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Flash Physics: Jupiter’s swirling poles, ordered droplets, no fifth force, topological magnetoelectric effect

Juno’s stunning portrait of Jupiter shows swirling storms

Jupiter's south pole as seen by Juno — A Juno masterpiece: Jupiter’s south pole covered in cyclones. (Courtesy: NASA / JPL-Caltech / SwRI / MSSS / Betsy Asher Hall / Gervasio Robles)

NASA’s Juno mission has sent back stunning images of Jupiter’s poles. The above image shows the gas-giant’s south pole. The spacecraft’s JunoCam instrument took multiple pictures from an altitude of 52,000 km on three separate orbits, allowing researchers to create full enhanced colour projection. The images of both poles reveal that they are covered in Earth-sized swirling storms up to 1000 km across, but do not look like each other. “We’re questioning whether this is a dynamic system, and are we seeing just one stage, and over the next year, we’re going to watch it disappear, or is this a stable configuration and these storms are circulating around one another?” explains Juno’s principal investigator Scott Bolton. As well as the images, Juno sent back a huge array of results from its first data-collection pass. They are presented in two Science papers and 44 papers in Geophysical Research Letters. “There is so much going on here that we didn’t expect, that we have had to take a step back and begin to rethink this as a whole new Jupiter,” says Bolton.

Small water droplets show unexpected order

Ordered water: researchers studied tiny water droplets in oil. The method involves overlapping ultrashort laser pulses in a mixture of water droplets in liquid oil and detecting photons that are scattered only from the interface. (Courtesy: EPFL / Julia Jacobi, Laboratory for Fundamental BioPhotonic)

Tiny water drops have surprisingly ordered surfaces, according to Sylvie Roke from École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland and colleagues. The team looked at droplets with a diameter of around 200 nm. Such nanoscale beads of water are everywhere – in the air, rocks, oil fields and even our bodies – and therefore understanding their behaviour may provide insights into atmospheric, geological and biological processes. To study the tiny droplets, the scientists look at how their curved surfaces interact with the surrounding water-repellent environment. “The method involves overlapping ultrashort laser pulses in a mixture of water droplets in liquid oil and detecting photons that are scattered only from the interface,” explains Roke. “These photons have the sum frequency of the incoming photons and are thus of a different colour. With this newly generated colour we can know the structure of only the interface.” The team discovered that the surfaces of these tiny pockets of water at room temperature are much more ordered than that of normal water. The enhanced tetrahedral structure is instead comparable to super-cooled water – liquid water below the freezing point – which has very strong hydrogen bonds between the water molecules. The results, presented in Nature Communications, suggest the nano-droplets may have reduced reactivity, and further studies will investigate how this affects real-world systems.

Study places limit on a “fifth force”

A new way of working out whether a “fifth force” exists has been developed by an international team led by Andrea Ghez and Aurélien Hees at the University of California, Los Angeles. The group looked at the motions of two stars (S0-2 and S0-38), which orbit the supermassive black hole (SMBH) at the centre of the Milky Way. The stars were monitored for 19 years, which is roughly the time it takes the stars to complete an orbit of the SMBH. The team looked for deviations from the trajectories predicted by Einstein’s general theory of relativity, and no discrepancies were seen. This suggests that the strength of a fifth force is less than 1.6% the strength of gravity. Modern physics includes four forces: gravity, and the electromagnetic, strong and weak forces. A hypothetical fifth force appears in some theories that try to unify gravity with quantum mechanics or to explain dark matter and dark energy. While much stronger exclusions of a fifth force have already been obtained by studying forces on masses on Earth and also on objects within the solar system, this is the first study to look at large objects in the huge gravitational field of a SMBH. Writing in Physical Review Letters, Ghez, Hees and colleagues point out that their measurement should be improved next year when one of the stars makes its closest approach to the SMBH, where a deviation from general relatively could be strongest.

Topological magnetoelectric effect rotates light

New twist: topological magnetoelectric effect in action. (Courtesy: Technical University of Vienna)

A new interaction between light and a material has been observed by physicists in Austria and Germany. The team shone a polarized beam of terahertz electromagnetic radiation through a thin film that included a topological insulator in an applied magnetic field. The researchers found that the polarization of the beam is rotated by a specific angle as it travels through the material. At first glance, this rotation is similar to the well-known magneto-optical effect that occurs when light passes through a magnetic material. However, Andrei Pimenov and colleagues at the Technical University of Vienna and the University of Würzburg found that the angle is independent of the thickness of the topological insulator – which is not the case for the magneto-optical effect. Furthermore, they found that the angle is fixed at a specific value that is related to the fine-structure constant. This is a dimensionless quantity that defines the strength of the electromagnetic interaction. According to the team, the polarization is rotated by a fixed value every time it passes through a surface of the topological insulator. The researchers say this is related to the peculiar properties of a topological insulator, which is an electrical conductor at its surfaces but an insulator in the bulk. Writing in Nature Communications, the team says that this “topological magnetoelectric effect” could provide a way of defining three basic physical constants that are related to the fine structure constant: the charge of the electron, the speed of light and the Planck constant.

Laser-engraved graphene pixels work in extreme environments

With our persistent march towards nuclear fusion, the need for technologies that can operate in high-energy environments is becoming ever more urgent. Now, researchers in the UK and Spain have discovered a material that can allow us to take pictures inside a nuclear reactor.

In the past, graphene has been used in the design of flexible photodetectors that operate over a large range of frequencies. However, compared to current inorganic photodetectors, the resolution and power response just do not measure up.

Chemical doping of graphene can help some of these problems by increasing the density of charge carriers (electrons or holes). Adding iron-chloride (FeCl₃) molecules between a few layers of graphene has been shown to lead to extremely high concentrations of carriers in the material. This FeCl₃-intercalated few-layer graphene (FLG) is also stable in ambient conditions. It is 1000 times more conductive than pure single-layer graphene, while retaining an equally low absorption in the visible frequency range.

Starting with this material as a base, Saverio Russo and his group at the University of Exeter, UK, used a laser to engrave regions of lower carrier concentration. The laser removes some of the FeCl₃ molecules, creating photoactive junctions between highly doped and laser-treated regions of the material. When light shines on this junction, a current is detected across the material, like in a pixel in a camera sensor.

The recipe for extreme photodetectors

The cleverness of this particular material lies in the ultra-high carrier concentration and accelerated cooling of the carriers. Unlike in previously studied devices, the response of the carriers is purely photovoltaic. In FeCl₃-intercalated FLG, when the carriers are generated by the incident photons, the electric field created by the difference in charge density on either side of the junction leads to a separation of electrons and holes. This in turn leads to a current.

To the left, a schematic shows the iron-chloride intercalated graphene device. A laser engraves regions of lower doping, creating photoactive junctions between the p p’ p regions, illustrated in the band diagram below. To the right is a plot of the response of the photodetector with power of incident photons. This demonstrated the enhanced linear dynamic range (blue shaded region) of this material (red line) compared to pure graphene (black line). Image courtesy of A De Sanctis.

In other graphene-based photodetectors the carrier behaviour is dominated by the photothermoelectric (PTE) effect; the difference in Seebeck coefficients either side of the junction is responsible for the diffusion of hot carriers when illuminated, as in a common thermocouple. This means the response is spread across an area of a few μm, which limits the size and packing density of the pixels. Using FeCl₃-intercalated FLG instead has a huge advantage here, as the miniaturization of pixels is not hindered by a need for thermal isolation.

“This is truly a wonder material; our results show for the first time that graphene-based photodetectors are not always dominated by the PTE,” says Adolfo De Sanctis, lead author of the paper. A reduction of the PTE effect, in fact, results in another marvel. Pixels made from FeCl₃-intercalated FLG have a linear (and therefore predictable and reliable) response to photons over a range of incident powers that reaches around 4500 times higher than for other graphene-based devices. This huge linear dynamic range holds true for frequencies from mid-IR to UV-A. This makes these pixels ideal for extreme environments, such as within nuclear reactors, or for working with the high-energy lasers needed for nuclear fusion.

Breaking the diffraction limit

In the absence of the PTE effect, the size of the junctions is instead constrained by the diffraction limit of the laser used to create them. In collaboration with a team at the Institut de Ciències Fotòniques (ICFO), Spain, led by Frank Koppens, the researchers pushed beyond the limits of diffraction when creating the junctions. Using near-field optical nanoscopy, they were able to carve away the FeCl₃ molecules and create a junction with a width of only 250 nm, more than halving the diffraction limit of the laser.

The next challenge that faces the researchers lies in producing larger sheets of this exciting material. They can then pattern extensive arrays of photoactive junctions, creating an imaging device suitable for the extreme environments that modern research is leading us towards.

You can read more about this work in the original paper, published in Science Advances.

Diamond sensors boost NMR resolution

A new way of boosting the resolution of quantum magnetic sensors has been developed independently by three teams of physicists. The technique has already been used to achieve a huge improvement in nuclear magnetic resonance (NMR) spectroscopy.

Quantum sensing is used to measure frequencies in multiple areas of physics, but for a quantum sensor to measure anything, it must interact with its environment. This degrades its quantum properties very quickly – and this limits the measurement accuracy. Now, however, three research groups have independently synchronized multiple quantum measurements using a classical clock, allowing frequency measurements up to 100 million times more accurate than previously possible with a quantum sensor. One group then went on to demonstrate unprecedented accuracy in micron-scale NMR spectroscopy.

All three groups – at ETH Zürich in Switzerland, Ulm University in Germany and Harvard University in the US – made use of negatively charged nitrogen-vacancy (NV) centres in diamonds. These occur when two adjacent carbon atoms in a carbon lattice are replaced by a nitrogen atom and a vacant site. The spin states of NV centres can be controlled and measured using light, and are also exquisitely sensitive to magnetic fields. Whereas the traditional coil detectors used in NMR spectroscopy and magnetic resonance imaging (MRI) require bulk samples, atomic-scale NV centres can be placed right next to molecules in “nano-NMR” experiments, which are becoming widespread. In 2016, the Harvard and Ulm researchers detected individual protein molecules on the surface of an NV-implanted diamond and even inferred some structural features by studying changes in the frequencies of the fields detected by the NV centres.

Spatial versus spectral

To determine the structure of large molecules using nano-NMR requires even better spectral resolution to allow more precise measurement of the precession frequencies of nuclei, and thus their chemical environments. “The length of time over which you can sample a signal limits the resolution with which you can determine its spectrum,” explains Kristian Cujia, a member of the ETH Zürich team. Unfortunately, the coherent quantum state of an NV centre collapses after a few microseconds because of environmental interactions. Such a short measurement carries significant uncertainty. Worse still, to improve the spatial resolution of diamonds, researchers often implant NV centres more densely or place them closer to the surface. This brings the NV centres closer to the sample, making them more sensitive to its magnetic field, but it also makes them less isolated, causing decoherence to occur more quickly, further reducing the spectral resolution.

Researchers can improve the magnetic sensitivity of NV centres by simply making multiple measurements. As the errors on successive measurements are uncorrelated, the precision improves as more measurements are made. However, the spectral resolution does not improve with such repeated, uncorrelated measurements. The three teams have surmounted this problem by synchronizing repeated NV magnetic measurements to an external clock. This allows them to keep track of time even after decoherence occurs.

“Normally, you would have to take your next measurement as an independent measurement,” explains Ulm’s Liam McGuinness. “When we did our next measurement, we already had a clock that was keeping track of time. That let us stitch together a sequence of measurements.” Indeed, the researchers could make a measurement on an NV centre that could be monitored indefinitely, effectively eliminating the limitation of NV decoherence. All the groups were able to measure megahertz-scale frequencies with sub-millihertz precision – nearly a million times better than the spectral resolution of other NV measurement protocols.

Diffusion difficulties

McGuinness and colleagues used their measurement protocol to perform NMR spectroscopy on a nanometre-sized sample of polybutene. However, the researchers encountered a problem: “Our molecules diffuse past our NV centre,” explains McGuinness. This restricted the length of time the researchers could observe a single molecule, preventing them from obtaining a resolution better than about 1 kHz.

The Harvard group, however, came up with a solution to this problem by getting the measurement protocol to work for ensembles of NV centres in the same diamond. This means that their sample volume is slightly larger (micron sized) and their measurements suffer much less from the effects of molecular diffusion. “With current technology, you can’t use the synchronized readout technique usefully for high-spectral-resolution NMR at the nanoscale, because of the random fluctuation of the sample’s spin polarization [which impedes coherent detection of the small NMR signal],” says Harvard’s Ronald Walsworth. “At the micron scale you can.”

The Harvard researchers obtained resolutions as good as 3 Hz – nearly 100 times smaller than ever seen before in NMR using NV centres. They also observed many of the crucial features used to interpret NMR signals for the first time – including J-couplings. “That opens up a whole new world of micron-scale NMR – potentially for intracellular NMR, for example,” says Walsworth. The next step, says Walsworth, will be to try to perform genuinely new science using NV-centre NMR.

McGuinness says the new sensing protocol is a “general technique” and could find application well beyond NV centres and NMR. “We draw parallels to heterodyne, or beat-note, detection. If you have a weak laser and you want to measure its frequency, you take another very strong laser, join them together and measure the beat note. Here, instead of taking a classical laser, we take a quantum sensor.”

“Important technique”

Theoretical physicist Andrew Jordan, who was not involved in the research, says that the ETH Zürich and Ulm University papers represent “a nice advance in this field…Maybe the most important parameter we have is frequency, because that sets the precision of our timekeeping devices. I think this is going to be an important technique going forward, if nothing else to calibrate people’s systems before they go on to do other applications”. He declined to comment on the Harvard research because it has not yet been through the peer-review process.

The ETH Zürich and Ulm University papers are published in Science. The Harvard research is described in a preprint on arXiv.

Simulating the universe

When Einstein proposed his general theory of relativity a century ago it meant that scientists could in theory describe the universe’s behaviour with mathematics. However, Einstein’s equations are so fiendishly hard to use that researchers were only able to apply the equations to approximations of the real universe. Now, however, two independent groups have finally used Einstein’s equations to describe reality. Find out more by reading this feature article from the May issue of Physics World.

Flash Physics: Flytrap robot catches prey, doughnut-shaped ‘planets’, EU dishes out £55m for UK physics

Flytrap soft robot catches prey

A Venus flytrap’s autonomous insect-catching ability has been replicated by a tiny soft robot. To create the device, Arri Priimägi and team from Tampere University of Technology in Finland attached a strip of light-responsive liquid-crystal elastomer to the tip of an optical fibre. Mimicking the Venus flytrap’s head, the strip of elastomer is about 10 mm long, 1 mm wide and 20 μm thick. It contains layers of ordered molecules that have a different orientation in each layer – those in the “insect-facing” layer are horizontal while those on the opposite side are vertical. The molecules in between are at an intermediate angle. When light is shone on the elastomer, the molecular alignment becomes random. This causes the insect-facing layer to contract and the other side to expand – in other words, the strip of elastomer bends like a flytrap closing. Usually a light-responsive elastomer requires external illumination, but by attaching the strip to an optical fibre, Priimägi and colleagues integrated a light source. Light shone through the optical fibre and elastomer creates a cone of illumination. When an object such as an insect enters this field of view, light is reflected back in the direction of the elastomer. This thereby triggers the elastomer to bend and close around the object. To release the object, the light is simply turned off. The autonomous device, presented in Nature Communications, could be used for intelligent micro-robotics as well as handling delicate small objects.

Could huge doughnut-shaped “planets” exist?

Schematic of a synestia — Hot doughnut: schematic of a synestia. (Courtesy: Simon Lock / Harvard University)

Huge doughnut-shaped objects made from vapourized rock could be orbiting stars other than the Sun. That is the conclusion of Simon Lock of Harvard University and Sarah Stewart at the University of California, Davis, who have done calculations that suggest a new type of planetary object called a synestia could form when rocky planets collide with each other. Such an object would be about four times the diameter of Saturn’s rings and would comprise a ring of rapidly rotating vapourized rock. It would resemble a doughnut, but instead of having a hole in the middle, a synestia would have a dense planet-like object at its centre. Lock and Stewart say a synestia would form when the debris from planetary collisions was both very hot and carrying large amounts of angular momentum. They also suggest that most planets could have been synestias early in their lifetimes. Small planets such as Earth would only spend a few hundred years in this phase before condensing into solid objects. However, larger or hotter objects such as gas-giant planets or even small stars could spend much longer times as synestias. Although synestias have not been observed, the calculations could encourage astronomers to look for huge doughnut-shaped objects alongside rock and gaseous exoplanets. The research is described in the Journal of Geophysical Research: Planets.

European Union dishes out £55m for UK physics

UK physics received £55m in 2014/2015 from the European Union (EU) according to a report by Technopolis Group – an independent policy research organization. Commissioned by the UK’s four national academies – the Academy of Medical Sciences, the British Academy, the Royal Academy of Engineering and the Royal Society – the report looked at how reliant UK research is on EU funding. The EU’s Seventh Framework Programme, which ran from 2007 to 2013, provided UK organizations with around €7bn and its successor – Horizon2020 – is providing around €1.1bn per year. This figure amounts to more than 10% of total UK government support for research and is around 5% of the UK’s gross domestic expenditure on R&D. The report finds that UK universities received around £725m in research grants from EU government bodies in 2014/2015, of which £55m was received by both physics and chemistry while the biosciences got £90m. As the top 10 UK universities receive almost half the £725m funding, the report warns that this will be “difficult to replace” after the UK leaves the EU in 2019.

Search for the ‘perfect’ theory

The term “theory of everything” was a common turn of phrase among high-energy particle theorists during the 1980s, used with varying degrees of irony. Physicists from other fields were often not amused, seeing this terminology as yet more evidence of the hubris of particle physicists. In his new book Theories of Everything: Ideas in Profile, author Frank Close uses the term unapologetically, outlining the current state of our best attempt at a unified theory that should apply to “everything”.

Currently, the closest such theory that we have is commonly known as the Standard Model of particle physics, although Close also uses an alternate name some favour – the Core Theory. He describes some of the features of the theory, leading up to the vindication of one of its central ideas – that of a Higgs field – with the first-ever observation of the Higgs boson, made by researchers using the Large Hadron Collider (LHC) at the CERN particle-physics laboratory in Geneva in 2012. For a more detailed account of this story, Close’s 2013 book, The Infinity Puzzle, is an excellent source.

The great success of the Standard Model has left particle physicists in a difficult position; with not just the Higgs, but all other results from the LHC and other particle-physics experiments so far agreeing perfectly with the theory. This has crushed hopes that something unexpected might be found, which would ultimately indicate a way forward to a better, more complete theory. A major goal of Close’s latest book is to put this situation in historical context, describing earlier “theories of everything” and the theoretical advances that gave new, fundamental insight into the nature of physical reality.

A crucial question about our current situation is whether we really are at, or near, the end of our search for what theoretical physicist and Nobel laureate Steven Weinberg refers to as a Final Theory, or whether there is another revolution in our understanding still to come. One often reads quotes attributed to Albert Michelson (“the grand underlying principles have been firmly established”, 1894) and Lord Kelvin (“there is nothing new to be discovered in physics now”, 1900), indicating that they, like many now, thought they were near the end of the road. That of course would have been a huge mistake, with the great revolutionary discoveries of modern physics – relativity and quantum mechanics – just a few years off.

Close points out that Kelvin’s actual 1900 speech was much more prescient, as he described “two clouds” on the horizon, pointing out that experimental results were in direct conflict with the accepted theory of that time (the Michelson–Morley experiment and the black-body radiation spectrum). For anyone trying to look for a lesson from history applicable to today’s “theory of everything”, a key question is whether any analogue of these “two clouds” can be found.

Close takes up this question and argues that there are good candidates for our “two clouds”. The first is the energy density of the vacuum, also known as the cosmological constant. Cosmological observations appear to indicate that this is a non-zero number, with an order of magnitude so small that it doesn’t fit at all with what one might expect from the Standard Model and general relativity.

The second cloud, according to Close, is the so-called “hierarchy problem”. This is a theoretical problem with our now experimentally confirmed theory of the Higgs field, which is strongly sensitive to very-short-distance phenomena. We seem to lack a convincing idea that would consistently describe what is happening at unobservably short distances, without requiring an unmotivated and very special choice of parameters in order to get the Higgs physics seen at the LHC.

An increasingly popular tactic for theorists frustrated by not having an answer to these problems is that of postulating a “multiverse”, in which our universe is but one disconnected component, born out of some process that left it with some essentially random choice of fundamental parameters. In this scenario there’s no point in worrying about why these parameters have the scales they do, since somehow the “multiverse did it”, in a manner constrained only by the “anthropic principle”, which says that the parameters must have values consistent with our existence. Close quite rightly raises the issue of whether this is really a valid explanation, since it’s one currently lacking any means to subject it to experimental test.

He then goes on to explain that the current “two clouds” seem to have a root in the same fundamental issue – the lack of a viable general quantum theory of gravity that would unify the theory of relativity with the quantum field theory of the Standard Model. For quite a few decades now theorists have put great hopes in certain speculative ideas proposed back in the 1970s that were supposed to lead towards such a unified theory. Recent years have not been kind, though, to these proposals, with results from the LHC killing hopes for experimental evidence of one of them – supersymmetry – and the great complexity needed to get anything not obviously inconsistent with experiment making the other – string theory – less and less appealing.

I think Close is on the right track with his final argument where he concludes “My conjecture is that in some future theory of everything, space and time will turn out not to be fundamental and will emerge from some deeper concept. Whoever first establishes what this is will enter the pantheon of science, along with Newton, Maxwell and Einstein.”

The lack of a compelling, unified theory that can explain how the degrees of freedom fundamental to the Standard Model and its forces fit together with those describing space, time and the gravitational force is a major hole in today’s “theory of everything”. Perhaps the future will bring a new idea that tidily fills that hole, thereby dispersing Close’s clouds. It’s also possible that the clouds are indications of a storm to come, with new ideas tearing apart the Standard Model, replacing it with a quite different new “theory of everything”. I hope we’ll soon find out which route the future of physics will take.

2017 Profile Books 176pp £8.99pb

Web life: Errant Science

So what is the site about?

Errant Science is a blog about being a scientist and working in academia today. Posts cover a range of topics such as “how to plan your science conference schedule”, “a cynic’s guide to academic papers” and even “how to fund your research after #Brexit: a flow diagram”. As the blog’s author Matthew Partridge puts it, the site is about “life as a university researcher [and is] a strange mixture of sarcasm, cynicism and giddy enthusiasm for science”. Partridge is also a skilled cartoonist and illustrator and most posts involve a graphic of some sort. A particularly commendable feature of the site is that it does not suffer from the irregularity that so many other blogs succumb to – it has a new post every week.

Who is behind it?

Partridge, a postdoc at Cranfield University, UK, has been writing Errant Science alone since 2012. Based at Cranfield’s Department of Engineering Photonics, Partridge began blogging when he set up a departmental website (openoptics.info). Looking for a more suitable space to talk about the wider aspects of academia, he created Errant Science. In March a sister blog named Errant Science Clutter was launched as a space for regular guest contributors, run by Michelle Reeve from the Royal Veterinary College, London. “The whole idea is to show that science doesn’t have to be stiff and serious, it can also be self-deprecating.”

What are some of the topics covered?

Pretty much anything that can come up in the life of an academic in the 21st century. There is a definite trend towards “how to” posts that cover everything from writing papers and theses to coding and big data, as well as presenting data and conference talks. Infographics and comics are included in most posts, as are flow diagrams to help you navigate issues such as “what to do when your experiment goes wrong”.

Who is it aimed at?

Absolutely anyone with an interest in academic life – whether you are a student, early-career researcher or established scientist, Errant Science will either help you or make you laugh. For anyone not in science, the blog is a great peephole into the complex and occasionally perplexing world of academia.

Can you give me a sample quote?

From a post published in March titled “How to get any work done while working from home with kids”: “Without colleagues and coffee breaks to distract me I generally found working from home more productive. I’d set myself a list of things to do and be finished by 10:30, leaving me with the moral dilemma: do I work the same number of hours or do the same amount of work?…But that was back when I had a quiet house. Things are different now – I have two noisy children, neither of whom understand the difference between daddy who can play and daddy who’s drafting a paper. Also the surly cat has got a lot more needy in his old age and insists on being anywhere that will either stop me using the keyboard or the mouse, preferably involving sleeping on one or the other.”

Enjoy the rest of the May 2017 issue of Physics World in our digital magazine or via the Physics World app for any iOS or Android smartphone or tablet. Membership of the Institute of Physics required

Ultracold atoms shed light on the Fermi-Hubbard model

New insights into a popular and potentially useful model of how electrons behave in solids have been provided by an experiment involving ultracold atoms. Markus Greiner and colleagues at Harvard University in the US studied the behaviour of lithium-6 atoms that are held in an optical lattice and interact according to rules set out by the Fermi-Hubbard model.

They found that the system becomes magnetic at low temperatures – and that the magnetism disappears when the density of atoms is reduced. The team can now use its atomic simulator to explore regimes of the Fermi-Hubbard model that could harbour very interesting physics including high-temperature superconductivity.

The electronic properties of solid materials arise from quantum-mechanical interactions between large numbers of electrons. It is notoriously difficult to calculate these properties, so physicists rely on simple models to simplify the mathematics – but even models have significant computational challenges. One such scheme is the Fermi-Hubbard model, which represents electrons as Fermi–Dirac particles (fermions) that hop between fixed sites on a lattice and only interact with each other when they occupy the same lattice site.

Dimensional difficulties

Despite its simplicity, the quantum nature of fermions means that meaningful calculations are only possible for 1D chains of lattice sites. Even calculations on 2D lattices – which could by very useful for understanding high-temperature superconductors – have proven extraordinarily difficult to achieve.

One possible way around this problem is to use a physical system of real particles to simulate the Fermi-Hubbard model – effectively doing an experiment to mimic a model that describes another physical system. Greiner and colleagues have used an ensemble of lithium-6 atoms, which are fermions and therefore obey the same quantum-mechanical rules as electrons. The team created their simulation by criss-crossing laser beams to make a square lattice of potential wells, each of which can hold an atom.

While this approach is not new, it had previously been very difficult to reduce the temperature of the atoms such that they behaved like electrons in a solid. Although previous attempts had chilled the atoms to just a tiny fraction of a kelvin, their thermal motions were on par with electrons in a solid heated about 1000 K. This is much hotter than the 100–200 K below which high-temperature superconductivity occurs, and is also too hot for the emergence of magnetism.

Under the microscope

Greiner and colleagues overcame the temperature problem by surrounding the optical lattice with a sea of atoms that act as a coolant. They also used an optical system dubbed a “fermionic microscope” to monitor individual lattice sites.

The team found that when the lattice was full – or nearly full – of atoms, the system behaved as an antiferromagnetic insulator. According to Thierry Giamarchi of the University of Geneva in Switzerland, who was not involved in the experiment, it is the first time that a system has been cooled sufficiently to create a magnetic state with long-range order. As the number of atoms is decreased, the magnetic state is seen to disappear.

It is in this low-density regime that a state resembling a high-temperature d-wave superconductor is expected to exist – albeit at a lower temperature than is currently accessible to Greiner’s team. Writing in Nature, the team points out that it should be possible to further cool the atoms to reach the superconducting state.

Female academics do more admin than their male colleagues

Female academics do significantly more internal administrative work than their male counterparts, according to an analysis of surveys performed at US institutions. Carried out by researchers at the University of California, Riverside, and Indiana University, the study found that the gender imbalance is highest in science, technology, engineering and mathematics (STEM) fields. While such internal work is vital for the day-to-day running of institutions, it is less valuable for promotions and salary increases than research and teaching, possibly hindering female career progression.

In one survey, which included 6875 tenure and tenure-track faculty at 140 US institutions, female academics reported spending, on average, 0.6 hours more per week than males on admin. The researchers also looked at 2012 data from a mandatory performance reporting system at two campuses belonging to a large public university. Covering 1378 faculty, it showed that women perform 12.4 admin activities per year, while men do just 10.9. In STEM subjects, women reported performing three more admin activities per year than men, compared with 2.5 for liberal arts and 0.3 for social sciences.

Internal imbalance

The researchers found that the imbalance was driven by internal admin – i.e. work related to the running of departments, schools or universities. Men perform an average of 6.1 internal-admin activities per year, while women do 7.3. There was, however, no significant gender difference in “external” admin work – performed for national or international communities.

Cassandra Guarino, professor of education and public policy from the University of California, Riverside, who led the work, told Physics World that women might be doing more internal-admin work because they are less likely to say no and are being asked more often.

Institutional norms

“Research shows that in negotiations women have more difficulty being assertive and they are more penalized for doing so,” says Guarino, adding that individuals can find it hard to gauge how much of such work is normal. “To me, this is the solution to the problem: making it more transparent so that everyone can see what everybody is doing every year,” she says. “It should be required of department chairs to monitor it, to make sure it doesn’t become unbalanced.”

Patricia Rankin, chair of the American Physical Society’s committee on the status of women in physics, explains that if institutions consider it is up to the individual to deal with this on their own, the problem will persist. “If institutions want to retain their women faculty, they can help by setting norms,” says Rankin. “You only have so much time – if you do more [admin] work there is less time for research. Unless the [work] is valued equally to research, this will slow women down in their career progression.”

The study is presented in Research in Higher Education.

Flash Physics: Silver boosts optical computers, debris ring orbits young star, Ernest Moniz joins fusion firm

Silver boosts optical computers

Tiny particles of silver could boost the performance of tomorrow’s optical computers. That is the claim of Tim Liedl and colleagues at Ludwig-Maximilians-Universitaet in Munich and Alexander Govorov and team at Ohio University, who have shown that the addition of silver nanoparticles to a chain of gold nanoparticles makes the chain much more efficient at conducting plasmons. Computers could be much faster and more energy efficient in the future if they used light to transmit and process information, rather than the electrical signals used today. However, the light that is most efficient at transmitting data over optical fibres has a wavelength greater than 1 μm, which is huge compared to the current size of computer circuits. One way of creating tiny optical circuits is to “shrink” the wavelength of the light by converting it into a plasmon – an oscillation in the conduction electrons of a metal that occurs when the material interacts with light. Once converted to plasmons, data within an optical signal could be processed in high-density chips. Plasmons can be conducted through a circuit using a chain of tiny gold particles, with diameters measuring just tens of nanometres. One problem, however, is that plasmon transmission in gold results in the generation of a significant amount of heat – making such conductors no more efficient than those found in conventional computer circuits. Liedl, Govorov and colleagues have shown that putting a silver nanoparticle (diameter 30 nm) between two gold nanoparticles (diameters 40 nm) results in plasmons being conducted along the chain with almost no energy lost to heat. The research is described in Nature Physics.

Solar-system-like debris spotted around young star

A composite image of the Fomalhaut star system - ALMA data (orange) shows the distinct ring, the central dot is the star and the optical data (blue) comes from the Hubble Space Telescope. The dark region is the coronographic mask filtering the star's light — Ring of ice: chaotic destruction surrounds Fomalhaut. Composite image of the Fomalhaut star system – ALMA data (orange) shows the distinct ring, the central dot is the star and the optical data (blue) comes from the Hubble Space Telescope. The dark region is the coronographic mask filtering the star’s light. (Courtesy: ALMA (ESO / NAOJ / NRAO), M MacGregor; NASA / ESA Hubble, P Kalas; B Saxton (NRAO / AUI / NSF))

An icy debris ring surrounding a neighbouring planetary system has a chemical kinship with solar-system comets. An international team reached this conclusion after making the first complete image of the rubble ring using the Atacama Large Millimeter/submilimeter Array (ALMA) in Chile. The planetary system is 25 light-years from Earth and a tenth the age of the solar system. Orbiting Fomalhaut – a young star with twice the mass of the Sun – the system contains one of only 20 planets that scientists have imaged directly. Debris rings are common features for young stars and are thought to be caused by collisions between comets and planetesimals during the system’s chaotic early life. Light from Fomalhaut is absorbed by the rubble and re-emitted as radio waves before being captured by ALMA. The new image shows Fomalhaut’s ring in full, revealing an elongated band of icy dust. “We can finally see the well-defined shape of the disc, which may tell us a great deal about the underlying planetary system responsible for its highly distinctive appearance,” says Meredith MacGregor of the Harvard-Smithsonian Center for Astrophysics in the US. The researchers estimate the band is about two billion km wide and around 20 billion km from Fomalhaut. They also found that the ring’s relative abundance of carbon monoxide and carbon dioxide resembles comets found in the solar system. This suggests the system is going through its own Late Heavy Bombardment – a period four billion years ago when the solar-system planets were frequently struck by asteroids and comets left over from the system’s formation. Two papers presenting the work have been accepted for publication in The Astrophysical Journal.

Former US energy secretary joins fusion power firm

Photograph of Ernest Moniz — Directing fusion: Former US energy secretary Ernest Moniz has joined Tri Alpha Energy. (Courtesy: MIT Energy Initiative)

The nuclear physicist and former head of the US Department of Energy (DOE) Ernest Moniz has joined the board of directors of Tri Alpha Energy. Based in Foothill Ranch, California, the privately held company is trying to develop an “aneutronic” fusion power system that is based on nuclear-fusion reactions that do not produce large amounts of neutrons. If it can be made to produce energy on a commercial scale, the company’s ion-beam-based system would not have to contend with the damaging neutron radiation that would be generated in other fusion power schemes. Moniz served as US energy secretary under Barack Obama in 2013–2017 and is currently an emeritus professor of physics and engineering at the Massachusetts Institute of Technology.

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