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Home » Page 1343

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.

‘Fano switch’ for colour displays

Researchers in the US have created the first “plasmonic Fano switch” made from tiny gold nanoparticles and liquid crystals. The device might be used as an active filter that reflects/transmits light of a certain wavelength and would be ideal in applications such as colour displays because it is more stable than traditionally employed organic chromophores.

Nanoplasmonics is a new and upcoming field of research that focuses on using metallic nanostructures to make tiny optoelectronics devices. Tiny metallic structures show great promise because they interact strongly with light via localized surface plasmons, which are collective oscillations of electrons on a metal’s surface.

The plasmonic Fano switch was developed by a team led by Naomi Halas and Stephan Link at Rice University in Texas. The device consists of a specifically designed cluster of gold nanoparticles fabricated using electron-beam lithography. The cluster comprises a large hemi-circular disc surrounded by seven smaller nanodiscs. Interactions between localized surface-plasmon resonances of the individual nanoparticles within a cluster lead to a so-called Fano resonance. This resonance comes about thanks to near-field coupling between collective “bright” and “dark” plasmon modes of the cluster.

Symmetry breaking

“By breaking the symmetry of the nanoparticle cluster through the hemi-circular centre disc, the Fano resonance is polarized and can only be observed for one polarization of incident light,” explains Link. As a result, no Fano resonance appears in the light spectrum, for incident light that is polarized at 90° to this direction.

The nanoparticle clusters are incorporated into liquid crystals in which the molecules at the device interface can be rotated in plane by 90° when an AC voltage of about 6 V is applied. The field creates a twist in the overall alignment direction of the crystals, which leads to a “homogenous nematic” (voltage off) to a “twisted nematic” (voltage on) phase transition.

Fano-resonance switching

“Thanks to the birefringence of the liquid crystal, the voltage-induced phase transition causes an orthogonal rotation of the scattered light from the plasmonic clusters as it travels through the device,” Link says. “This results in switching between the optical response with and without the Fano resonance, so we are thus able to switch the Fano resonance on and off in a voltage-dependent manner.” The presence or absence of Fano resonance affects how light is transmitted through the device and therefore the system could be used as an optical switch.

The device might be used as an active filter that reflects/transmits light of a certain wavelength and that could then be turned on or off by applying an external voltage, he adds. It could be ideal for use in colour displays because plasmonic nanostructures are much more stable than the organic chromophores typically employed as colour pigments today. Replacing these organic molecules, which photobleach over time, with plasmonic nanostructures could thus dramatically increase the lifetime and brightness of colour displays.

The Rice team says that it is now working on Fano-switch devices that operate at lower voltages. “We are also further optimizing the cluster geometry to manipulate the polarized Fano resonances so that we can achieve a larger contrast of the on–off modulation for a narrow spectral range,” says Link.

The current work is detailed in Nano Letters.

How many water molecules does it take to make ice?

How many water molecules does it take to make the smallest possible ice crystal? Around 275: that is the conclusion of researchers in Germany and the Czech Republic, who have developed the first-ever technique for probing large clusters of water molecules. Their findings could help to shed light on the formation of ice high in the atmosphere.

Water clusters are assemblies of water molecules that are held together by intermolecular hydrogen bonds. Until now, most studies have focused on small clusters with 12 molecules or less and the structure of these objects bears little resemblance to bulk ice. In the past few years, researchers in Japan have developed a spectroscopy-based technique to probe water clusters containing up to 50 molecules. However, detailed structural analysis of clusters with 100–1000 molecules, where ice crystallization was thought to occur, was beyond the reach of these studies.

The main difficulty in analysing large water clusters is knowing exactly how many molecules they contain. This is done by mass spectrometry, which involves ionizing the clusters by hitting them with high-energy radiation, which can smash the delicate clusters into fragments. Furthermore, researchers would rather study neutral than charged water clusters because these are involved in most of the ice-crystallization processes in nature.

Doped water clusters

Now, researchers including Thomas Zeuch at the Institut für Physikalische Chemie in Göttingen, Germany, have found a way to analyse neutral water clusters containing hundreds of molecules. Their success lies in two clever tricks. First, each water cluster is doped with a single sodium atom. Using this highly reactive metal means that the doped water clusters are ionized more easily than pure clusters and ensures that the electron is liberated from the sodium atom rather than the neutral water cluster.

Second, before being ionized, the doped clusters are excited with infrared radiation. This increases their temperature, thereby altering their structure in such a way that further lowers their ionization potential. The clusters can then be ionized with a 390 nm ultraviolet laser, which has low-enough energy to avoid fragmentation. The sizes of these ionized water clusters are determined using time-of-flight (TOF) mass spectrometry.

Then, in order to probe their structure, the infrared spectra of the water clusters are calculated. Infrared radiation with wavenumbers between 2800 and 3800  cm–1 is used, corresponding to the vibrational (stretching) frequencies of oxygen–hydrogen bonds. This vibrational spectroscopy provides an insight into the arrangement of water molecules inside the cluster. For instance, it is known that crystalline ice has an absorption maximum at wavenumbers around 3200 cm–1, whereas amorphous ice and liquid water have maxima at approximately 3400 cm–1.

Turning water into ice

Zeuch and colleagues obtained infrared spectra for cluster sizes ranging from 85 to 475 molecules. As expected, there was a shift in the spectrum maxima towards lower wavenumbers as cluster size increased. The transition from 3400 to 3200 cm–1 began at around 275 molecules, with the first crystalline ice occurring in the centre of the cluster, forming a ring of six hydrogen-bonded water molecules in a tetrahedral configuration.

As the cluster size increased further, the crystalline core gradually grew. By 475 molecules, the infrared spectrum was dominated by the ice structure: the formation of the ice crystal was all but complete. This behaviour matched theoretical predictions made by a different group of researchers in 2004.

“It’s not such a surprise that water crystallizes when you bring together a certain number of water molecules,” says Zeuch. “But the question was ‘Where does this happen?’ We’ve now developed a technique that pinpoints the size range where crystallization takes place.”

Going stratospheric

This new technique could help scientists to understand cloud-formation processes in the Earth’s atmosphere. “There are regions in the stratosphere without any nucleation sites where ice crystals are formed directly from water molecules,” says Zeuch. “The dynamics of this process could now be modelled in more detail.”

“These are really exciting results,” says Francesco Paesani, a chemist at the University of California, San Diego, who studies water clusters. “Nanometre-sized water particles play an important role in the atmosphere and ice crystals can be found in many types of clouds. Therefore, understanding how water clusters crystallize provides fundamental insights into cloud formation and properties, which in turn influence the Earth’s radiation budget and climate.”

Zeuch also believes that the research will help scientists to better model the interactions between water clusters in molecular-dynamics simulations. Understanding exactly how these water clusters behave in bulk water is one of the key goals of these models and one of the great unsolved problems in chemistry.

The research is described in Science.

India's innovators of tomorrow

Students at IISER, Pune, India


Post-dinner discussions.

By James Dacey, reporting from India

One of the most interesting visits I made during my time in Maharashtra was to the Indian Institute of Science Education and Research (IISER) in Pune. This is one of five such institutes set up in the past few years by the Indian government in an attempt to generate more interest among students in pursuing careers in fundamental research.

I met the dean of research, L S Shashidhara, who told me that students can enrol on a five-year integrated Master’s programme designed to provide a broad scientific education including a wide exposure to research. In the first two years, students take modules and lab work in physics, biology and chemistry, before having the choice to specialize in their third and fourth years. Then students have the entire final year to carry out a research project of their choice. This could be at the university facilities or it could be in industry, and it could even be in science policy.

Shashidhara, a bioscientist who studied for his doctorate at Cambridge University in the UK, told me that the government is pouring a lot of resources into the institution because it recognizes the need for more innovation. “The current problem of the pharma-industry, automotive industry and IT industry is they do not have sufficient numbers of people trained to do R&D work,” he says. “So, for example, they are happy to manufacture any number of cars in this country but to design a new car they don’t have the people available with sufficient knowledge.”

India’s focus on engineering education at the expense of the fundamental sciences is a topic that will be explored in the podcast I am producing on physics education in India, to appear on physicsworld.com in the next couple of months. In recording interviews for this podcast, I also met a number of the students at IISER, including the ones in the photograph above with whom I went for dinner. They seem to be thriving on the flexibility they have been allowed during their studies and have even found time to produce a college magazine. Though one of them did joke that when he tells his friends that he is studying science, a common reaction he gets is “Why didn’t you get engineering?” Unperturbed, he already knows he wants a career in physics research.

Ganesha galore!

A Ganesha idol in Pune, India


A colourful Ganesha in Pune.

By James Dacey, reporting from India

Earlier this week was the first day of the Hindu festival Ganesh Chaturthi and the city of Pune was a spectacular sight as a multitude of colourful Ganesha idols cropped up in temples across town. This 11-day festival takes place each year in honour of Ganesha, the elephant-headed god who is said to remove obstacles from the lives of those who worship him. I had been staying in Pune at the Inter-University Centre for Astronomy and Astrophysics (IUCAA). I think various people were concerned (quite rightly) that if I went to see the idols alone, then I would get horribly lost among the throngs of people. So Shruti, one of the astrophysics PhD students, kindly offered to join me on a rickshaw tour of some of the temples.

Shruti told me a version of the story of how Ganesha came to possess his unforgettable elephant head at the hands of his father Shiva, the god of gods. The story goes that Shiva’s wife Parvati had created the boy Ganesha out of earth and asked him to guard a room in her home, forbidding anyone to enter. When Shiva returned to find his path blocked by Ganesha, he was furious, so he chopped off the boy’s head. Parvati was distraught at sight of her headless son; so to appease his wife, Shiva went to fetch a new head, the first one he could find.

These days, Ganesha is celebrated by Hindus as a god of the people and Ganesh Chaturthi is one of the most significant festivals in his honour. During Ganesh Chaturthi, families and communities create decorative Ganesh idols out of clay and erect them in permanent and pop-up temples, returning each day to pray to the idol and share offerings. The festival comes to a close with “Immersion”, when idols are released into large bodies of water, commonly the sea or lakes. In fact, I’ve noticed several articles in the local papers about efforts this year to keep the Ganesha decorations environmentally friendly by using things like non-toxic paints.

On my fly-by tour today, things were still warming up; but there were still throngs of people and some fantastic idols on show, including the one pictured above. I also learned from a local that Pune is of particular importance because it is the home of the modern form of the festival, which began in the late 19th century. The man told me that small local celebrations got the support of the nationalist politician Lokmanya Tilak, who promoted the festival as a means of bringing people together of different castes and Hindu faiths to create unity against the British rule. He assured me that there are no hard feelings today!

From my experiences talking with Indian physicists and engineers, it has been fascinating to hear them describe Hindu traditions in the same academic way that they describe their work. For instance, it was funny yesterday during my temple tour when Shruti was telling us about the “logic” of the Ganesha-losing-his-head story. “It shows that you should always listen to your parents,” she joked. I was also given a more serious lesson about the meaning of Ganesh Chaturthi by a group of astronomers a couple of days ago when I visited the Giant Metre Wave Telescope about 80 km north of Pune. Over lunch they were talking about how Hindu stories often involve the concepts of renewal and cycles, and that this could explain the idea behind the immersion at the end of the festival.

One problem with the Ganesh idols, however, is that they are often coated with toxic paints that dissolve in the water to create environmental hazards. There have been attempts by the authorities to persuade people to buy eco-friendly alternatives, as this article in the Times of India explains, but other reports suggest that sales of the traditional version – being cheaper – remain stubbornly high.

Yesterday I said farewell to Pune and headed back to Mumbai, where I’m sure plenty more Ganesha madness awaits!

The physics of cancer

A new point of view
A new point of view

Sykes is based at the Curie Institute in Paris – an interdisciplinary research hub including laboratories and a hospital. In addition to her own research, Sykes has recently been involved in the formation of a special collection of academic articles for New Journal of Physics. Further contributions will be added to this collection throughout this year and next.

Should the convention for awarding the Nobel Prize for Physics be changed so that it can be given to a large collaboration?

By Hamish Johnston
Facebook poll

Four years ago Physics World's Jon Cartwright asked "Who will get a Nobel if the Higgs is discovered?" – and pointed out that under the current convention, the Nobel Prize for Physics is awarded to a maximum of three people.

Back then, this was a purely hypothetical question. But now the Royal Swedish Academy of Sciences (RSAS) will have to decide what to do about recognizing the discovery of a particle that some say was predicted by more than three people – and discovered by thousands working on the LHC.

In his piece four years ago, Jon pointed out that the RSAS could, in principle, choose to award the prize to an institution or collaboration – it just hasn't done so since the first prize was announced in 1900.

Could a Higgs Nobel be the first awarded to a large group? Back in 2008 Jon spoke to the physicist Anders Bárány, who is senior curator at the Nobel Museum and was secretary of the Nobel Committee for Physics for 14 years, and he said he was willing to put money on it.

"If Ladbrokes was taking bets on whether the RSAS will give the prize to 'an institution or a society' such as the LHC, I would bet a considerable sum on it...Physics has changed so much since the RSAS discussed this issue in 1900 that it would really be a limitation on the prize if it continued to be given only to individuals."

This week's Facebook poll is inspired by Bárány's wager:

Should the convention for awarding the Nobel Prize for Physics be changed so that it can be given to a large collaboration?

Yes
No

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 "Which scientific issue should be of greatest importance to politicians?" The most popular response was "science education" with nearly 40% of the vote. The runner-up was "science's role in economic growth" with about 23% and "climate change and energy security" at 16%. The least popular of the six options was "space exploration" with only 3% of the vote.

Philip Gibbs was one who didn't vote for education and he explained why: "Scientists are very mobile so if you support education but not research you will just educate people who go abroad. On the other hand, if you support research you will have experts coming here to form good science departments."

Does noise improve a bird’s spin-based compass?

According to the "radical pair" model, some migratory birds exploit the quantum phenomenon of electron spin to navigate using the Earth's magnetic field. This idea has now been bolstered by a new study from physicists in Singapore, who have shown that the spin-based process and the dynamics of a proposed "compass" molecule take place over similar timescales. The team also shows that the sensitivity of the avian compass can be enhanced, rather than degraded, by environmental noise. Others in the field, however, take issue with the latest results.

Many birds are believed to navigate long distances using the Earth's magnetic field. Some species, such as the European robin, are thought not to exploit the geomagnetic-field's polarity but instead its orientation relative to the horizontal, from which the directions north and south can be derived. It is believed that these birds' magnetic sense organ is embedded in their eyes.

The radical-pair hypothesis says that incoming photons excite molecules in the birds' retinas, causing an electron to transfer between two adjacent molecules and leave each of those molecules with an unpaired electron spin. The tendency for those spins to point in either the same or opposite directions while the molecules remain excited – states known as triplets and singlets, respectively – depends on the orientation of those molecules to an external magnetic field.

Singlets and triplets

Molecules lying along the field lines tend to favour the singlet state. A bird can therefore determine the orientation of the geomagnetic field by comparing the effect of the field on molecules arranged at different angles across the retina. According to one popular version of the hypothesis, singlets and triplets yield different chemical messengers that travel to the bird's brain.

Last year, Erik Gauger and colleagues at the University of Oxford and the National University of Singapore calculated how long these pairs of molecules typically remain excited, using data from behavioural studies that showed how oscillating magnetic fields can disrupt the sense of direction of European robins. Taking the value for the smallest field strength shown to disable orientation, Gauger's group worked out the minimum "radical-pair lifetime" to be around 100 μs and, taking into account possible sources of environmental noise, found that the quantum states generated by the excited molecules should persist for around the same length of time (Phys. Rev. Lett. 106 040503). The researchers pointed out that this exceeded the longevity of quantum states achieved in the laboratory and, as they saw it, weakened the view that life is too "warm and wet" to sustain delicate quantum phenomena.

However, in the new research (Phys. Rev. Lett. 109 110502), Dagomir Kaszlikowski and colleagues at the National University of Singapore argue that this earlier work falls down because it fails to take into account another behavioural study that considered the effect of an artificial static field on European robins' orientation. That study showed that the birds become disorientated when they experience a total field strength at least a third larger or smaller than the natural geomagnetic field. Kaszlikowski's group argues that when this study is taken into account, the radical-pair lifetime is just 5–7 μs.

The shorter the better

The researchers say that this figure agrees well with the excitation time of cryptochrome, the pigment molecule that is believed to generate the radical pairs within the retinas of European robins, according to independent experimental data. Kaszlikowski and colleagues also show theoretically that the robin's compass can, under certain circumstances, be made more sensitive to changes in the relative orientation of the Earth's magnetic field when there is environmental noise compared with when there is none. As a result, they conclude, longer-lasting quantum states may sometimes impede navigation.

Responding to the latest research, Gauger and his colleague Simon Benjamin of the University of Oxford say that the new figure for the radical-pair lifetime is not reliable, arguing that Kaszlikowski and his co-workers have carried out their analysis using a "pick and mix" of data points. Had the rival group selected its data differently, Gauger and Benjamin maintain, its figure "would have not have been very different from ours".

Important unanswered question

Benjamin says that in any case the result does not alter what he considers to be an important unanswered question: why do the robins take so long to get their magnetic reading? He argues that the field orientation could be established in just tens or hundreds of nanoseconds, rather than in microseconds, and that it is not in the birds' interest to take any longer, since a quicker process means more information, and therefore a better signal, in any given time interval.

The answer, he believes, might lie in a mechanism based on physics rather than chemistry. Describing a model that he and others developed with the recently deceased Marshall Stoneham, he explains that each pair of molecules excited by an incoming photon acts as a tiny electrical dipole, so the many molecules in the eye collectively generate an electric field. Such fields can directly affect normal vision, leading to darker or lighter patches. Triplet states generate more fields since they last longer. "A longer excitation time would lead to a stronger visual effect, improving the compass rather than making it worse," he says. "This seems to provide a clearer evolutionary path."

Exhibition will showcase the latest vacuum technologies

 

Encompassing nanotechnology, big science and lots in between – Vacuum Expo is coming to the Ricoh Arena in Coventry, UK, on 17–18 October. The only event of its kind in the UK, this year's exhibition includes the 3rd Vacuum Symposium, which includes scientific and technical sessions along with practical courses on vacuum technology.

The symposium will feature three technical programmes, one running on Wednesday and the other two on Thursday. The Wednesday programme is entitled "Vacuum and plasmas for Industry – essential ingredients for manufacturing success". It will offer presentations on a range of industrial process including a discussion of freeze-drying by Kevin Ward of Biopharma Technology. The use of vacuum technology in the large-area coating of glass will be covered by John Oldfield of the Pilkington Technology Centre, while Niall Macgearailt of Intel Ireland will talk about plasma process control in the semiconductor industry.

Big science on the agenda

One programme running on the Thursday will look at various aspects of vacuum technology for "big science". Speakers include CERN's Giulia Lanza, who will discuss the vacuum system of the Large Hadron Collider (LHC). She will cover everything from its design to how the system is performing under the exacting conditions of the particle-physics experiments done at the Geneva-based facility. Other accelerator-related presentations include one by Dimo Yosifov of TRIUMF in Canada, who will provide an update on the facility's cyclotron vacuum system.

Paul Flower of the Culham Centre for Fusion Energy in the UK will talk about the significant demands that fusion reactors put on their vacuum systems. The vacuum requirements of experiments done in extreme conditions using super-intense lasers will be covered by Steve Blake of the Central Laser Facility in Harwell, UK, and Matthew Cox of the Diamond Light Source – also in Harwell – will talk about the not-insignificant demands of running the vacuum system of a major synchrotron facility.

Also on Thursday is a programme that will focus on the use of vacuum technology in the creation of nanostructured metal-oxide thin films. Divided into three sessions, the first part of this programme will look at the techniques used for the fabrication and characterization of thin films. The second session focuses on optical films and includes an invited talk by Alfons Zöller of Leybold Optics in Germany, who will discuss the use of plasma-assisted reactive magnetron sputtering to manufacture high-performance interference filters.

Industrial processing

The final session of the thin-film programme will cover industrial processing and will begin with an invited talk entitled "Traceable measurements of water-vapour transmission rate for high-performance barrier layers" by Paul Brewer of the UK's National Physical Laboratory.

Delegates can also take part two training courses. On Wednesday and Thursday mornings, Austin Chambers of the University of York will teach a half-day course on "Basic vacuum principles". This course will begin with a basic description of a typical vacuum system and then move on to the discussion of specific topics including how a vacuum is specified, a comparison of the fluidic and molecular description of gases, and the fundamentals of pumping.

An afternoon course called "Creating and measuring vacuum" will also run on both days. Led by Ron Reid of Daresbury Laboratory, this session will look at the various techniques for creating and maintaining a vacuum – and how to determine its pressure.

Meet the experts

The Vacuum Symposium event is co-located with Vacuum Expo, which is organized by Xmark Media and will include representatives from many of the vacuum industry's leading companies. As well as having ready access to an extensive pool of vacuum knowledge, delegates will also get a close look at a wide range of new technologies at the exhibition. On display will be surface-engineering equipment such as plasma-enhanced chemical-vapour-deposition sources and sputtering systems. Visitors will also be able to see a variety of pumping technologies, including ion, turbomolecular, rotary-vane and root-pumping systems.

Photograph of a vacuum pump made by Busch Vacuum Pumps and Systems
All hands to the pump at Vacuum Expo

A wide range of vacuum-related equipment will be on show, including sample-handling systems, isolation valves and analytical instruments such as mass spectrometers and residual gas analysers. Power supplies and other electronics systems for vacuum systems will also be on display.

Vacuum Expo is held alongside the Photonex exhibition, which is the UK's largest event dedicated to optics, photonics and vision technologies. This year, biomedical and biophotonic applications of optical technologies will be a highlight of the event, which will include a one-day meeting on biomedical sensing on Thursday 18 October.

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