Physics World that McDonald made SNO possible because he was able to convince a senior executive at the Canadian mining company Inco to allow physicists to build the laboratory in a working nickel mine.

Kajita was born in 1959 in Higashimatsuyama, Saitama Prefecture and completed a BSc in physics at Saitama University in 1981. He then pursued MS and PhD degrees at the University of Tokyo, completing his studies in 1986. Two years later Kajita joined the Institute for Cosmic Ray Research at the University of Tokyo, where he is currently director.

Superconductor induces magnetism in non-magnetic gold

Physicists in the UK have shown that a superconductor can transfer a magnetic field from a magnet to a non-magnetic metal without becoming magnetized itself. The surprising effect is not predicted by any prevailing theory of superconductivity and could have important applications in the emerging field of superconducting spintronics.

In a conventional superconductor, electrical current is carried by “Cooper pairs” of electrons. The electron spins in a pair point in opposite directions and therefore the pair has zero net spin. The application of a strong magnetic field destroys superconductivity by encouraging both spins to point in the same direction, which tears the Cooper pairs apart. Weak magnetic fields cannot exist within a conventional superconductor, which acts to expel magnetic-field lines. As a result, superconductivity and magnetism are usually seen as mutually exclusive phenomena.

Pairs with spin

However, recent calculations suggest that when a superconductor is placed right next to a magnet, both spins in a Cooper pair can point in the same direction – thereby giving the pair a net spin of one. This intriguing theoretical development has given birth to the nascent field of superconducting spintronics, which seeks to create electronic devices that use the spin of Cooper pairs. While several experimental groups have claimed success in observing spin-one Cooper pairs, physicists have not been able to detect the expected magnetic fields within superconducting materials that are in close proximity to magnets.

Machiel Flokstra of the University of St Andrews and colleagues set out to map the field inside a device called a superconducting spin valve – a device that is designed to measure the interaction between magnetism and superconductivity. They used an exquisitely sensitive technique called low-energy muon spin rotation, in which muons are passed through a sample. The muon spins rotate around the local magnetic field until they decay, each emitting a positron along their spin axis as they do so. Detecting the positron reveals the muon’s spin-rotation rate, which in turn gives the local magnetic field.

The team’s spin valve comprised two ferromagnetic layers separated by a thin layer of a normal metal, all placed underneath a layer of superconducting niobium just 50 nm thick. Expecting it would make no difference to the outcome of the experiment, the researchers also included a layer of gold on the top of the spin valve: “There wasn’t any specific reason,” explains Flokstra, “but we thought ‘why not look into it?'”

Golden discovery

What the researchers found surprised them. They found no evidence of a magnetic field inside the superconductor, but they did see a magnetic field in the gold – even though gold is not normally magnetic. In other words, a magnet on one side of a superconductor can induce a magnetic field on the other side of the superconductor – even though there is no field within the superconductor. Furthermore, the researchers found that the induced magnetic field in the gold depended on the relative orientation of the fields in the two magnetic layers on the opposite side of the superconductor. When the two fields were perpendicular, a strong field was induced. When they were parallel, however, the effect was almost zero.

The researchers have several ideas for how the spin might be transferred, including the transfer of spin through the superconductor via spin-polarized Cooper pairs.

Jacob Linder of the Norwegian University of Science and Technology and Jason Robinson of the University of Cambridge both see the work as an important achievement. “Although this particular finding is surprising because it has no known theoretical explanation, it confirms a suspicion that it may be possible to do completely new physical things using superconductors and ferromagnets,” says Linder.

‘Really nice result’

Robinson says that of the three explanations the researchers posit, the most likely is the generation of spin-polarized Cooper pairs and their transmission through the superconductor. “It’s a really nice result,” he says, “and, for any of the people out there doubting that you could generate a spin triplet superconducting state then, I hope this will clarify that.” Linder and Robinson agree with Flokstra that, whatever its detailed explanation, the phenomenon could be important in superconducting spintronics. “This potentially opens a window for doing conventional, non-superconducting spintronics with much lower dissipational energy,” Linder says.

The research is published in Nature Physics.

Who was Emmy Noether?

Tomorrow sees the announcement of the 2015 Nobel Prize for Physics, as one or more researchers joins the elite club of scientists whose achievements are scribed into the history of human innovation. Physics laureates come from a variety of different countries and research fields, as shown in this year’s infographics and infographics from last year. But one feature these people do tend to have in common is that they are men. In fact, since the prize was first awarded in 1901, only two of the 199 individuals to win the prize have been women – Marie Curie in 1903 and Maria Goeppert-Mayer in 1963.

Among the female researchers who should arguably have been awarded a Nobel prize is Emmy Noether, the German mathematician whose life and work is introduced in this video for our 100 Second Science series.

The video’s presenter – Ruth Gregory of Durham University in the UK – explains how Noether’s eponymous theorem relates conservation laws to symmetries in nature. Noether did her groundbreaking work around a century ago but its significance has became increasingly apparent over the intervening years. For instance, the mathematical concepts of symmetry were applied to the study of fundamental particles, which became a key feature of the Standard Model of particle physics.

Sadly, Noether was not around to appreciate the full impact of her work, as she died at the relatively young age of 53, which is perhaps one of the reasons why she never won a Nobel.

The announcement of the 2015 Nobel Prize for Physics will be made tomorrow at 11.45 a.m. local time in Sweden (CEST). Stay tuned to this website and our Twitter feed @physicsworld for coverage tomorrow as events unfold. If we are to finally see another female physics laureate, then one worthy recipient would be Deborah Jin for her work on fermionic condensates. Jin is among the researchers predicted to win this year’s prize by the Physics World team.

If you enjoyed this video explainer, then check out more from our 100 Second Science series.

Pulsars speed up by tapping into superfluid core

Pulsars are known to be the most precise cosmic timekeepers, but occasional “glitches” or a sudden increase in their spin rate disrupts the stars’ otherwise regular behaviour. A new study of the glitching process by an international team of researchers suggests that superfluid matter in the core of a pulsar may cause the poorly understood effect. The work combines radio and X-ray data to determine pulsar masses, and successfully explains glitches that are documented in 45 years’ worth of observational data.

Pulsars are highly magnetized, rapidly rotating neutron stars that emit “pulses” of broadband electromagnetic radiation at very regular intervals. Born in the collapse and subsequent supernova explosion at the end of a massive star’s life, pulsars are small and extremely dense. They normally have a radius of about 25 km and a mass greater than that of the Sun. They are mainly made up of neutron-rich matter that is tightly packed at densities greater than that of an atomic nucleus.

Speed up or slow down?

The spin periods of observed pulsars range between 1.4 ms and more than 1 s, but their rotation periods are normally exceedingly stable. Indeed, many pulsars rival the precision of an atomic clock. A pulsar’s rotation period is also known to decrease with time, as it loses energy through electromagnetic radiation. But in many young pulsars, this long-term slowdown is occasionally interrupted by a sudden increase in speed, called a glitch. After the speed-up, which can occur in less than one minute, the pulsar gradually returns to its previous speed, but this can take up to about a year. Just as rotation speeds vary from pulsar to pulsar, the magnitude of the speed-up can also be different for different pulsars.

Our current understanding of pulsar dynamics suggests that glitches occur because of some interaction between superfluid matter and normal matter within the star. Pulsar structure comprises three different regions – the outer crust, the inner crust and the core. While the core is made up of free neutrons in a liquid state, the inner crust contains neutron-rich nuclei swimming in a sea of free neutrons, which are expected to be in a superfluid state.

Spin reservoir

While the rest of the pulsar slows down, the superfluid does not – instead it acts as a reservoir of angular momentum. Previous theoretical models work on the principle that, when the difference in speeds between the superfluid and the rest of the pulsar reaches a critical (but currently unknown) value, the superfluid transfers some of its angular momentum to the normal matter, thus jolting the pulsar and making its speed of rotation increase. However, more recent work has shown that there is a sizable amount of uncertainty in this theory because the amount of superfluid available in the crust is not enough to create the glitch – a lot more would be required.

Using computer simulations, Wynn Ho of the University of Southampton in the UK, together with colleagues in Chile and Netherlands, has taken a closer look at pulsar superfluity and developed a model wherein the glitches gain the necessary momentum by tapping into the superfluid core. “We used the best current models of superfluids and pulsars, which are based on nuclear-physics experiments and calculations, to calculate how strong glitches should be as a pulsar ages,” explains Ho. “The age is important because the amount of superfluid in a pulsar depends on its temperature, and thus as the pulsar ages, it cools, and more of it becomes superfluid.”

Weighing up

Ho told physicsworld.com that while glitches are seen primarily in radio data, they also crop up in X-ray data, which gives the pulsar’s temperature. By comparing a pulsar’s observed temperature and glitch size, the researchers can also measure its mass. “This last point is what is really exciting about our work, because it allows us to measure the pulsar’s mass even if it is not in orbit near another star or planet,” says Ho. “Or in other words, we can measure mass using nuclear physics, not gravity. This is also extremely useful because most pulsars are isolated stars, not in binary systems.” The ability to measure the masses of such lone pulsars has not been demonstrated before.

In their latest work, the researchers looked at glitches in nine pulsars, but they plan to look at more with their new model in the months to come. Ho and colleagues hope that their research inspires more theoretical work on superfluids and promotes connections between astronomers and nuclear physicists. “We think that the potential of the Square Kilometre Array to discover and monitor many more pulsars could really help to revolutionize our understanding in the respective fields of study,” says Ho.

The research is published in Science Advances.

And the winner is: our 2015 Nobel-prize predictions

Psychic world: we have another go at predicting the Nobel winners. (Courtesy: iStockphoto/shutter_m)

By Tushna Commissariat and Hamish Johnston

 

Update: Looks like we were quite spectacularly wrong this time around with our predictions as this year’s Nobel has been awarded to Arthur McDonald and Takaaki Kajita “for the discovery of neutrino oscillations, which shows that neutrinos have mass”. While Physics World’s news editor Michael Banks did predict this in 2013, we did not think this would be the year. Clearly, as our “Which physics disciplines attract the most Nobel prizes” infographic suggests, the field of particle physics still seems to be the most Nobel-worthy one.

It’s a mug’s game, we know, but come the start of October we just can’t resist trying to predict who will win the Nobel Prize for Physics, which this year will be announced on Tuesday 6 October.

With the exception of 2013 – when most pundits were right in thinking that the prize would be related to the 2012 discovery of the Higgs boson – predicting the next Nobel winners (or winners) is a tough call. If you want to take an analytical approach, check out the infographic we published last year: “Which physics disciplines attract the most Nobel prizes”. It suggests that the field of atomic, molecular and optical physics is due a prize, and one of us (Hamish Johnston) thinks an excellent bet is Deborah Jin for her work on fermionic condensates. If Jin were to win, she would be only the third woman ever to win a physics Nobel – the other two being Marie Curie in 1903 and Maria Goeppert-Mayer in 1963.

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New infographics show that more than one-quarter of physics Nobel laureates are immigrants

Migration of minds: maps showing the movement of Nobel laureates (click to see full infographic). (Design: Paul Matson)

By Hamish Johnston

Next Tuesday the Nobel Prize for Physics will be announced at 11:45 CEST and I am making the bold prediction that the winner – or one of the winners – will be an immigrant. Why? Because this year’s Physics World Nobel-prize infographics show that of the 198 people who have won the prize, 51 are immigrants – so I reckon there is a reasonable chance that I will be right.

What do we mean by an immigrant? This is a tough question, especially in science, where people tend to move around a lot and don’t always settle in one place. For the purposes of these infographics, we have used a rather crude definition of an immigrant laureate: someone who died or currently lives in a country other than that of their birth. There is more about how we made the infographics later in this post – but first, what do they tell us?

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Between the lines

Anatomy of a disaster

Early in the morning of 27 April 1986, operators at the Chernobyl nuclear-power plant set in motion a chain of events that led to the world’s worst nuclear disaster. In the immediate aftermath of the explosion (which, ironically, happened during a test of one of the plant’s safety features), much of the blame fell on the operators themselves. Later investigators probed deeper, pointing out design flaws in the plant’s reactors and deficiencies in its safety culture, but few have analysed these issues with as much care as Sonja Schmid does in her book Producing Power: the Pre-Chernobyl History of the Soviet Nuclear Industry. Schmid begins with a detailed explanation of how the Soviet Union chose to develop its nuclear-power industry, and how two government ministries assumed competing and sometimes conflicting responsibilities. The machinations of these rival bureaucracies are important to Schmid’s argument, but unfortunately, they do not make exciting reading. General readers may wish to skip ahead to the third chapter, which deals with how the Soviet nuclear workforce was trained. At this point, what was previously a rather dry tale comes to life, with fresh insights on almost every page. The early US nuclear-power programme, Schmid explains, experienced “recurring incidents of human error”, which were addressed by increasing the degree of automation and engineered safety features. Early Soviet efforts, in contrast, encountered problems with unreliable instrumentation, leading to a greater reliance on human intelligence and ingenuity. Neither approach is inherently wrong, and Schmid avoids making hindsight-based judgements. Gradually, however, the Soviet nuclear cadre developed “a rigid and contradictory structure of accountability that…envisioned expert judgement as the ultimate redundancy feature, while simultaneously restricting operators in their actions and undermining their preparedness to exercise their judgement”. That is a recipe for disaster in any field, and there is something compelling in the way Schmid marshals her facts to explain how this and other factors contributed to the Chernobyl disaster.

  • 2015 MIT Press £26.95/$38.00hb 384pp

Back to the imagined future

The astrophysicist Jayant V Narlikar is best known among scientists for his contributions to cosmology. His long career as a writer of science-fiction stories will be less familiar to many, perhaps because these stories have, until now, been published only in Narlikar’s native India. His “new” (at least to readers living abroad) three-part collection The Return of Vaman begins with a short story, “The rare idol of Ganesha”, about an obsessive physicist called Ajit and his old university pal John. This ingenious tale, written in 1975, cleverly combines mathematics, physics, cricket and Indian mythology as Ajit repeatedly risks his life while trying to perfect his potentially game-changing research. Some of the scientific terminology could confuse non-scientists, but Narlikar captivates via his vivid descriptions of each scene and his ability to raise interesting questions about research ethics. Ethical questions are also on display in the novel that forms the book’s core (and its title). The action here revolves around a team of experts (including physicists) assigned to probe the contents of a strange container found deep underground, and several criminals, who are likewise set on obtaining the secrets held within. Narlikar creates believable characters and skilfully weaves in real-life physics and realistic scientific scenarios. While some may guess a few of the impending plot twists, the fast-paced tension keeps it an enthralling read. Certain aspects of the novel, penned in 1986, do seem rather dated; for instance, one of the criminal characters uses an offensive term for Japanese people that may grate on modern ears. But the science-fiction elements have generally aged well, and the final part of the book – an autobiographical essay encompassing details of the Hindu mythology integral to each tale, as well as Narlikar’s thoughts on science communication and science fiction – makes an interesting conclusion.

  • 2015 Springer International Publishing £15.00pb 142pp

Popular cosmology

The discovery of the cosmic microwave background (CMB) in the mid-1960s was a seminal event in modern cosmology. As a subject for a popular-science book, then, the CMB is an excellent choice, and The Cosmic Microwave Background: How It Changed Our Understanding of the Universe has a lot of things going for it. Author Rhodri Evans, an astronomer at Cardiff University, UK, has a pleasantly informal writing style, and the book would make an accessible guide for (say) first-year undergraduates in astrophysics. However, his stated audience is “all those people who want to learn more about where our universe comes from”, and his view of what these non-scientist readers will understand often seems optimistic. On page 73, for example, he provides a detailed explanation of the electromagnetic spectrum, complete with a nice analogy comparing the different parts of the spectrum to the keys on a piano. Bright 13 year olds or adults who have long since forgotten their school science classes could follow this passage easily; indeed, more advanced readers may wish to skip past it. However, on the next page Evans seems to think his readers will understand, without explanation, what is meant by “an electron in the ground state” and “the spin of the proton in the nucleus”. A separate concern is that the book’s coverage of the observations by the team behind the Background Imaging of Cosmic Extragalactic Polarization (BICEP2 ) telescope appears to be a victim of bad timing. Although the book has a 2015 copyright, Evans completed it in April 2014, when cosmic dust had only just begun to cloud BICEP2’s purported discovery of evidence for cosmic inflation in the form of “B-mode polarization” of the CMB – a debate that has rumbled on since. The timing is hardly Evans’ fault, but it makes parts of the discussion feel weirdly out of date – a drawback in a book that otherwise has much to recommend it.

  • 2015 Springer £31.99/$34.99pb

Ferroelectricity discovered on the nanoscale

Ferroelectricity can exist in a sheet of material just a few nanometres thick. This new and unexpected discovery by researchers in the US and South Korea could help in the development of new materials for nanoscale electronics.

Ferroelectric materials have permanent electric dipole moments – much like their ferromagnetic counterparts, which have permanent magnetic dipole moments. Ferroelectrics have the potential to be used in a wide range of devices because their dipole moments can be oriented using electric fields, which are much easier to create than the magnetic fields used to manipulate ferromagnetic materials. One possible application is memory chips that store data in terms of the polarization of ferroelectric thin films. A major problem, however, is that these materials cease to be ferroelectric as they become very thin, which limits their usefulness in modern electronic devices.

The researchers, led by Chang Beom Eom of the University of Wisconsin–Madison, have found that a thin film of a material that is normally not electrically polarized can be made polar by taking advantage of existing tiny polar nanoregions within the material. “This happens when the film is made so thin that its whole volume is occupied by these nanoregions,” explains Eom. “When these are electrically aligned in one direction, this leads to a net polarization – and the material becomes ferroelectric.”

The flexoelectric effect

The team’s new discovery stems from its earlier work, in which the researchers discovered naturally occurring polar nanoregions in strontium-titanate films and crystals, which are neither polar nor ferroelectric. “We found that we could reverse the polarization in this material without any applied voltage by simply exerting pressure on the film through the tip of an atomic force microscope,” says team member Alexei Gruverman of the University of Nebraska–Lincoln. “Such voltage-free ferroelectric switching is possible thanks to the ‘flexoelectric effect’, whereby a mechanical-strain gradient induces electrical polarization.”

The researchers wanted to see if the same thing happened in much thinner films of the non-polar material. “We found that we could induce a large flexoelectric effect in this material, but only if the films were very thin,” Eom and Gruverman explain. “To our surprise, we saw that these films behaved almost like ferroelectric ones – that is, they could be polarized not only by applying mechanical strain to them, but also with an applied voltage, and that this polarization was stable. The striking discovery was that the thinner the film, the more stable the polarization, and ferroelectrics typically tend to behave in the opposite way.”

Nanoregion takes over

The researchers say that they immediately connected this observation with their previous work on polar nanoregions, and can now clearly explain how the strain-free ultrathin films of otherwise non-ferroelectric strontium titanate become ferroelectric, says Eom. “As mentioned, when the film’s thickness becomes as small as individual polar nanoregions (which are several nanometres across), the whole volume of the film is occupied by them and the film starts to behave like a ferroelectric,” he says.

The researchers performed ferroelectric measurements, piezoresponse force microscopy and scanning transmission electron microscopy on their samples to confirm their results.

Strontium titanate is an important building block for oxide electronics and has superconducting, 2D gas and magnetic properties, and so is useful for a range of device applications. It also appears to be a good material for solar cells.

Unique or ubiquitous?

Eom and Gruverman say that they do not yet know whether the effect they have observed is unique to strontium titanate. However, they hope that it is valid for other perovskite dielectrics, in which polar nanoregions could be controlled by carefully engineering defect structures in these materials. If this is the case, nanoscale devices in which ferroelectricity is coupled to other properties such as magnetism might be possible.

The discovery is described in Science.

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