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Home » Page 1313

Focus on: Big Science

By Michael Banks

POct12-BigScience-cover200.jpg

Just a couple of weeks ago the European Commission kick-started the Extreme Light Infrastructure Nuclear Physics Facility (ELI-NP) project by announcing €180m towards its construction.

ELI-NP, costing €350m and to be built near Bucharest in Romania, will generate laser pulses with a power of some 10 petawatts (1016 W) – intense enough to study nuclear transitions in unprecedented detail.

The facility is one of four centres planned as part of the huge ELI project – the others being a centre in Hungary for attosecond physics, a third working on laser-based particle-beam production in the Czech Republic, and a fourth on ultrahigh-powered lasers. The latter’s location is still up for grabs.

Along with the ITER experimental fusion reactor in Cadarache, France, and the European Spallation Source in Lund, Sweden, ELI is just one of a whole host of “big science” facilities set to come online in the coming decade. Indeed, the Square Kilometre Array is now nearer to construction following a decision in May to split the facility between Australia and southern Africa.

It doesn’t stop there, with physicists looking even further ahead such as to a successor to CERN’s Large Hadron Collider, a muon collider, as well as an electron–ion collider that would be able to study gluons in unprecedented detail.

In a special focus issue accompanying the October edition of Physics World, and available to view free here, we take a look at the technical challenges in building and designing some of these big science facilities.

I hope you find this focus issue stimulating and please do let us have your comments by e-mailing pwld@iop.org.

Here’s a rundown of what’s inside:

• A phased approach – Jon Cartwright looks at the technology behind phased arrays – a key part of the planned Square Kilometre Array

• Planning the world’s next collider – An interview with linear collider director Lyn Evans on what comes next after CERN’s Large Hadron Collider

• The attraction of superconductors – Development of a magnet built from high-temperature superconductors will be at the heart of a proposed muon collider, as Tim Wogan reports

• Exploring “the mass that matters” – Peter Gwynne describes plans for an electron–ion collider – a new kind of facility that would study the properties of gluons

• New eyes for a dark world – Technology based on superconducting circuits will allow astronomers to detect every photon that arrives at a telescope’s lens, as David Appell explains

• Turkey accelerates ahead – Michael Banks travels to Ankara to hear plans for a Turkish Accelerator Centre

• Illuminating new frontiers – Brian Stephenson, director of Argonne National Laboratory’s Advanced Photon Source, gives his opinion on why the future is bright for light sources

Read the Focus on Big Science now

Survival of the fittest nanoantenna

Physicists in Germany have used evolutionary algorithms to help them pinpoint the best geometry for a nanoantenna. As well as zeroing in on the optimal design out of more than 10132 alternatives, the technique has provided unexpected new insights into the complex optical properties of nanostructures.

Nanoantennas convert light to electrical power and vice-versa, and are essential in the design of tiny electro-optical devices. They have diverse potential applications in just about anything based on light–matter interaction, including optical sensing and signalling, microscopy, solar-power conversion and quantum cryptography.

An antenna’s ideal size is dictated by the wavelength of the radiation that it handles. For radio waves, this is of the order of metres, and for light it is hundreds of nanometres. Most research so far has focused on designs that are essentially miniaturized radio antennas. However, tiny metal components interact with light in a much different way from how radio waves interact with larger components – and this means that shrinking radio designs is not necessarily the best approach to take.

Employing evolution

Instead, Thorsten Feichtner and colleagues of the University of Würzburg have used evolutionary optimization to wheedle-out the most structurally well adapted nanoantenna for their purposes. Inspired by natural selection, evolutionary optimization algorithms work towards an ideal design rather than evaluating the performance of all possible designs. For the problem tackled by Feichtner’s team, the latter would be impossible because more than 10132 antenna designs would need to be evaluated using a process that takes 20 minutes per structure.

The team’s goal was to find a geometry that would enhance the near-field intensity of an illuminating beam of light as much as possible, so they chose this as the “fitness parameter” that they would judge each design against. Their antennas consisted of 21 by 21 matrices of gold cubes measuring 10 nm on the side. Beginning with patterns that were generated randomly, they ran simulations on batches of 20 designs to establish which were best adapted to the design goal.

Just as in nature, the fittest patterns got the chance to pass on their characteristics to the next generation, while the weaker specimens were discarded. The highest-performing five from each batch were used to build a new generation of 20 structures via crossing techniques and mutations. The new structures were in turn pitted against one another, so the overall fitness of the designs improved generation by generation – over 100 generations – until the near-field intensity enhancement registered almost twice that of the reference antenna.

Puzzling patterns

“In the end, we found that the best shape was a very random-looking pattern of gold blocks,” says Feichtner. The central part of that design was reminiscent of a classical antenna though: two rod-like structures separated by a gap. When the researchers took a close look at the currents flowing in that area, they discovered that the rods were connected by a single gold block, positioned just below the gap between the rods.

“If you look at the problem purely geometrically, you should have no conductive link between two blocks which are just touching at the edges. But the simulation approach we used allowed current to flow along the edges from one cube to another,” explains Feichtner. “This was a bit of a surprise to us … and when we removed just this one block, the fitness decreased by a factor of two.”

“Of course, this whole structure is very complex and we’re far away from understanding what is happening everywhere,” he adds. Rather than pursue that answer too doggedly, the team instead came up with a stripped-down version of their fittest candidate, which might feasibly be built with today’s technology – essentially a hybrid of a two-wire antenna and the split-ring resonator geometry that they discovered – and showed that it, too, boasted the same two-fold increase in fitness factor.

Hot topic

This is certainly not the first study to demonstrate that nanoantennas can be optimized using evolutionary algorithms but,” says Feichtner, “[previous studies] were restricted to changing the shape of one particle, or using a defined-shape particle and moving them round or changing the size. Our approach allows one to build up arbitrary geometries made of one or multiple particles, and also gives the possibility to see how large connected structures will work. The split-ring geometry would never have been found with the approaches that have been used thus far.”

The study indeed “introduces a very original way of designing optical antennas”, according to Sébastian Bidault, who investigates DNA-based fabrication of nanoantennas at the Institut Langevin in Paris, France, and was not involved in the research.

He points out, however, that one of the main drawbacks of metal-based nanoantennas is strong losses due to local heating. “Optimizing local intensity enhancement by adding more metal that will induce more ohmic losses [fails to] consider what happens to this energy afterwards.”

The research appears in Physical Review Letters.

Hair’s where it’s at

Humans have a long and fruitful history of looking towards nature for ideas to build new technologies or solve problems. From Leonardo Da Vinci studying the flight of birds to develop the earliest “flying machines”, to the Swiss engineer George de Mestral developing Velcro after studying the surface of burrs, nature has long been influencing technologies.

A recent review paper published in the journal Smart Materials and Structures takes an in-depth look at the different “hairy” sensors that a whole host of animals possess. This could help us to develop our own sensors to serve a multitude of purposes from gauging flow turbulence to more efficient liquid-dispensing methods to developing robots that can successfully navigate underwater or underground and other biomedical applications. Such sensors would require many capabilities such as short response times and low detection thresholds – capabilities that already exist in animals.

Many life-forms live in conditions that are constantly changing and so have adapted a wide range of sensory strategies to survive. In the paper, the authors point towards many examples. Mexican blind fish rely on a” lateral line system” to detect movement and vibration in the surrounding water. Crickets use their hairy “cerci” or feelers that provide them with flow information that let them know if another creature is approaching them. Caterpillars also use cerci to detect airborne disturbances that let them know when predators, like flying wasps, are overhead. Meanwhile, bats control their flight by monitoring air-flow conditions via hairs on their wings.

According to the paper, “Among the various flow sensors in nature, the instinctive flow sensors of aquatics and arthropods are the most intensively studied.” In the initial sections the researchers look at the “morphology, function and biomechanics of the lateral line neuromas of aquatics and the fusiform hairs of arthropods are examined to shed light on the development of their artificial counterparts”. In later sections they divide the types of sensors that can be developed into six categories: thermal, piezoresistive, capacitive, magnetic, piezoelectric and optical sensors. They look at various groups around the world that are currently developing some of the different types of sensors considered and the different methods they use. The final discussion looks at how to best optimize such sensors, process the information they would provide and how the field will progress in the future.

So for an insightful look into all sensors hairy, take a look at the paper here.

Origins of galactic jet seen for the first time

Astronomers have for the first time observed the base of the jet emanating from the M87 galaxy. The result reveals a spinning black hole at the centre of the galaxy and future refinements of the technique could provide the sternest test yet of Einstein’s general theory of relativity.

Most galaxies, including our own Milky Way, are thought to harbour a supermassive black hole at their heart. In some galaxies, matter falling into the black hole forms an accretion disc that generates huge amounts of radiation across the electromagnetic spectrum. Such galaxies are called “active galaxies”. Even more spectacular are the 10% of active galaxies that also have a relativistic jet of matter emerging from their cores.

A prime example is M87, a huge elliptical galaxy located just over 50 million light-years from Earth. M87 sends a long, thin jet of plasma 5000 light-years into space. However, the radiation at the centre is so intense that the photons of light often scatter off each other, blocking astronomers’ views of the base of the jet. Despite being one of the most studied of all relativistic jets, this has left unanswered questions regarding its exact formation mechanism.

A grapefruit on the Moon

Now, astronomers, led by Sheperd Doeleman of the MIT Haystack Observatory in Massachusetts, US, have managed to glimpse the base of the jet for the first time using the Event Horizon Telescope (EHT). “We’ve fashioned an Earth-sized virtual telescope by linking radio dishes in California, Arizona and Hawaii,” Doeleman tells physicsworld.com. Combining widespread telescopes in this way is known as very long baseline interferometry (VLBI). Such a long baseline meant that Doeleman and colleagues could achieve unprecedented resolution. “It is about the same as resolving a grapefruit on the surface of the Moon,” he explains.

The base of the jet was revealed to be only 5.5 Schwarzschild radii in extent – significantly smaller than the accretion disc. The Schwarzschild radius is the distance from the centre of the black hole to where the velocity required to escape its gravitational pull exceeds the speed of light. The diminutive size of the jet-base allowed the team to infer some properties of the black hole producing it. “Models suggest the jet would appear 7.4 [Schwarzschild radii] across for a non-spinning black hole,” says Doeleman. “For a spinning black hole, where the accretion disc orbits in the opposite direction to the spin, we’d expect it to be more than 9,” he adds. The fact that it is as small as 5.5 implies the black hole at the heart of M87 is spinning, but that the accretion disc follows the direction of spin.

It has long been thought that spinning black holes might create the relativistic jets in active galaxies, but proof had remained elusive. “People had assumed, but we’ve now found evidence and a technique to back it up,” Doeleman explains. “We’ve formally linked very small-scale regions, where general relativity is important, to large-scale jet structures,” he adds. Einstein’s theory could come under further scrutiny as the fledging EHT telescope is developed in the coming years. It is hoped that the Atacama Large Millimetre Array (ALMA), currently under construction in the Chilean desert, could join the other EHT telescopes as soon as 2015. “Adding ALMA would double our resolution overnight,” says Doeleman.

Shadowing general relativity

Such an upgrade would allow astronomers to observe the black hole’s “shadow”. Because of the extreme warping of space by the intense gravity of the black hole, light travelling away from the Earth is bent back round to form a ring with a dim patch – or shadow – in the centre. The exact size and shape of this shadow is governed by Einstein’s equations of general relativity. Comparing prediction to measurement under these extreme conditions could be the ultimate test for the theory. “We’ll soon have the tools to push Einstein to the limit. If his theories are going to break down anywhere then it will be here,” Doeleman suggests.

Rob Fender of the University of Southampton, UK, is excited by the findings. “This work is absolutely fantastic,” he says. “That they’ve managed to see the base of the jet is quite incredible.” Despite not being entirely convinced by the team’s conclusions about the black hole’s spin, Fender sees this as a potential breakthrough in observing the immediate shadowy environment around black holes. “We’re now only a few steps away from being able to directly image the effects of the black hole at the centre of our own Milky Way,” he explains. “Many scientists are still sceptical about the existence of black holes. A direct image of the area right up close to one would provide some pretty weighty evidence,” he concludes.

The research is described in Science.

Physicists in Japan have discovered element 113. What should they call it?

Facebook poll

By Hamish Johnston

Nine years after physicists in Japan caught the first glimpse of element 113, researchers working at the RIKEN Nishina Center for Accelerator-based Science have obtained conclusive evidence that they have produced the elusive element.

They spotted six consecutive radioactive decays that begin with an isotope of 113 and end at mendelevium-254. According to a statement from RIKEN, the discovery can be claimed because all six of the decay products were identified unambiguously by the team – which was led by Kosuke Morita.

The statement also says that the discovery “promises to clinch [the Japanese physicists’] claim to naming rights for the 113th element”.

In this week’s Facebook poll we ask:

Physicists in Japan have discovered element 113. What should they call it?

Rikenium (honouring Japan’s national research labs)
Nishinium (honouring pioneering nuclear physicist Yoshio Nishina)
Japonium (honouring Japan)
Other – please suggest with a comment

Have your say by visiting our Facebook page, and please feel free to explain your response by posting a comment below the poll.

Last week we asked “Should the convention for awarding the Nobel Prize for Physics be changed so that it can be given to a large collaboration?” The majority of you (77%) said yes – including Peter Cuttell, who said “The way in which physics is done has changed since the Nobel Foundation was established. The scale, complexity and expense of experiments has required that large (and sometimes multinational) collaborations undertake them.”

Slow-cooling super Earths could lack life

Rocky exoplanets with masses that are between 2–10 times that of the Earth may not have the long periods of volcanic activity that are thought to be essential for life to evolve. That is the conclusion of a new study by scientists in the US and Germany into the rate with which these “super Earths” cool. The research also suggests that understanding the behaviour of a super planet’s interior over long periods of time can provide important information about conditions on its surface.

Astronomers have so far found more than 600 super Earths orbiting stars other than the Sun – and many more are expected to be discovered by the Kepler space telescope and other future missions. Super Earths currently offer astronomers the best opportunity to look for signs of extra-terrestrial life because they are easier to find and study with today’s telescopes than are Earth-sized exoplanets.

If a super Earth were in the habitable zone of its star – where, in principle, the surface of the planet could be at the right temperature to have liquid water – such an exoplanet could sustain life. Being in the habitable zone alone is, however, not enough because the conditions on the surface of the super Earth must be conducive to life.

Volcanoes needed

The new study is by Vlada Stamenkovic of the Massachusetts Institute of Technology and colleagues from the DLR Institute of Planetary Research in Berlin. It suggests that super Earths are unlikely to have the long periods of volcanism that is believed to play an important role in making the surface of a planet suitable for the emergence of life. In the case of our own planet, heat transferred from deep within the Earth via convection in the mantle makes the continents shift round like large plates, with this “plate tectonics”. This triggers long periods of volcanic activity, which spews out so much CO2 that it leads to greenhouse warming. This keeps the Earth’s surface cosily warm, with most of the water being liquid rather than frozen.

Ongoing volcanism also plays an important role in the geological carbon cycle, whereby CO2 held in rocks is put into the atmosphere by volcanoes and then incorporated back into the planet’s interior via rock weathering and plate tectonics. This cycle is believed to contribute to Earth’s relatively stable climate, which in turn is believed to be a prerequisite for life.

Stamenkovic and colleagues have looked at a number of different factors that could affect the conditions on the surface of a super Earth. They began by considering the viscosity of the interior of a super Earth, which determines how quickly heat moves via convection from the hot centre of a planet to its much cooler surface. Planets with high-viscosity interiors transfer heat more slowly than those with low viscosity.

Pressure-dependent viscosity

Most calculations to date have used a formulation for viscosity that is dependent only on temperature but not pressure – a formulation that doesn’t even work for the Earth, according to Stamenkovic. Instead, the team argues that at the greater pressures found in super Earths the correct formulation for viscosity should also include a term that is a function of pressure. The result is a much larger viscosity than used in previous studies of super Earths.

The team then looked at how the temperatures within a super Earth would change over its lifetime – because both the temperature and pressure define the viscosity inside a planet. They first calculated what would happen if super Earths were as “cool” as generally predicted by previous studies. At these relatively low temperatures Stamenkovic and colleagues predict that there will be no convection in the lower mantle. In this “stagnant” scenario, the exoplanet cools slowly.

However, more recent studies suggest that super Earths are much hotter when they first form – perhaps even molten. Under this hot-formation scenario, the team found that convection will occur, albeit at a sluggish rate. In both cases the cooling of the mantle and the core would be inefficient, according to Stamenkovic.

Armed with this information, the team then looked at the implications of this sluggish cooling on the habitability of a super Earth. In both cases, the slow cooling of the core and mantle of a super Earth suggest that plate tectonics is less likely to occur. Stamenkovic points out, however, that the water content of the lithosphere also plays an important role in determining whether plate tectonics will occur, and might reverse such trends.

Laboratory experiments needed

According to Stamenkovic, this latest research shows the importance of understanding the time evolution of super Earths, rather than trying to calculate their steady-state properties. To gain a better understanding of how heat is transferred in super Earths, Stamenkovic says that more high-pressure experiments need to be done in labs here on Earth and that more data on the densities and atmospheres of super Earths is required. To gather this information, he supports a number of low-cost space telescopes that would each focus on one nearby super Earth for the long periods of time needed to gather enough data. Such a project has been proposed by a team at MIT led by Sara Seager and is called ExoplanetSat.

The art of science blogging

The cosmologist Mark Trodden is one of the scientists to grab this opportunity, through his regular contributions to Cosmic Variance, a blog hosted by Discover magazine. In this audio interview with Physics World, Trodden talks about how his urge to start blogging was triggered by a long-standing interest in politics.

A flair for communication
A flair for communication

Trodden – who by day works at the University of Pennsylvania – shares his thoughts on what he believes is the key to a successful blog. He talks about how he draws his inspiration for new blog entries and the need to regularly produce new blog articles in order to generate a buzz. Trodden also addresses some of the challenges facing bloggers, such as moderating reader comments to generate productive discussions.

One amazing moment

"What was the single biggest turning point of modern science?"

My colleague Phil Allen, a condensed-matter theorist, greeted me with that question the other day, suggesting by his tone that obvious answers were incorrect. I tried some anyway.

"Rutherford's 1911 discovery of the nucleus?" He shook his head. Ernest Rutherford was right in concluding that an atom's mass resides mostly at its centre, but his model was not taken seriously for two years because orbiting electrons ought to radiate energy and fall into the nucleus, which does not happen.

"The Bohr atom of 1913–1914?" Phil conceded that Niels Bohr's model was an enormous leap, for it saved Rutherford's idea by demonstrating that the electrons could not radiate energy and were confined to specific orbits. But it did not work for any atom besides hydrogen because it neglected things such as electron–electron interactions.

"Planck, the quantum, 1900?" But Max Planck initially saw quantization as only a mathematical fix and five years elapsed before someone (Einstein) suggested the quantum governed not oscillators but light itself. It took another decade or so for that idea to spread.

"Compton scattering, 1923?" The finding that light quanta are particle-like in that they carry momentum as well as energy is the subject of a book, The Compton Effect: Turning Point in Physics by Roger Stuewer. But it affected only atomic physics, and Phil was asking about something with an impact on many scientific fields.

I was still drawing a blank when Phil helped me out. "This year's the centennial," he said.

It was 100 years ago

That gave it away: the discovery of X-ray diffraction in crystals, in what Einstein called one of the most beautiful discoveries in physics.

The year 1912 was an ambiguous time in the history of science, when it was not fully certain whether X-rays were electromagnetic waves, or even if crystals consisted of regularly spaced atoms. Early in 1912 the German theoretical physicist Max von Laue – in discussions with a graduate student, Paul Peter Ewald – realized that one way to test some of these ideas was to shoot X-rays through crystals and look for a diffraction pattern.

On 21 April 1912 the experimentalists Walter Friedrich and Paul Knipping irradiated copper-sulphate crystals, large pieces of which were easily available, and soon obtained a pattern of regular points on photographic plates, now called "Laue diagrams". Laue, Friedrich and Knipping announced the result to the Bavarian Academy of Sciences on 4 May, and two years later Laue won the 1914 Nobel Prize for Physics "for his discovery of the diffraction of X-rays by crystals".

X-ray diffraction provided two things: a way to find out where atoms in crystals are, and a way to measure X-ray wavelengths, and thus determine frequencies and energies. Suddenly X-rays became a highly productive tool with a huge scope. Spectroscopy, for instance, can only be done by using a crystal as a diffraction grating to provide monochromatic X-rays of known energy. "In the truest sense of the word," science historian Armin Hermann once wrote, "scientists began to cast light on the structure of matter."

Events moved swiftly. William Henry Bragg and his son Lawrence masterfully exploited diffraction, discovering "Bragg's law" and creating the first X-ray spectro-meter, for which they shared the 1915 Nobel prize. Information provided by diffraction enabled Henry Moseley to conduct his famous studies of the X-rays emitted by atoms. The Dutch lawyer and amateur physicist Antonius van den Broek – one of the quirkier figures in nuclear physics who was fascinated by numerical regularities – pointed out that the "atomic number" Z, not the "atomic weight" A, was the key parameter in diffraction, which implied the existence of a positive, charge-carrying particle. In other words, X-ray diffraction proved that Z is the number of electrons in an atom, thereby giving theoretical meaning to Bohr's picture of the atom.

The critical point

Despite these positives, the discovery of X-ray diffraction is rarely heralded as a turning point in science. Even the venerable Abraham Pais gave it just a one-sentence nod in his authoritative history of 20th-century physics Inward Bound. So why the neglect? One reason is the false impression that X-ray diffraction is a mere technique of crystallography rather than a discovery with a broad and significant impact on physics. Another is our tendency to overvalue theoretical breakthroughs. Finally, condensed-matter physics is not seen as being as "sexy" or fundamental as other areas of physics.

The discovery is, however, familiar to science historians as an example of the fight between "internalists", who take guidance from memoirs and recollections of participants, and "externalists" who view science history as the province of professional historians. In 1962, on the discovery's 50th anniversary, Ewald published a book called Fifty Years of X-ray Diffraction that included recollections by participants. Shortly afterwards, the maverick historian Paul Forman attacked science history based purely on recollections, which he said tended to involve "gross misrepresentations" that neaten conceptualizations and purify motives (1969 Arch. Hist. Exact Sci. 6 38). Ewald's reply blasted Forman's article as "worthless except as an example to which conclusions the study of literature under a biased point of view can lead" – the bias being a desire to prove the thesis that the scientists were self-aggrandizing. The exchange in turn inspired the journal's editor, a historian, to remark that "physicists are as pig-headed and superstitious as any monk of the dark ages".

These disputes apart, it is clear to me that the discovery of X-ray diffraction was a central event in modern science. "That discovery brought it all together," said Phil. "It suddenly made the atom real and accessible to accurate measurement, showed us what it 'looked like' – and how it was arranged in molecules and solids. For me, the year 1912 in science was the amazing turning point."

Spin friction seen for a single atom

An international team of researchers says that it has measured the spin-dependent component of friction of a single atom as it slides across a magnetic surface, for the first time. The team used a combination of spin-polarized scanning tunnelling microscopy and single-atom manipulation to push individual magnetic atoms over a magnetic template. Comparing experimental results with simulations, the researchers were able to conclusively say that spin-dependent friction contributes substantially to the overall frictional force experienced by an atom. They say that their results provide an essential step towards developing a fundamental theory of friction.

Although friction is one of the most pervasive forces in the universe, and is one of the oldest-known physical phenomena, the intricacies of the fundamentals of friction and its origins have eluded scientists. The study of friction, or "tribology", looks at surfaces in contact from the macro to the nanoscale. A basic issue is to single out and connect an observed frictional force between two contacting materials with the physical, chemical and mechanical properties of the materials that cause the friction. But this is easier said than done.

Spinning free

Over the years, scientists have determined two factors that contribute substantially to friction at the macro level – namely, electronic degrees of freedom and lattice vibrations. "So if the electron system in the atom plays such a crucial role in friction, it is clear that we must consider spin too, as an electron possess not only charge but also spin," explains Roland Wiesendanger of the University of Hamburg, Germany, who is one of the authors of the new work. He, along with lead author Boris Wolter and colleagues, has been interested in spin-dependent phenomena at the atomic scale for more than 25 years and has found very little experimental interest in spin friction. "This is probably because it is very difficult to find a system that allows us to identify spin friction easily," says Wiesendanger. If one considers a ferromagnet, such as iron, for example, all the magnetic moments or spins are perfectly parallel, "so it is virtually impossible to distinguish between spin-dependent and normal frictional contribution", Wiesendanger explains. "You must design a model system where the electronic properties are different from the magnetic properties," he says.

That is precisely what the team did – it considered using an anti-ferromagnetic material, wherein neighbouring spins always point in opposite directions. In fact, the researchers created a manganese-on-tungsten-substrate material that is anti-ferromagnetic on the small scale and shows a spin-spiral state of about 170° between adjacent rows on the larger scale. (See image part (a)). They then studied the friction of this system by using a spin-polarized scanning tunnelling microscope (SP-STM) with a cobalt (Co) atom at the tip that slides over the manganese layer, with the Co atom moving from one lattice site to the next. The spins in the anti-ferromagnetic manganese layer are alternating, and the Co atom's spin aligns with the nearest manganese atom, giving an alternating pattern. Bright and dark lines indicate atomic rows with parallel and antiparallel magnetization components relative to the tip magnetization.

Polarized tip

An SP-STM tip is used because if a normal metal tip was used to drag the Co across the manganese surface, the image shows only the symmetry of the atom lattice (see image part (c)). But when the team used a spin-polarized tip, it showed the weak magnetic coupling to the cobalt, and it is this image that reveals the magnetic order of the structure, showing the spin-dependent component. "Another appealing point about the magnetic manipulation imaging technique is the extremely high spatial resolution and large magnetic corrugation, although at the cost of slightly increased experimental and analysing effort," says Wolter. "The first experimental discovery was a little bit of serendipity while working on SP-STM manipulation experiments with the sample, and the technique was then refined to improve the quality of the images for further study," he explains.

To confirm the fact that the spin degree of freedom contributes towards the friction of the system, the team then carried out Monte Carlo simulations, using a system very similar to the experimental one. The simulated systems agreed. "It was the choice of the right model system that gave us the right to say that the spin-dependent component is definite," says Wiesendanger. He further explains that the magnitude of the spin-friction contribution can be of the same order as that of the electronic or lattice vibration-dependent friction, and this is surprising considering the orders of magnitude of difference between chemical and magnetic coupling energies. They also expect the spin friction to be a general phenomenon occurring in a large class of systems, not just magnetic systems.

Model behaviour

The researchers' findings are relevant for many reasons. First, that they have helped to better understand the fundamentals of friction, and their study serves as a starting point towards gauging the importance of the spin degree of freedom in "surface phenomena" such as the diffusion of magnetic atoms on magnetic substrates. On a more practical level, their work is relevant for magnetic data-storage systems such as hard drives, or magnetic motors with sliding parts.

"What is also very important is that our work," says Wiesendanger, "demonstrates how important it is to find and build the proper model systems that give clear answers. The idea [of magnetic spin friction] has been around theoretically for a few years, but to pin it down, the model was essential, and this is equally important in all fields of physics."

The research was published in Physical Review Letters.

Bursting bubbles drive micromotors

With one face looking back to the past and the other peering forward to the future, Janus was the two-faced Roman god of change. The name, however, is also used in modern science to describe tiny spheres coated on one side with one material and on the other side with another. What is interesting about these "Janus spheres" is that some of them can actually propel themselves in a specific direction when placed in a chemical solution, although why this occurs has been something of a mystery.

Now, however, Manoj Manjare and Yiping Zhao of the University of Georgia, along with Bo Yang of the University of Texas, have shown that Janus spheres actually propel themselves because one of the faces – but not the other – blows bubbles. Researchers are interested in such spheres because they are an example of a kind of "micromotor" that can propel itself through chemical media. Studying such motors could not only shed light on how simple living cells propel themselves but also help scientists to create tiny machines that could, for example, deliver drugs to specific parts of the body.

One family of designs that have shown great promise as micromotors are structures made from two different materials that interact differently with their environment. These could be rods with different materials at either end, or Janus spheres with hemispheres coated in two different materials. One of the materials is often a catalyst such as platinum, which speeds up the rate of conversion of hydrogen peroxide to water and oxygen, for example. The chemical reaction will occur at a much faster rate on the catalyst side of the motor – and this asymmetric release of chemical energy is converted into kinetic energy.

Mysterious mechanisms

However, for many micromotors no one quite knows how the conversion takes place. In the case of the Janus particles, scientists had thought that propulsion involved the creation of oxygen bubbles at the catalyst surface that then leave the surface and impart momentum to the motor. While bubble propulsion has been spotted in tubular micromotors, it had not been seen in spheres – something that is backed up by calculations that suggest that bubbles are unlikely to form on very small spheres because of their relatively large curvatures.

In their new work, Manjare, Zhao and Yang have tried to catch Janus particles in the act of blowing bubbles. The team carried out their experiments in a 5% solution of hydrogen peroxide in water and captured the behaviour of the micromotors using a fast CCD camera. They looked at Janus particles with diameters ranging from 2–50 μm – with one face coated with titanium and the other with platinum.

Minimum diameter for bubbles

Using the camera, the trio found that no bubbles formed on spheres smaller than 10 μm and that bubbles formed more readily as the diameter of the spheres increased – confirming previous calculations. Focusing on individual spheres, the team watched as a bubble formed over about 0.1 s. In the case of a 45 μm sphere, the bubble reached a diameter of about 73 μm before bursting in a process that occurs within 50 μs.

As the bubble grows, the bead is pushed slightly away from the centre of the bubble. But when the bubble bursts, the sudden drop in pressure sucks the bead back, giving it a push in the direction of the platinum face. Although bubble-driven motion has been seen in much larger structures, the team claim that this is the first time this rocking motion has been seen in micromotors. Closer inspection of the camera images reveals that some beads reached a top speed of about 14 cm/s and remained in motion for about 0.2 s before being stopped by the water's drag.

The trio then created a theoretical description of the propulsion mechanism by combining equations describing the growth of bubbles, the viscous drag of the water and the pressure drop expected when a bubble bursts. The team believes that its theoretical framework could be useful for describing the propulsion mechanisms of other bubble-propelled micromotors and nanomotors. The research is described in Physical Review Letters.

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