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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.”

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.

Skyrmions: a twisted future in data-storage technology

Having a particle or even a whole class of particles named after you is one of the great legacies in physics. Those who share this rare honour include Enrico Fermi, Satyendra Nath Bose and Peter Higgs. But have you ever heard of the British physicist Tony Skyrme? In the early 1960s Skyrme developed a theoretical model describing forces in atomic nuclei. Within his theory, topological solitons emerge as particle-like solutions of nonlinear field equations, which five years before Skyrme’s death in 1987 became known as skyrmions.

Recently, one variety of skyrmion, the magnetic skyrmion, has emerged as a hot topic in physics. This is partly due to its topological properties, which result in a rich variety of magnetic phenomena, and partly from its potential as a “bit” in future data-storage devices. Rather than being a fundamental particle, such as an electron, magnetic skyrmions are quasiparticles, which can emerge as a collective phenomenon in magnetic materials with broken inversion symmetry. Like real particles, such as atoms, they are localized, can move around, interact with each other, form lattices and have antiparticles. However, skyrmions are confined to the magnetic material as their carrier and have no existence outside it.

1 Magnetic knots

Sketch of a skyrmion and an antiskyrmion
(Courtesy: Kirsten von Bergmann and André Kubetzka)

Sketch of a skyrmion and an antiskyrmion. Each arrow represents the local magnetization direction, which varies from upwards (blue) in the centre through horizontal (white) to downwards (red) in the surroundings. While these two quasi-particles are topologically distinct, they both wrap a unit sphere exactly once (see inset). If you follow the arrows indicated by the white circles, they rotate in different directions, which means their topological indices have the opposite sign: +1 for skyrmions and –1 for antiskyrmions.

In a magnetic material, the atomic magnetic moments are coupled with each other and so produce a vector field of magnetization that is almost continuous. In a ferromagnet, for example, all magnetic moments point in the same direction. In a particle-like skyrmion, however, the magnetic moments in its centre point in the opposite direction to those of its ferromagnetic surrounding (figure 1). These magnetic configurations are characterized by a quasi-continuous rotation of the direction of the atoms’ magnetic moments relative to their neighbours. In fact, there are two basic types: the skyrmion and the antiskyrmion. The difference between them is the way they locally twist: skyrmions have rotational symmetry, whereas antiskyrmions have a two-fold rotation axis, i.e. they look the same only when rotated by 180°. The shape of such skyrmions and antiskyrmions is very robust and in some sense they behave like particles, living in a sea of parallel magnetic moments pointing opposite to the one in the centre.

Skyrmions and antiskyrmions, which are like knots in the magnetization, are classified by their topological index. Annihilating a knot turns out to be easy when using a knot with opposite topological properties

Skyrmions and antiskyrmions, which are like knots in the magnetization, are classified by their topological index, which can be viewed as the number of windings around a unit sphere. Unwinding such a single knot would be impossible, if, in an imaginary world, magnetic materials were continuous rather than being composed of atoms. However, annihilating a knot turns out to be easy when using a knot with opposite topological properties. A skyrmion and an antiskyrmion, if brought together, can smoothly unwind to form the topologically trivial ferromagnetic state, similar to the situation in particle physics where electrons and positrons annihilate when brought together.

Keeping stable

In the real world, topology alone cannot stop skyrmions or antiskyrmions from collapsing to the ferromagnetic state. Instead, their stability depends on an energy barrier related to the so-called Dzyaloshinskii–Moriya interaction (DMI), a phenomenon that occurs due to spin–orbit coupling (the interaction of spins with their motions). The DMI stabilizes skyrmions and stops them collapsing, but usually it does not contribute to the energy of an antiskyrmion, which causes the latter to be unstable.

The DMI in general favours adjacent magnetic moments having a twist between them, rather than them being parallel, but importantly it also selects a material-specific rotational sense. Analysing the two magnetic objects in figure 1 demonstrates that the rotation direction is coherent across a skyrmion, but within the antiskyrmion it is changing: following the indicated yellow lines from left to right, the rotation is clockwise, whereas for the perpendicular green line across the antiskyrmion, the rotation is anticlockwise. Let’s consider a case where the DMI favours a clockwise rotation of the magnetization. Speaking in terms of energy, the antiskyrmion gains DMI energy in one direction and pays the same energy penalty perpendicular to it. In contrast, the skyrmion gains DMI energy in every direction, making it the energetically more favourable state.

The DMI is able to impose a unique rotational sense but is itself subject to selection rules: it may only occur in systems that lack inversion symmetry; in all other systems the effective DMI vanishes. Typical skyrmion materials are thus either those with chiral crystal structures, or magnetic films, which lack inversion symmetry due to the presence of a surface or interface.

Harnessing skyrmions

The current interest in skyrmions is fuelled by their promise for future information technology. Their potential to be harnessed in this way is down to their magnetic twist, which interacts strongly with electrical currents. Electrons traversing through such spin “textures” can transfer their spin to atomic magnetic moments and thus induce a torque, resulting in a movement of the spin texture through the material. Pure spin currents generated in an adjacent material can also trigger a motion, which is then sensitive to the magnetization rotation direction, i.e. clockwise rotating skyrmions move in one direction and anticlockwise twisting states in the other.

2 Under the microscope

This spin-resolved scanning tunnelling microscopy measurement shows skyrmions in an ultrathin magnetic film
(Courtesy: Kirsten von Bergmann and André Kubetzka)

(a) This spin-resolved scanning tunnelling microscopy measurement shows skyrmions in an ultrathin magnetic film made of palladium and iron atoms on a highly ordered iridium surface in a magnetic field, B, of 1.1 T, at a temperature of 8 K. Colours have the same meaning as in figure 1. (b) The precise orientations of the atomic magnetic moments that form the indicated tiny magnetic knot were derived from such a measurement.

The experimentally realized sizes of skyrmions range from the micron- down to the nanometre-scale, eventually comprising barely a few dozen atoms. An example of particularly small skyrmions is those that can form in a bi-atomic layer of palladium and iron on a highly ordered iridium single-crystal surface (figure 2a). Due to the magnetization of the probe tip, the tunnel current is sensitive to the magnetization of the sample, and in this case a magnetization parallel to the tip magnetization is coloured red, whereas the opposite magnetization component is blue. The diameter of a skyrmion (the distance between opposite in-plane spins) depends on the magnetic interaction parameters and the external magnetic field and for those displayed is about 3.5 nm, which corresponds to about 13 atomic distances. Figure 2b shows the atomically precise sketch of the spin structure, which was derived from the experiment.

As for how magnetic particle-like states could be used in information technology, in 2008 IBM fellow Stuart Parkin and colleagues proposed that magnetic domain walls could be used as information carriers (Science 320 190). This idea was extended to magnetic skyrmions in 2013 by Albert Fert and colleagues at the Université Paris-Sud (Nature Nanotechnology 8 152). The presence of a skyrmion at a specific position could mean a “1” and the absence a “0”.

In this so-called “racetrack memory”, magnetic domain walls, or alternatively skyrmions, are moved through a magnetic material by electrical currents (figure 3). In contrast to a conventional computer hard disk where the recording medium is moved mechanically with respect to the read/write heads, in racetrack memory the magnetic bits move along the racetrack, but the racetrack itself (as well as the read/write elements) remains stationary. Lacking any moving mechanical parts, such a device is extremely shock-resistant and therefore also ideal for mobile applications. In addition, the racetrack concept can be a 3D information storage medium, whereas hard drives are 2D. This memory concept, in combination with small skyrmion sizes, has the potential to boost the storage density of data.

Several challenges need to be tackled, however, before skyrmions can be used in racetrack memory for everyday applications. Ordered skyrmion lattices in chiral magnetic bulk crystals can be moved smoothly by electrical currents, but for technology, independent magnetic skyrmions are crucial for the representation of a bit. Such individual magnetic skyrmions are found in magnetic thin-film skyrmion materials, which are compatible with current magnetic multilayer device technology. However, in many of these materials the skyrmions get stuck or “pinned” at defects such as grain boundaries or imperfections, which is a problem. Also, the skyrmions that are stable at room temperature are still one or more orders of magnitude larger than the small isolated nanoscale skyrmions, such as those in figure 2, that have been observed at low temperatures.

3 Memory of the future

A sketch of how a skyrmion-based racetrack memory would be set up.
(Courtesy: Kirsten von Bergmann and André Kubetzka)

A skyrmion-based racetrack memory would involve equidistant “bits”, where the presence of a skyrmion indicates a “1” and the absence of a skyrmion indicates a “0”. These bits would be moved along a magnetic track (red), using electrical currents, and they would pass a stationary read/write element. The element depicted here consists of a contact (yellow) that is separated from the track by a tunnel barrier (transparent).

Towards skyrmion devices

Size does matter. Bits need to be small, but the tinier they get the harder it is to do reading and controlled writing. However, it has been shown in scanning tunnelling microscope experiments that it is possible to write nanoscale skyrmions reliably using electric-field-driven switching. This proof-of-principle experiment exploits the electric field between the probe tip and the sample. An electric-field-driven mechanism is also compatible with the racetrack memory concept by using a metallic contact, separated from the magnetic track by a tunnelling barrier. This could serve as a combined element that both reads and writes the information, as in figure 3.

In principle, every operation needed for a racetrack memory based on magnetic skyrmions – the all-electrical movement, and the reading and writing of the information – has been demonstrated. However, the individual tasks were realized in different materials, for different length scales of skyrmions, and in different temperature and magnetic-field regimes. The challenge now remains to unify all of these operations into a single prototype device.

Einstein, Hawking and Rees set to music, singing about virtual particles, tiny satellite will soon blast off

“What I wanted to write was something about the universe and our place in it: from the Big Bang, through our insignificance in the vastness of it all, our need for exploration and where space travel will take us, to the nature of light or the make-up of electrons, and finally ideas about multiverses and infinity.”

That is the motivation behind the “secular oratorio” Space Time Matter Energy by Simon McEnery, which premieres at St Mary le Strand Church in London on 10 June. The piece melds the words of famous physicists such as Stephen Hawking, Martin Rees and Albert Einstein with music and song from the Salisbury Chamber Chorus, the percussion ensemble Beaten Track and the pianist Peter Toye.  If you can’t be in London on the 10th, there is also a performance in Salisbury on 17 June.

Sticking with the musical theme, theoretical physicist Sabine Hossenfelder’s career as a singer-songwriter looks set to take off with the release of three music videos in one month. Her latest song is about virtual particles and you can watch it above.

He may be just 15 years old, but India’s Rifath Shaarook has designed and built what is claimed to be the lightest satellite ever to be launched by rocket. Shaarook made the external shell of his 64 g satellite from 3D-printed carbon fibre. “It will have a new kind of onboard computer and eight indigenous built-in sensors to measure acceleration, rotation and the magnetosphere of the Earth,” he told the Daily Telegraph. The satellite was a winning entry in the Cubes in Space design competition and will be launched in June by NASA. The rocket will follow a non-orbiting parabolic trajectory before returning to Earth.

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